Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

One Moses; Read B Mapeo; Joyce G Maphanyane

doi:10.1080/24749508.2023.2219492

Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

Moses, One; Mapeo, Read B; Maphanyane, Joyce G 2023-06-21 00:00:00 GEOLOGY, ECOLOGY, AND LANDSCAPES INWASCON https://doi.org/10.1080/24749508.2023.2219492 RESEARCH ARTICLE Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques One Moses, Read B Mapeo and Joyce G Maphanyane Department of Environmental Science, University of Botswana, Gaborone, Botswana ABSTRACT ARTICLE HISTORY Received 8 November 2022 This study used morphometric techniques to generate new information describing the evolu- Accepted 25 May 2023 tion and hydrogeological behaviour of the Limpopo River Basin in Botswana, based on the analysis of drainage surface features, form, and size. Drainage basins provide basic information KEYWORDS on their evolution, which, when quantified, yield information on the interaction between Morphometry; geospatial tectonics, climatic, and surface processes. Drainage networks were extracted from the analyses; active tectonics; Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM) (90 m × 90 m), and drainage basin; Limpopo subsequently, morphometry indices were computed using ArcGIS 10.5. Drainage network River Basin; Botswana extraction was performed using the Arc Hydro extension in ArcGIS 10.5. ArcGIS 10.5 image processing technique was used to extract lineaments and create rose diagrams. The results showed that the Limpopo sub-basins in Botswana were drained by fourth- and fifth-order streams, with a total drainage area of 107, 871 km . Additionally, the basin asymmetry and mean bifurcation ratios showed tilting in the sub-basins, suggesting tectonic instability and structural control in a low – to-moderate active tectonic zone. The sub-basins also had a coarse texture, indicating a high infiltration capacity. These results are essential for planning and managing watershed systems, flood risk assessment, and potential groundwater assessment for the different sub-basins of the Limpopo River Basin in Botswana. Introduction topographic maps, aerial photographs, satellite data, The morphometric analysis of a watershed can provide and digital elevation models and by quantifying various valuable information regarding its characteristics, morphotectonic indices (Bhatab et al., 2020; Keller, regional topography, drainage pattern, basin geometry, 1986; Mohanty et al., 2004; Radaideh et al., 2016). nature of the bedrock, and potential groundwater Analysis of drainage basins in response to tectonic zones, thus aiding the effective planning and manage- processes can provide insight into the history of ment of natural resources (Chaitanya et al., 2021; a particular region alongside any recent deformational D. S. Singh & Awasthi, 2010; Radwan et al., 2017; events (Joshi et al., 2022; Matoš et al., 2016; Menier Shelar et al., 2022; Vincy et al., 2012). The morphotec- et al., 2017; Psomiadis et al., 2020). Drainage networks tonic analyses of drainage basins reveal the relationships are the most active and sensitive elements that can be between the properties of a drainage basin and climate, used as powerful tools for understanding the tectonic basin relief, lithology, and tectonics (Bhatt et al., 2020; activity of various regions (Ahmed & Rao, 2016; Chaitanya & Moharir, 2017; Lone, 2017; Różycka & Karami et al., 2018; Mumipour et al., 2012; S. Singh Migoń, 2021; S. Singh et al., 2019, 2021). These analyses et al., 2018; Sedrette & Rebai, 2022). Streams are also provide a means of assessing natural resources powerful indicators of neotectonic changes and cli- within a basin, including options for the sustainable matic conditions since climatic conditions, lithology, management of water resources and groundwater and geologic structures control stream processes. exploration (Strahler, 1964). The quantitative morpho- These features influence flow, erosional dynamics, metric properties (derived from remote sensing ana- and sediment transportation. Therefore, by studying lyses in a Geographic Information System (GIS) the nature and types of drainage patterns, the rock environment (Chaitanya & Moharir, 2017; Chaminé types and geological structures responsible for the et al., 2021; Issa & Saleous, 2019; Maphanyane, 2016; development of a drainage network can be interpreted Pauly, 2009); are used to evaluate the neotectonic activ- alongside the river dynamics over time (Twidale, ities within basins. Information regarding the tectonic 2004). This is due to the knowledge that the evolution history of a given region can be obtained by analysing of landscapes, including river systems, results from CONTACT Read B Mapeo mapeorbm@ub.ac.bw Department of Geology, University of Botswana, Private Bag UB00704, Gaborone, Botswana © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 O. MOSES ET AL. complex interactions involving climate, tectonics, and rainfall trends in the watershed are decreasing over surface processes (Willett et al., 2006). time (Maruatona & Moses, 2022, and references Morphometric studies performed in the Limpopo therein). In the Notwane sub-basin, the annual rainfall River Basin in southern Africa have primarily focused decreased between 1974 and 1999, ranging between on flood risk assessment, climate change effects on the 600 and 500 mm a year but has since decreased to 500 basin, and the livelihoods of individuals residing along to 400 mm. Furthermore, in the Mahalapye and the basin (Petrie et al., 2015; Alemau et al., 2018). Bonwapitse sub-basins, the annual rainfall decreased However, these studies lacked the application of techni- from 450 mm in 1982 to approximately 380 mm ques utilising morphometry, remote sensing, and GIS, all between 1994 and 2018. Data from the Shashe sub- of which can address essential questions regarding the basin show that the total annual rainfall decreased hydrogeological behaviour of the basin in relation to from an estimated 530 mm in the 1970s to approxi- recent earth deformations (SARDC, 2002; Spaliviero mately 450 mm between 2008 and 2017. The data for et al., 2011; WMO, 2012). In southern Africa, morpho- the Lotsane sub-basin shows that the annual rainfall metric analysis has previously been used to characterise increased from 300 mm in 1988 to 400 mm in 2017. the history of the Cape Fold Belt region in South Africa, Furthermore, the annual average minimum and max- which has undergone reorganisation as a result of the imum temperature data showed a marginal increase of exhumation of the belt and the retreat of the great approximately 1.0–1.5°C between 1970 and 2019 escarpment in the Western Cape of South Africa (Moses, 2017, and references therein). The entire (Richardson et al., 2016). Alemaw (2018) examined Limpopo River Basin (LRB) lies in the eastern portion flood hazards in the LRB using the geospatial character- of southern Africa between 20°–26°S and 25°–35°E. istics of basin geomorphology. In this study, DEMs and The LRB straddles four nations (Figure 1): South the geomorphic instantaneous unit hydrograph (GIUH) Africa (47%), Botswana (17.7%), Zimbabwe (16%), were used to simulate hydrological responses to rainfall and Mozambique (19.3%) (Chilundu, 2008; Petrie in the basin. The GIUH depends on Horton’s morpho- et al., 2015). The LRB is spread across the above metric parameters and rainfall data. The results revealed nations, whilst its mainstream marks the international that fast-flowing floods in the LRB were generated in the boundary between South Africa and Botswana and upland region of the basin; thus, an early warning system between South Africa and Zimbabwe. The Limpopo was developed for the sub-basin. River flows between South Africa and Botswana and is The main objective of this study was, therefore to joined by more tributaries from Zimbabwe in the east use morphometric parameters extracted from an SRTM before entering Mozambique and draining into the digital elevation model (DEM) in a GIS environment to Indian Ocean. The LRB flows through several climatic understand the evolution of the Limpopo River basin in zones but generally lies within the Köppen climate Botswana through quantitative analysis. Such studies zone BSh (semi-arid, dry, hot). The Limpopo River are essential for basin management and prioritisation Basin primarily experiences an arid, hot, and dry cli- for soil and water conservation along the span of the mate except for the mouth of the river basin in basin. The Limpopo River Basin lies in eastern Mozambique, which experiences a tropical climate. Botswana and comprises six sub-basins, with rivers However, in most of Botswana and Zimbabwe, it lar- that drain to the east separated from rivers draining gely drains in arid environments (CAR, 2010). to the west (Figure 1). We computed parameters such as basin asymmetry, hypsometric integral, transverse Geological setting topographic symmetry factor, sinuosity index, stream length gradient, and longitudinal river profiles. Based The Limpopo River in Botswana trends NE – SW and on these parameters, we assessed the role of tectonics in cuts through several different geological formations, shaping the basin and described several features of the including lithological units related to the Kaapvaal and geomorphic development of the Limpopo Basin. Zimbabwe cratons and the Limpopo Belt, between the cratons (Chinoda et al., 2009; McCourt, 2004) (Figure 2a,b). The Limpopo Belt follows an east – Study area northeast trending granulite gneiss terrane that This study was carried out across six sub-basins of the forms a major geological structure developed in Limpopo River Basin in Botswana. The Limpopo Botswana, Zimbabwe, and South Africa. This is River drains a large area of six sub-basins, namely, believed to have resulted from a collision between Mahalapswe, Bonwapitse, Lotsane, Notwane, Zimbabwe and the Kaapvaal cratons at approximately Motloutse, and Shashe, all of which contain major 2.6–2.0 Ga (Blenkinsop & Rollinson, 1992; Van rivers and tributaries (Figure 1) (Table 1). The study Reenen et al., 1992). The centre of the belt consists of area was irregular and bounded by the latitude 20.448° strongly deformed granulite facies rocks derived from S to 25.419°S and longitude 25.111°E to 29.391°E, granite-greenstone that are similar to those found in defining a large watershed of 107 871 km . The annual the Zimbabwe Craton (Aldiss, 1991). Both the GEOLOGY, ECOLOGY, AND LANDSCAPES 3 Figure 1. Location map and DEM of the Limpopo River Basin (LRB) in southern Africa. A detailed DEM of the LRB subbasins in Botswana is shown on the right. Kaapvaal and Zimbabwe cratons consist of high-grade However, two major seismic events have occurred here, metamorphic rocks, including lithologies of the gran- initiated by two earthquakes in September and ite-greenstone terrains (Aldiss, 1991; Carney et al., October 1952 with ML Richter magnitudes of 6.1 and 1994; Ranganai et al., 2002) (Figure 2a,b). Stratified 6.7, respectively, in northern Botswana in the Okavango lithologies comprising amphibolite, tonalitic gneiss Rift Zone (ORZ). The Okavango Rift Zone (ORZ) is with metamorphosed basic/ultrabasic intrusions, and constructed by a series of horst and graben structures granitoid gneiss underlie much of this area (Figure 2a, in northwest Botswana at the southern tip of the East b). The southern portion of the LRB in South Africa is African Rift System (EARS) (Modisi et al., 2000; Modisi, underlain by mafic and intrusive ultramafic rocks, 2000). Rifting on the ORZ began after the emplacement including granite and felsic lavas (Chinoda et al., of the 179 Ma Karoo dyke swarm as the dykes are dis- 2009). The lower portion of the LRB in Mozambique placed by rifting (Le Gall et al., 2005). Several north- is an erosional plain formed from consolidated and easterly trending rift faults occur parallel to the unconsolidated sedimentary rocks, including argillite, Okavango Rift Zone, terminating in the Okavango fluviatile sandstones, and mudstones (SARDC, 2002). Delta in the south (Modisi et al., 2000; Modisi, 2000). Due to the different characteristics of the lithologies, Another region of active faulting is the Makgadikgadi their effect on the drainage patterns, including the Rift Zone, located to the southeast of the ORZ. The deformations experienced, can differ. Makgadikgadi Rift system occurs northwest of Sua Pan, with faults following the structural trend of the Paleoproterozoic Magondi Belt with faults that also strike Seismicity in the Limpopo River Basin (in NNE to NE (R. B. M. Mapeo et al., 2001; Eckardt et al., Botswana) 2015, and references therein). Nthaba et al. (2018) showed that between 1966 and 2012, when records Tectonically, a major part of the basin lies on the became available, 327 microseismic events were Limpopo Belt terrane, a primarily aseismic region. 4 O. MOSES ET AL. Figure 2. (a) a regional geological map of southeastern and central Botswana covering areas of the Notwane Subbasin of the LRB (source Mapeo & Tschirhart, 2022). (b) a regional geological map of southeastern and central Botswana covering areas of the Bonwapitse, Mahalapswe, Lotsane, Motloutse and Shashe Subbasins. Note the major structures that define the geological terrain of the Limpopo Belt (source Mapeo & Tschirhart, 2022). recorded, implying that this region of Botswana has low ancient faults within the Limpopo Belt, in a zone stress dominated by minor-to-moderate events. This bounded by two major shear zones/faults, the Phalala previous study showed that micro-seismic activities ran- and the Sunnyside Shear Zones (Figure 2b). Towards the ging from 1.3 to 5.7 in magnitude were distributed in the south, between the Notwane and Bonwapitse rivers, northern and southern parts of Botswana. In April 2017, Mulabisana et al. (2021) previously analysed InSAR an Mw 6.5 (moment magnitude) earthquake occurred in Sentinel-1 images to map an NW – trending active Central Botswana with the epicentre at Moiyabana fault that was in line with the location of the (25.134°E, 22.565°S; depth 22 ± 3 km). This earthquake Moiyabana Earthquake (Figure 2c). The Limpopo River was associated with 15 events with magnitudes up to Mw basin has a complex history of shifting and changing its 5 (Moorkamp et al., 2019; Mulabisana et al., 2021). These course, which is said to have been due to the splitting of events also attest to intraplate earthquake activity asso- the Gondwanaland supercontinent (Goudie, 2005). The ciated with reactivation along major NNW trending Limpopo River formed along a rift during the opening of the Mozambique channel (McCarthy, 2013). Here the triple junction failed, whilst only two branches formed Table 1. A table showing the subbasin of the Limpopo river in where Africa and South America had separated. Botswana and their major rivers. The East African rift system is also an example of LRB-BASIN RIVER LENGTH (km) AREA (km ) the evolution of drainage systems in the East African Shashe Thalogang 44 29612.2 Rift Valley system, which disrupted the upper tribu- Shashe 36.2 Tati 156.3 taries of the ancestral Limpopo River and diverted Ramokgwebane 43.0 some of this into the lower Zambezi River. For exam- Notwane Dikolakolare 81.4 17423.7 Sekhukhwane 34.4 ple, the Kwando River in northern Botswana was Notwane 21.5 diverted by faulting associated with the EARS into Metsimotlhaba 75.8 the lower Zambezi River (McCarthy, 2013). Evidence Taung 27.8 Motloutse Motloutse 288 19589.6 has shown that the southern African paleo drainage Moenyenane 46 system consisted of three major rivers: The Karoo and Thune 138 Makutlagaga 25 Kalahari rivers in the south-flowing to the Atlantic, Mhwana 35 and the Limpopo on the west coast flowing eastwards Mahalapye Mahalapye 74 13106 Taupye 77 to the Indian Ocean. Moore and Larkin (2001) sug- Bonwapitse Bonwapitse 132 13015 gested that during the Jurassic and Lower Cretaceous Serorome 129 Lotsane Lotsane 119 15124.5 periods, the Kafue and Luangwa Rivers were tribu- Susubela 76 taries of the upper Zambezi and flowed initially into GEOLOGY, ECOLOGY, AND LANDSCAPES 5 Figure 2. (c) a DEM image showing faults scarp of an NW trending fault. The fault is located north of the Dikolakolane and Sekhukwane rivers (Table 1). The fault crosses the A1 Road to the north of Botswana at 24.004°S, 26.376°E. All subbasins are largely located on the hanging wall of this fault (Notwane, Bonwapitse, Bonwapitse, Mahalapye, Lotsane, Motloutse and Shashe) (source Mulabisana et al., 2021). the Limpopo via the present Shashe River. The are designated as first-order, the confluence of two Limpopo River originally flowed towards the south- first-order channels gives channels segments easterly direction, implying a reversal of the flow of second-order, two second-order streams join to direction to the north-easterly direction (Moore, form a segment of third-order and so on. The areal 1999). or shape indices were computed using Horton’s, Strahler and Schumm’s laws. The indices estab- lished were the drainage density, drainage fre- Materials and methods quency, and elongation ratio. Horton’s laws and Strahler and Schumm’s methods were used to Morphometric analysis of the Limpopo River Basin establish the following indices: basin asymmetry, drainage system in Botswana required a delineation hypsometric integral, transverse topographic sym- of all existing rivers and streams and applied metry factor, sinuosity index, stream length gradi- a quantitative method to extract drainage networks ent, and longitudinal river profiles. Furthermore, and morphometric indices from the Shuttle Radar some indices were used to identify regions of activ- Topographic Mission (SRTM) Digital Elevation ity tectonics. The flow diagram below shows the Model (DEM) (90 m) (data available from https:// process of generating the morphometric and mor- earthexplorer.usgs.gov) using the Spatial Analyst photectonic parameters (Figure 3). tool of ArcGIS 10.5 software. The DEM was pre- processed using the fill tool to remove null data and sinks. Flow accumulation and direction were Basin asymmetry and sinuosity computed using DEM. Subsequently, streams and The basin asymmetry factor indicates deformation sub-basins were delineated. The drainage network caused by tectonic tilting that results in warping or was analysed through morphometry analysis flexing, thus leading to longer streams possessing (Horton’s laws and Strahler method) to extract a larger area on one side of the basin (Mathew, 2016). the morphometry indices and drainage structures. The basin asymmetry factor of the different sub-basins The morphometric parameters under linear and suggests that the basins were developed under active shape characteristics were computed using standard tectonic settings or that there was lithologic and struc- methods and formulae (Horton, 1932, 1945; Smith, tural control. This factor is used to assess the tilt and tilt 1950; Strahler, 1964) (Table 2). The linear para- direction of the basin. The Af is given by meters included stream order, length, length ratio, Ar and bifurcation ratio. The stream order is Af ¼ � 100;where Ar is the area of a basin on the At a hierarchical positional measurement of a stream right of the mainstream (looking downstream), whilst within its associated tributaries (Leopold et al., At is the total drainage basin area. The asymmetry 1964). The smallest, un-branched fingertip streams factor should be 50 or closer in a stable environment, 6 O. MOSES ET AL. thus indicating no, or only a small amount of tilting of 2002; Pavano et al., 2016; Pérez-Peña et al., 2009; the main drainage line. Basin sinuosity deals with the Pérez‐Peña et al., 2009). Several previous studies channel pattern of a drainage basin. In general, its value have demonstrated that hypsometric integrals can varies from 1 to 4 or more. Rivers with a sinuosity of 1.5 provide information regarding lithological resis- are called sinuous, whilst those above 1.5 are called tance and tectonic uplift, both of which assess meandering (Ahmed & Rao, 2016). the disequilibrium balance between erosion and tectonic forces in each basin (Hurtrez et al., 1999; Lifton & Chase, 1992). The shape of the Basin transverse topographic symmetry factor hypsometric curves and integral values provide (TTSF) valuable information regarding the erosional stage of the basin and the tectonic, climatic, and This is another quantitative index used to evalu- lithological factors controlling landform develop- ate basin asymmetry (Keller & Pinter, 2002). The ment (Huang & Niemann, 2006; Moglen et al., absence of tectonics would cause the main river 1998; Willgoose & Hancock, 1998). Strahler to flow evenly from both sides, forming (1952a, 1952b) classified drainage basins according a perfectly symmetric basin with a TTSF value to their hypsometric integral (HI) and curve (HC), of 0. TTSF is defined by TTSF = Da/Dd, where where HCs are convex upwards with high HIs > Da is the distance from the midline of the drai- 0.6 are of the youthful stage, the S-shaped HCs nage basin to the midline of the active meander and HIs between 0.35 < HI < 0.6 are in the matur- belt whilst Dd is the distance from the basin ity stage, and concave upward HCs and low HIs < midline to the divide. The basin midline is the 0.35 are in the old stages. symmetry line of the basin, with values closer to 1 indicating basin tilt. Longitudinal profile analyses and lineaments Hypsometric integrals A longitudinal river profile is a powerful tool The hypsometric curve and integral are accepted required to draw relevant conclusions regarding indicators of active tectonics in terrains under- the geological setup and structural and tectonic going denudation and indicate erosional processes deformations of a watershed (Singh & Awasthi, and patterns along a river profile (Mathew, 2016). 2010; Dar et al., 2014; Pavano et al., 2016; Roy Hypsometric analysis of drainage basins has been & Sahu, 2015; Snyder et al., 2000). Longitudinal applied to areas experiencing rapid tectonic activ- profiles in nature are depicted by a concave ity and allows for comparisons between different upward curve that is generally not smooth. sub-basins (Hurtrez et al., 1999; Keller & Pinter, Table 2. A table showing morphometry and morphotectonic indices used in this study and their formulas. Morphometric/ morphotectonic parameter Formula Reference Linear parameters Stream order Hierarchical rank Strahler (1964) Stream length Length of the stream Strahler (1964) Stream number Nu= N1 +N2 + . . . . Nn Horton (1945) Lu Mean stream length Nu Strahler (1964) Lsm ¼ Lu Stream length ratio Horton (1945) RL ¼ Lu 1 Nu Bifurcation ratio Schumm (1956) Rb ¼ Nuþ1 Mean bifurcation ratio Rbm = Average of bifurcation ratios of all orders Strahler (1964) Areal parameters Lu Drainage Density Dd ¼ Horton (1945) Nu Stream frequency Horton (1945) Df ¼ Form factor R ¼ Horton (1945) f 2 Elongation ratio Re= √A/π/L Drainage texture Dt = Nμ/P Where, Nμ = No. of streams in a given order and P = Perimeter (Kms) Horton (1945) Relief parameters Relief Hb ¼ H h Schumm (1956) Ruggedness number Rn=Dd*Hb Schumm (1956) Hypsometric integral Hi ¼ ðh h Þ=ðh h ÞHi Strahler (1952b) mean min max min Morphotectonic parameters Ar Basin Asymmetry factor Keller and Af ¼ � 100 At Nichola (1996) Transverse Topographic T = Keller and symmetry Factor Da/Dd Nichola (1996) where, Da is the distance from the midline of the drainage basin to the midline of the active meander belt and Dd is the distance from the basin midline to the basin divide Sinuosity index SI=AL/SL, AL =actual channel length between source and mouth, SL =straight line length Schumm (1956) GEOLOGY, ECOLOGY, AND LANDSCAPES 7 Figure 3. A schematic flow diagram for the methodology used for the extraction of morphometric and morphotectonic parameters from the STRM DEM for the Limpopo River subbasins in Botswana. According to the concave-upward power function, Bonwapitse, and Notwane Rivers. The objective the long profile deviation of a steady-state river of the field survey was to identify the knick- from this ideal shape can be used to infer active points along the river profiles that were inter- tectonic deformation in the basin (Hack, 1957; preted as river terraces. River terraces are rem- Roy & Sahu, 2015). This is affected by several nants of abandoned floodplains, riverbeds, or factors, including discharge, amount of river valley floors formed during previous erosion, load, bed and bank material, topographic relief, deposition, or both, along river valleys. They depth and depth width, and river gradient. The express the previous river channel and floodplain profiles may also include convexities due to the levels that correspond to valley aggradation and local steepening of the channel from resistive rock river incision (Maddy, 1997). The formation of strata, larger loads, and tectonic activity. The river terraces requires a sustained change in base lineaments were identified using DEM and exist- level that can be achieved by uplift, sea-level ing geological maps. Using the spatial analyst tool changes, and climate changes (Bridgeland & in ArcGIS 10.5, a hill shade was created, which Westaway, 2008). Furthermore, river terraces pos- enhanced linear features on the DEM; the linear sess a flat surface called the tread, a steep slope features were then traced and saved as a new separating it from adjacent flood plains or ter- shapefile. The geological maps were used to vali- races, and an erosional base called strath date the structures, whilst the extracted linea- (Schirmer, 2020). ments included faults, dykes, and joint systems. Results and analyses Field survey The drainage basin morphometric parameters of A field survey was conducted to verify the drai- the Limpopo River basin in Botswana have not nage characteristics obtained from the digital ele- previously been used to understand the basin’s vation model analysis along the Mahalapye, hydrological and morphological characteristics as 8 O. MOSES ET AL. well as structural controls if any (Lifton & Chase, (Table 3). The main truck segments of the four 1992; Rai et al., 2017; Prakash et al., 2017, 2019). sub-basins (Bonwapitse, Lotsane, Motloutse, and The results of different hydrological and morpho- Notwane Rivers) are fourth-order, whilst Shashe is logical characteristics derived from different mor- of order five. In total, 1145 streams are present in phometric parameters are discussed below. the basin and define the various catchments. Linear morphometry indices Stream length Stream order The stream length distances within the basin were measured along the various channels from the The stream order is a hierarchical positional mea- river source to an outlet or given point. The indi- surement of a stream within its associated tribu- vidual stream lengths in all orders were added to taries (Leopold et al., 1964; Rai et al., 2017). The determine the total stream length for the six smallest, un-branched fingertip streams are desig- basins, which are 1465, 1276.50, 919.99, 2102.20, nated as first-order, the confluence of two first- 1843, and 3434.40 km, respectively (Table 3). The order channels gives channels segments of second individual stream lengths decreased with increas- order, two second-order streams join to form ing order. The graphical representation of the a segment of third order and so on. Four of the logarithm of stream length versus stream order six sub-basins (Notwane, Bonwapitse, Lotsane, and for Notwane, Bonwapitse, and Lotsane (Figure 5) Motloutse) in the study area are fourth-order exhibits a certain amount of deviation from basins; Shashe is a fifth-order basin, and the a linear trend, thus suggesting variation in lithol- Mahalapswe sub-basin is a third-order (Figure 4). ogy and topography. In contrast, the Mahalapswe The number of streams decreases with increasing and Shashe basins display a linear pattern, thus order, with the first-order basins generally posses- indicating that they are incised through sing a significantly higher number of streams Figure 4. DEM of the Limpopo Subbasin showing the stream orders for the watershed of the study area. GEOLOGY, ECOLOGY, AND LANDSCAPES 9 Table 3. A table showing the stream orders, number of streams, stream length and stream length ratios in the six subbasins. Sub-basin Stream Order No of Streams Stream Length (Km) Mean stream Length Stream Ratio Total stream Length(Km) Notwane 1 146 973.8 6.7 2 20 482.9 24.1 0.5 3 13 144.6 11.1 0.3 4 7 241.7 34.5 1.67 1843 Bonwapitse 1 114 809.5 7.1 2 21 416.1 19.8 0.51 3 7 97.6 13.9 0.23 4 2 141.8 70.9 1.46 1465 Mahalapswe 1 76 498 6.6 2 20 291 14.6 0.58 3 3 130.9 43.6 0.45 919.99 Lotsane 1 99 656 6.6 2 17 393.3 23.1 0.59 3 7 101.5 14.5 0.26 4 1 125.7 125.7 1.23 1276.5 Motloutse 1 175 1093.9 6.3 2 16 475.7 29.7 0.43 3 7 202.2 28.9 0.43 4 3 330.4 110.1 1.63 2102.2 Shashe 1 202 1512.3 7.5 2 95 1120.9 11.8 0.74 3 62 484.2 7.8 0.43 4 8 127.3 15.9 0.26 5 24 189.7 7.9 1.5 3434.4 homogeneous underlying material, perhaps com- 1.67, suggesting a late youth to maturity stage of geo- prising weathered basement rocks. morphic development. Stream length ratio Bifurcation ratio The mean stream length in Botswana ranges from 6.0 This factor describes the ratio between the number of to 125 m and increases with increasing stream orders. streams in one order and the next and has been calcu- The mainstream trunk has the highest mean stream lated for the six basins (Table 2). The mean bifurcation length, ranging from 15.9 for the Shashe sub-basin to ratio in the six sub-basins ranges from 2.9 to 5.3. 125.7 for the Lotsane sub-basin (Table 3). The stream A high bifurcation ratio indicates a short concentra- length ratio ranges between 0.5 and 1.7; these ratios tion time and a high probability of flooding. The mean are inconsistent, likely reflecting differences in slope bifurcation ratio is 5.16, which is larger than the stan- and topography (Table 3). The stream length ratio dard ratio (Table 4). The bifurcation ratio varies between the third and fourth order is significantly between 2 in flat and rolling surfaces to 4 or 5 in higher than the others. The values range from 0.43 to mountainous or highly dissected drainage basins NOTWANE SUBBASIN MOTLOUTSE SUBBASIN SHASHE SUBBASIN 3 4 4 3 3 y = -0.2845x + 3.0236 2 y = -0.1931x + 3.1181 y = -0.2339x + 3.1388 R² = 0.9828 2 2 R² = 0.6503 R² = 0.7018 1 1 0 0 0 1 2 3 4 1 2 3 4 1 2 3 4 STREAM ORDER STREAM ORDER STREAM ORDER a) b) c) MAHALAPSWE SUBBASIN LOTSANE SUBBASIN BONWAPITSE SUBBASIN 3 3 4 y = -0.2741x + 3.0646 2 2 y = -0.2899x + 3.142 R² = 0.8251 y = -0.2901x + 3.0063 R² = 0.7851 1 1 R² = 0.9874 0 0 0 1 2 3 1 2 3 4 1 2 3 4 STREAM ORDER STREAM ORDER STREAM ORDER d) e) f) Figure 5. Plots showing the relationship between the logarithmic of stream length and stream order. The graphs (a, b, c, d, e and f) show a relationship between stream length and stream; the stream length increases with increasing stream order. LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH 10 O. MOSES ET AL. Table 4. A table showing the geomorphic indices of the six subbasins in the study area. Notwane Bonwapitse Mahalapswe Lotsane Motloutse Shashe Geomorphic Indices Basin 1 Basin 2 Basin 3 Basin 4 Basin 5 Basin 6 Basin Asymmetry 43.23 (high) 32.07 (high) 10.7 (moderate) 27.08 (moderate) 60.08 (high) 18.75 (moderate) Sinuosity Index 1.39 0.81 0.61 1.28 1.33 1.21 Bifurcation Ratio (mean) 3.6 (moderate) 3.9 (moderate) 5.3 (moderate) 5.1 (moderate) 5.2 (moderate) 2.9 (low) Transverse Topographic Symmetry Factor 0.27 (low) 0.48 (moderate 0.12 0.34 (low) 0.37 (low) 0.47 (moderate) Hysometric Intergral 0.49 (high) 0.49 (high) 0.5 (high) 0.5 (high) 0.5 (high) 0.49 (high) Basin Elonagtion ratio 0.623 (high) 0.763 (moderate) 0.742 (moderate) 0.472 (high) 0.423 (high) 0.409 (high) Hysometric Curves Convex(high) Convex (high) Comvex (high) Convex (high) Convex (high) Concave (high) (Verstappen, 1983). The minimum bifurcation ratio density in these sub-basins is low and likely reflects indicates whether it is a flatter or rolling drainage a low dissected catchment with a relatively slow basin. The study watershed is highly susceptible to hydrogeological response to precipitation events. The flooding due to the high bifurcation ratio. The stream frequency provides detailed information Mahalapswe sub-basin exhibits the highest mean regarding a basin’s permeability, infiltration capacity, bifurcation ratio of 5.3, possibly due to the observation and relief. Stream frequency depends on rainfall in the that it possesses the lowest number of streams. The area, relief, and erosive resistivity of the underlying Shashe sub-basin shows the lowest mean bifurcation rocks (Mahala, 2020). The stream frequency values in ratio of 2.9 (Table 3). The mean bifurcation values sub-basins are low and range from 0.008 to 0.012, indicate structural disturbance in the basin, which suggesting a low run-off component in the sub- could reflect the hilly terrain with less permeable basins alongside a high infiltration likelihood. The rocks in the basin. drainage texture values in sub-basins range from 0.085 to 0.201; in this respect, all the basins are coarse- textured (Table 5). Although the infiltration capacity Textural morphometry indices has not yet been derived for any of the sub-basins, judging by the low drainage density and the low Drainage area, density, stream frequency, and stream frequency, the infiltration numbers would drainage texture likely be extremely low, suggesting a higher infiltration The Shashe sub-basin possesses the largest drainage potential within the sub-basins. area of 29,612.2 km ; however, of this, only 10,790.2 km is located in Botswana, whilst the remaining is in Zimbabwe. The next largest is the Notwane sub-basin Elongation ratio which possesses a moderate drainage area of 17,423.7 km . The Bonwapitse sub-basin has the smallest drai- The elongation ratio of watersheds is defined as the nage area of 13,015.3 km (Table 5). The drainage ratio of the diameter of a circle of the same area as the density could be related to the state of dissection of watershed to the maximum watershed length. The the catchment areas as well as the response to preci- numerical value varies from 0 (for highly elongated pitation events. Regarding behaviour, high magni- basins) to 1 (in the circular basin). Meanwhile, the tudes of drainage density indicate that the catchment elongation ratio values are close to 1.0 for regions of is highly dissected and responds quickly to precipita- tremendously low relief and are between 0.6 and 0.8 tion events. A low drainage density would indicate the for regions of strong relief and steep ground slope. In opposite scenario. The drainage density for the six this study, it was suggested that the Lotsane, Shashe, sub-basins ranges from 0.099–0.122, thus indicating and Motloutse sub-basins are more elongated (<0.5), that there is approximately 90 to 120 m of stream while the Notwane sub-basin is elongated (0.623). The length for each square kilometre area and is of the low category. The results suggest that the drainage Mahalapswe and Bonwapitse sub-basins are less Table 5. Table showing textural characteristics of the Limpopo River Subbasins Notwane Bonwapitse Mahalapswe Lotsane Motloutse Shashe Areal Indices Subbasin 1 Subbasin 2 Subbasin 3 Subbasin 4 Subbasin 5 Subbasin 6 Drainage area (Km ) 17423.7 13015.3 13106 15124.5 19589.6 29612.2 Drainage Density (Km/Km ) 0.113 0.122 0.106 0.101 0.108 0.099 Drainage Texture 0.201 0.194 0.164 0.14 0.194 0.085 Form Factor 0.305 0.456 0.433 0.175 0.14 0.131 Relief (m) 646 516 620 832 722 872 Stream Frequency 0.011 0.012 0.011 0.009 0.01 0.008 Ruggedness Number 0.072 0.063 0.031 0.084 0.078 0.042 GEOLOGY, ECOLOGY, AND LANDSCAPES 11 elongated (0.7–0.8). These results suggested that the with AF > 50 are tilted to the left. The results showed that the Bonwapitse, Lotsane, Mahalapswe, sub-basins are in areas with high relief and steep and Shashe sub-basins are tilted to the right of the ground slopes and will be associated with high pick drainage basin. In contrast, the Motloutse sub-basin discharges. is tilted to the left of the drainage basin. The asym- metry factor for the Notwane sub-basin (Table 4) Form factor and relief suggests minimum tilting (Figure 7). Meanwhile, the analysis of lineaments (faults and dykes) shows The form factor of the sub-basins in the study area that most lineaments trend NW – SE. In contrast, ranges from 0.131 to 0.456, indicating that the catch- the Notwane basin has major dykes and faults that ment is moderately circular and somewhat elongated. trend E – W, with the main streams being super- The Bonwapitse and Mahalapswe sub-basins possess imposed on the same general trends as the linea- form factors of 0.433 and 0.456, respectively, thus ments within the rose diagrams, suggesting an indicating that the sub-basins are nearly circular elon- influence of tectonics on river development gated basins, implying low hydrograph peaks for (Figure 8a,b). The tilting in the sub-basins may be a longer duration or extended time (Table 5). In com- due to the active tectonics associated with recent earthquakes (Moorkamp et al., 2019; Mulabisana parison, the Shashe sub-basin will have lower pick et al., 2021). All sub-basins lie on the hanging wall flows of long duration, whereas the Mahalapswe and of a normal fault which is an extension of the Bonwapitse will have high peak flows of short dura- Moijabana earthquake fault (Mulabisana et al., tion. In general, the relief of the sub-basins increases in 2021) (Figure 2c). a northward direction from 646 m in the Notwane to 872 m in the Shashe sub-basin. The high relief also suggests the presence of faster runoff that may result Basin sinuosity in more peaked basin discharge and greater erosive The stream sinuosity index is a proxy for assessing power than in other basins. tectonic activity and describes the relationship between channel length and valley length. If the sinu- osity index of a stream is 1.3 or more, the stream is Ruggedness number considered meandering; a straight stream has The ruggedness number combines the slope steepness a sinuosity index of 1 and sinuous streams have and length, indicating the extent of the instability of indices between 1.05 and 1.3. The sinuosity results the land surface (Strahler, 1957). The estimated rug- show that Lotsane (1.28), Shashe (1.21), Motloutse gedness number value for the sub-basins in the study (1.33), and Notwane (1.39) are more sinuous than area ranges from 0.031 for Mahalapswe to 0.084 for the Bonwapitse (0.81) and Mahalapswe (0.61) rivers the Lotsane sub-basin. The ruggedness numbers for and can be considered semi-meandering. If we used the sub-basins are typically low, indicating that the the sinuosity index results as a proxy for neotectonics, sub-basin is mature and at its maximum denudation other than the Bonwapitse (0.81) and Mahalapye riv- erosive stage. The sub-basin regions are typically flat ers (0.61), most sub-basins would fall under the mod- with gentle slopes and possess small hills with flat tops erate to high tectonic activity zone category. at approximately 1200 m above sea level. The sub- basins typically possess low slopes in the range of 0– 4.0̊ (Figure 6). Medium slopes are observed within Transverse topographic symmetry factor (TTSF) sub-basins that have hilly areas, such as in the The Transverse Topographic Symmetry factor (TTSF) Shoshong and Tswapong hills, and the hilly terranes provides information about the tilt direction in a basin of southern Botswana (Carney et al., 1994; Ermanovics and is also a proxy for tectonic activity in the area. The & Skinner, 1980; Ermanovics et al., 1978; Mapeo et al., truly symmetric basins exhibit a transverse topo- 2004). graphic symmetry factor (TTSF) = 0 and TTSF > 0; generally, for asymmetric basins, the TTSF increases Tectonic geomorphology (morphotectonic as the asymmetry increases to values closer to 1. The indices) Limpopo sub-basins in Botswana possess TTSF values in the range of 0.12–0.48; TTSF > 0.38 suggest high Basin asymmetry active tectonics in the Bonwapitse and Shashe (0.47) The basin asymmetry factor computed for the sub- and Bonwapitse (0.48), whereas the Notwane (0.27), basins ranges from 18.75 to 60.08, indicating that Motloutse (0.37), and Bonwapitse (0.34) are asym- the sub-basins are tilted in different directions, metric and belong to the moderate tectonic zones basins with AF < 50 are tilted to the right; those (Table 4). Furthermore, the Mahalapswe Basin (0.12) 12 O. MOSES ET AL. Figure 6. A DEM map showing the slope characteristics of Botswana’s six subbasins of the Limpopo River basin. (a) Shashe, (b) Notwane, (c) Motloutse, (d) Mahalapswe, (e) Bonwapitse, and (f) Lotsane. is symmetrical and lies within a low active tectonic substratum, whilst many pick-up points along the zone. profile suggest the presence of knick-point zones. The Lotsane sub-basin also exhibits a rough profile curve with two sharp distinctive spikes at the 25,000 m Hypsometric integrals mark at an elevation of approximately 868 m and 850 The hypsometric curves and HIs indices were acquired m above sea level (Figure 10). The Mahalapswe sub- for the six basins and range between 0.49 and 0.50 basin has a much smoother profile curve than those of (Table 4), within 0.35 < HI < 0.60, which indicates that the Bonwapitse and Lotsane sub-basins. However, the sub-basins are in the mature stage. Considering the there is a sharp drop in the gradient (representing individual HCs, the Notwane and Mahalapswe sub- >100 m change in elevation) from just above 40,000 basins exhibit convex HCs, suggesting that they are in m to approximately the 34,000 m mark, with this the youthful stage and that the river experiences lateral being immediately followed by a pick-up point that erosion (Figure 9). The channels are V-shaped, wider, could potentially be a knick point (Figure 10). The and possess gentle slopes. In contrast, the Bonwapitse, Motloutse sub-basin exhibits a rough profile with ele- Lotsane, Motloutse, and Shashe sub-basins all possess vations ranging between 1000 and 550 m above sea S-shaped HCs, thus indicating a maturity state charac- level, which could be due to a rapid change in slope terised by U-shaped channels and more expansive val- resulting from the rough topography. A sharp spike is leys. This implies that they exhibit significant discharges also observed at the 210,000 m mark, which may be compared with those of young rivers. a knick-point (Figure 10). The Notwane sub-basins exhibit a steady decrease in the gradient, whilst the elevation ranges between 840 and 940 m. The long- Longitudinal profile analyses itudinal profile is rough between 0 and 30,000 m and All six sub-basins exhibit a concave upward longitu- between the 80,000 and 120,000 m marks, thus indi- dinal profile; however, the smoothness of the curves cating rough topography. The Shashe River profile differs. The Bonwapitse sub-basin profile is a rough elevation ranges between 550 and 900 m above sea curve that suggests the presence of a resistive level. There are breaks in the Shashe longitudinal GEOLOGY, ECOLOGY, AND LANDSCAPES 13 Figure 7. Maps showing the asymmetry factor for the subbasins. The red line shows the basin midline, which is theoretically where the river trunk should be; deviation from this line suggests shifting the river channel due to tilting. (a) Bonwapitse, (b) Motloutse, (c) Mahalapswe, (d) Notwane, (e) Lotsane, and (f) Shashe. profile, whilst the numerous spikes observed in the Bonwapitse and Notwane River profiles profile graph indicate rough topography (Figure 10). Terrace deposits in the Bonwapitse sub-basin were observed on the left bank facing upstream at locations −23.49072°S and 26.77469°E. The deposits are 110 m Field observation-pilot survey of knick-points thick from the current riverbed and consist of coarse on three river profiles red sandstone, pebble, angular, and irregular pebbles and cobbles, including quartz, jaspilite, and iron for- Mahalapswe river profile mation clasts cemented by calcrete. A well-exposed The knick-points observed in Figure 10 were investigated terrace deposit is exposed within the current channel in the field for the presence of river terraces and the along the Notwane River at 23.93094°S and 26.73955° development of new terraces. During field observations, E. The terrace deposits possess a thickness of 153 m it was observed that river terraces existed, as suggested by above the current riverbed and consist of cobbles and the profile analysis. There are clear benches that are now pebbles held by calcrete developed above weathered overgrown with tall grass. A three-step flat surface exhi- dolerite (Figure 11c). These deposits extend along the biting bench-like features was observed on the left side Notwane River and are incised by the river into the facing downstream. The terrace deposits were identified weathered dolerite. Google Earth images also show at 23.22861°S and 26.892028°E, comprising three- several meander scars along the Notwane River, indi- stepped flat surfaces with thick grass-covered soils. The cative of a well-developed meandering river system thickness of the recent deposits ranges from 1.7 m to 1 m, with oxbow lakes in the past. The meander scars whilst these deposits are comprised of unconsolidated represent neck cut-off, suggesting that the Notwane dark grey to grey soils (Figure 11a-c). Older terrace River previously had regular discharge with stable deposits incised by the river below consist of conglom- channels. Meanwhile, other river activity features erate pebbles and cobbles cemented by calcrete. show several river migration instances (Figure 12a-c). 14 O. MOSES ET AL. Figure 8. (a) a map showing lineaments (faults and dykes) on which the main rivers are superimposed within the subbasins. Discussion a fifth-order basin, whilst the Mahalapswe sub-basin is of the third order. It was observed that sub-basins Drainage morphometry- (morphometry indices) possessing larger areas also possess high-order The results of the stream order analysis revealed that streams. For example, at 29,612.2 km , the Shashe sub- the first-order streams are typically associated with basin is larger than the other basins, while the high-relief regions that tend to exhibit shorter lengths. Bonwapitse sub-basin covers the smallest area of A high number of segments in the lower order indi- 13,015.3 km . Meanwhile, the Mahalapswe sub-basin cates high erosion characteristics. Mahala (2020) pre- exhibits a lower number of lower-order streams, indi- viously observed that rivers possessing an increased cative of a mature topography. Mahala (2020) argued number of lower-order streams (I, II, and III) exhibit that the lower number of streams in higher relief areas higher sediment yields and receive a large amount of may indicate lower water regimes and water stress water, thus creating a large water flux in the lower conditions. The Shashe sub-basin possesses the largest plains. The highest orders are observed in low relief basin area, the highest stream order, and the highest areas (flat land), which typically account for the main stream number; this is in agreement with Strahler trunk of the river and possess longer lengths. (1957) that the stream orders increase with an increase The high mean stream length values (6.0–125) for in the basin area. the sub-basins indicate high stream orders. Four sub- The analysis of the bifurcation ratios in the sub- basins are fourth-order basins (Bonwapitse, Lotsane, basins reveals considerable variation in the values of the different stream orders due to variations in Motloutse, and Notwane). The Shashe sub-basin is GEOLOGY, ECOLOGY, AND LANDSCAPES 15 Figure 8. (b). Rose diagrams of lineaments and streams (i) Faults and dykes, ii) Rivers and streams. geological and lithological controls (Mahala, 2020; the mean stream length values between the orders Strahler, 1952a, 1952b). The Lotsane, Mahalapswe, result from slow changes that may lead to rapid and Motloutse rivers exhibited mean bifurcation ratios changes in flow characteristics. greater than 5.0, suggesting that these regions have The stream length ratio is higher in the high-order experienced structural disturbances that may have streams (Bonwapitse, Lotsane, Mahalapswe, influenced the present drainage patterns. The Motloutse, and Notwane rivers), whilst the stream Bonwapitse and Notwane sub-basins possess bifurca- length ratio between orders III and IV exhibits the tion ratios within the range of 3.6 to 5.3, indicating that highest stream ratio (1.23–1.67), while the lowest the drainage basin pattern may have been affected exists between orders III and II (0.23–0.45). For the mildly by tectonic deformation. The Shashe sub-basin Shashe sub-basin, the stream length ratio between also has a bifurcation ratio of less than 3, thus suggest- orders IV and V is the highest (1.50), while the lowest ing that it exhibits the lowest structural control among is between orders III and IV (0.26). These changes in all sub-basins. The high bifurcation ratio for the high- the stream length ratio indicate an early stage of geo- order streams indicates that the basin receives a large morphic development. amount of water in the high areas of the stream head- The low drainage densities (0.099–0.122) are attrib- waters. Furthermore, a low bifurcation ratio signifies uted to highly permeable subsoils that lead to low a high probability of flooding in some parts of the basin. surface run-off and soil erosion. Half of the sub- All six sub-basins (Bonwapitse, Lotsane, basins in Botswana are underlain by aerosols that Mahalapswe, Motloutse, Notwane, and Shashe) exhi- possess a loam-like and sandy texture (>70% sand), bit low mean stream length for the first order, thus indicating that they are highly permeable. Luvisols indicating young morphological development and underly much of the northern part of Botswana high erosion potential. The higher-order sub-basin (Shashe and Motloutse sub-basins) and possess (III, IV, and V) streams possess a high mean stream a coarse-grained sandy loam with a coarse-grained length since they have completed their channel length- sandy clay texture that makes them permeable and ening. These characteristics indicate that the sub- well-drained (Bangira & Manyevere, 2009). The basins (Bonwapitse, Lotsane, Mahalapswe, Motloutse, results suggest a lower surface run-off potential in Notwane, and Shashe) are in the early stages of geo- these regions, thus indicating the possibility of high morphic development. The inconsistent differences in infiltration in the area. 16 O. MOSES ET AL. Figure 9. Hypsometric curves of the six subbasins. The curves were generated from 90 m SRTM DEM released in 2013. The low stream frequency (0.009–0.012) in the circular. Sub-basins possessing this characteristic same region may have been due to less permeable feature generally experience a high peak flow of underlying rocks that can lead to less percolation. shorter duration, thus making them prone to This suggests that the area possesses a poor drai- flood hazards (Bali et al., 2012; Farhan et al., nage network. This value also indicates a very 2016,b). For example, catchments of drainage coarse drainage texture, as confirmed by the low basins in Ras en Raqbin, in Jordan, exhibited drainage densities for the sub-basins described a form factor of 0.4 and were affected by the above. In a study, Avinash et al. (2014) estab- May 2014 flash floods in the Aqaba area in lished that the Mulki-Pavanje in Karnataka, a manner consistent with this characteristic. Bali India, exhibited a very coarse texture in areas et al. (2012) previously demonstrated that sub- with high precipitation, leading to the develop- basins in the Pindari glacial-fluvial basin in ment of high groundwater potentiality. The find- India could be described by morphometric fea- ings in these studies were similar to those in the tures that include high peak flows over a short different sub-basins of Limpopo in Botswana, time. It has been shown that some sub-basins which are characterised by generally low relief possessing high form factors (0.6–0.72) can be areas (Table 5) and except for deficient rainfall. affected by flash floods due to the bursting of The elongation ratio (0.409–0.763) suggests that the supraglacial and englacial water bodies all six sub-basins are elongated. More elongated (Farhan & Anaba, 2016; Farhan et al., 2016, sub-basins may possess active tectonic influences 2017). related to high relief and steep slopes (Bali et al., The Mahalapswe and Bonwapitse sub-basins 2012; Mathew, 2016; Rawat et al., 2013). The form exhibit the same high form factor features, 0.433 factors for Mahalapswe and Bonwapitse are nearly and 0.456, respectively, whilst settlements along GEOLOGY, ECOLOGY, AND LANDSCAPES 17 Figure 10. Longitudinal profiles of Bonwapitse, Motloutse, Notwane and Shashe sub-basins. Morphotectonic Indices-Tectonic this basin, such as the Mahalapswe and Shoshong, Geomorphology are identified as flood-prone areas (Tsheko, 2004). For example, in December 2019, this area was The basin asymmetry factor is an important para- affected by floods that impacted the transporta- meter of basin drainage and determines the exis- tion system, causing a train derailment (Gaofise, tence of tectonic tilting in the basin, whilst the 2019; Kealeboga, 2020). Historical data indicates results indicate tilting within the sub-basins. several train derailments between the Bonwapitse Specifically, the Motloutse River is tilted to the River and the Mahalapswe River dating back to left, and the other five sub-basins are tilted to November 1987 (Shaw, 1988) that occurred due to the right. The basin asymmetry values (18.75– flooding. Shaw (1988) previously claimed that 60.08) suggested that the sub-basin had formed floods in this area have a recurring period of 20 under an active tectonic setting. The area has years and are significant due to their ability to undergone an uplift along the OKZ axis between cause structural damage (Shaw, 1988). The the Late Cretaceous and Early Tertiary stages Lotsane, Motloutse, Notwane, and Shashe sub- (Moore & Larkin, 2001; Moore, 1999), and this basins exhibit form factors of 0.175, 0.305, 0.131, may have contributed to the tilt of the river within and 0.140, respectively, suggesting that the sub- the different sub-basins. Overall, the region is basins are elongated. Information from the litera- aseismic, but recently, earthquake events along ture indicates that elongated basins exhibit low ancient faults have suggested that the region is peak discharge for a longer duration, thus making undergoing intraplate tectonic deformation it easy to manage flood flows in such areas (Nthaba et al., 2018; Moorkamp et al., 2019; (Farhan & Anaba, 2016; Farhan et al., 2016). Mulabiasana et al., 2021). 18 O. MOSES ET AL. Figure 11. Field photographs showing river terrace deposits identified in the cross-sectional profiles of the subbasins. (a) Mahalapswe, (Bonwapitse and (c) Notwane. The transverse topographic symmetry factor (Morgan, 1976; Rodríguez-ItRodri-Guez-Iturbe & (0.112–0.479) indicates the influence of new tectonic Escobar, 1982). Morgan (1976) observed that a high activities within the basins. HIs analyses revealed drainage density is required for a drainage network a range between 0.49 and 0.5, suggesting that the sub- with moderate to regular rainfall. Rodri-Guez-Iturbe basins are in their youth development stage, thus and Escobar (1982) suggested that the analysis of implying domination of vertical erosion. The long- drainage densities should remain general due to the itudinal profiles results for all the sub-basins variation in the variables influencing drainage density. (Bonwapitse, Lotsane, Mahalapswe, Motloutse, The drainage densities of the sub-basins are espe- −1 Notwane, and Shashe) exhibit rough topography, cially low (<0.25 km ), which differs from what is which is in agreement with the results of the drainage expected from the literature, which reports that semi- density and texture analysed for these sub-basins. arid areas possess high drainage densities. Carlston Cross-sectional profiles for the Bonwapitse, (1963) investigated 15 drainage basins in different Motloutse, Mahalapswe, and Notwane sub-basins mono-lithological areas in the Eastern United States indicated the presence of terraces, thus suggesting and observed no correlation between high drainage possible incision in the area. The Mahalapswe sub- density and increased rainfall intensity and run-off. It basin exhibits differential erosion, as indicated by the was found that drainage densities correlated with run- difference in the bank elevation (Figure 11). off in certain regions, a characteristic influence of cli- matic conditions. High temperatures also led to high Drainage density is a morphometric index that is drainage density because of their ability to increase the closely linked to climate change. This increases with evaporation rate, thus causing a decrease in vegetation decreasing precipitation, being high in semi-arid areas cover and leading to high erodibility (Eccker, 1984). with sparse vegetation and low in humid areas with The differences in drainage density of the current sub- dense vegetation (Rodri-Guez-Iturbe & Escobar, basins from those expected based on previous studies 1982). Semi-arid regions are known for their low may indicate that the drainage densities are not vegetation and soil cover, increasing drainage density GEOLOGY, ECOLOGY, AND LANDSCAPES 19 Figure 12. Google Earth images of the Lotsane, Notwand and Motloutse rivers showing various features, including meander scars and river migration. influenced by climatic conditions alone but by other (a) The sub-basins in eastern Botswana are drained variables such as tectonics, rock type, and relief. by fourth and fifth-order streams with high water flux in regions that have highly perme- able soils with a coarse drainage texture. The Conclusions slopes are low, suggesting water flows with less potential energy increase the possibility of high Based on Shuttle Radar Topographic Mission (SRTM) infiltration and groundwater recharge. DEM, we conducted a morphometric study on the (b) The mean stream length ratios show that all the Limpopo River Basin in Botswana to understand the sub-basins are in their early stages of geo- role of the underlying geology and tectonics on its morphic development, whereas an indication geomorphic and hydrogeological development. of mature topography is found in others. The Consequently, we arrived at the following conclusions: 20 O. MOSES ET AL. form factors allow us to designate the of Geology, Ecology and Landscapes for their valuable and constructive comments on the earlier draft of this Mahalapswe and Bonwapitse sub-basins as manuscript. flood-prone and requiring management. The HCs indicate that some river systems within the region are in their youthful stage, charac- Disclosure statement terised by wider V-shaped channels with gentle slopes. In contrast, others have S-shaped HCs, No potential conflict of interest was reported by the author(s). which are characteristic of a mature state with U-shaped channels and more expansive valleys. (c) Some sub-basins show a high frequency of Data availability statement NW – SE trending lineaments, with minor All data sets are available on request from the corresponding E – W trending lineaments that coincide with author (MapeoRBM). The data are not publicly available the drainage pattern orientations. This indi- because they are part of ongoing research yet to be wound cates that the drainage systems are largely down. structurally controlled. The Asymmetry Factor and TTSF analysis show that the sub-basins are References tilted due to tectonic activities. The tectonic activities started in the Jurassic and Lower Ahmed, F., & Rao, K. S. (2016). 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Quantitative Geomorphology of Drainage 10.1002/(SICI)1096-9837(199807)23:7<611:AID- Basin and Channel Network. In Chow, V. (Ed.), Handbook of ESP872>3.0.CO;2-Y Applied Hydrology (pp. 439–476). McGraw Hill. WMO. (2012). Limpopo River Basin: A Proposal to Improve Tsheko, R. (2004). Rainfall reliability, drought, and flood the Flood Forecasting and Early Warning System. 12(12). vulnerability in Botswana. Water South Africa, 29(4). http://www.wmo.int/pages/prog/hwrp/chy/chy14/docu https://doi.org/10.4314/wsa.v29i4.5043 ments/ms/Limpopo_Report.pdf http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geology Ecology and Landscapes Taylor & Francis http://www.deepdyve.com/lp/taylor-francis/evolution-of-the-limpopo-river-basin-in-botswana-based-on-morphometric-6BU7b03L04

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Publisher: Taylor & Francis
Copyright: © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON).
ISSN: 2474-9508
DOI: 10.1080/24749508.2023.2219492
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Abstract

GEOLOGY, ECOLOGY, AND LANDSCAPES INWASCON https://doi.org/10.1080/24749508.2023.2219492 RESEARCH ARTICLE Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques One Moses, Read B Mapeo and Joyce G Maphanyane Department of Environmental Science, University of Botswana, Gaborone, Botswana ABSTRACT ARTICLE HISTORY Received 8 November 2022 This study used morphometric techniques to generate new information describing the evolu- Accepted 25 May 2023 tion and hydrogeological behaviour of the Limpopo River Basin in Botswana, based on the analysis of drainage surface features, form, and size. Drainage basins provide basic information KEYWORDS on their evolution, which, when quantified, yield information on the interaction between Morphometry; geospatial tectonics, climatic, and surface processes. Drainage networks were extracted from the analyses; active tectonics; Shuttle Radar Topographic Mission (SRTM) Digital Elevation Model (DEM) (90 m × 90 m), and drainage basin; Limpopo subsequently, morphometry indices were computed using ArcGIS 10.5. Drainage network River Basin; Botswana extraction was performed using the Arc Hydro extension in ArcGIS 10.5. ArcGIS 10.5 image processing technique was used to extract lineaments and create rose diagrams. The results showed that the Limpopo sub-basins in Botswana were drained by fourth- and fifth-order streams, with a total drainage area of 107, 871 km . Additionally, the basin asymmetry and mean bifurcation ratios showed tilting in the sub-basins, suggesting tectonic instability and structural control in a low – to-moderate active tectonic zone. The sub-basins also had a coarse texture, indicating a high infiltration capacity. These results are essential for planning and managing watershed systems, flood risk assessment, and potential groundwater assessment for the different sub-basins of the Limpopo River Basin in Botswana. Introduction topographic maps, aerial photographs, satellite data, The morphometric analysis of a watershed can provide and digital elevation models and by quantifying various valuable information regarding its characteristics, morphotectonic indices (Bhatab et al., 2020; Keller, regional topography, drainage pattern, basin geometry, 1986; Mohanty et al., 2004; Radaideh et al., 2016). nature of the bedrock, and potential groundwater Analysis of drainage basins in response to tectonic zones, thus aiding the effective planning and manage- processes can provide insight into the history of ment of natural resources (Chaitanya et al., 2021; a particular region alongside any recent deformational D. S. Singh & Awasthi, 2010; Radwan et al., 2017; events (Joshi et al., 2022; Matoš et al., 2016; Menier Shelar et al., 2022; Vincy et al., 2012). The morphotec- et al., 2017; Psomiadis et al., 2020). Drainage networks tonic analyses of drainage basins reveal the relationships are the most active and sensitive elements that can be between the properties of a drainage basin and climate, used as powerful tools for understanding the tectonic basin relief, lithology, and tectonics (Bhatt et al., 2020; activity of various regions (Ahmed & Rao, 2016; Chaitanya & Moharir, 2017; Lone, 2017; Różycka & Karami et al., 2018; Mumipour et al., 2012; S. Singh Migoń, 2021; S. Singh et al., 2019, 2021). These analyses et al., 2018; Sedrette & Rebai, 2022). Streams are also provide a means of assessing natural resources powerful indicators of neotectonic changes and cli- within a basin, including options for the sustainable matic conditions since climatic conditions, lithology, management of water resources and groundwater and geologic structures control stream processes. exploration (Strahler, 1964). The quantitative morpho- These features influence flow, erosional dynamics, metric properties (derived from remote sensing ana- and sediment transportation. Therefore, by studying lyses in a Geographic Information System (GIS) the nature and types of drainage patterns, the rock environment (Chaitanya & Moharir, 2017; Chaminé types and geological structures responsible for the et al., 2021; Issa & Saleous, 2019; Maphanyane, 2016; development of a drainage network can be interpreted Pauly, 2009); are used to evaluate the neotectonic activ- alongside the river dynamics over time (Twidale, ities within basins. Information regarding the tectonic 2004). This is due to the knowledge that the evolution history of a given region can be obtained by analysing of landscapes, including river systems, results from CONTACT Read B Mapeo mapeorbm@ub.ac.bw Department of Geology, University of Botswana, Private Bag UB00704, Gaborone, Botswana © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 O. MOSES ET AL. complex interactions involving climate, tectonics, and rainfall trends in the watershed are decreasing over surface processes (Willett et al., 2006). time (Maruatona & Moses, 2022, and references Morphometric studies performed in the Limpopo therein). In the Notwane sub-basin, the annual rainfall River Basin in southern Africa have primarily focused decreased between 1974 and 1999, ranging between on flood risk assessment, climate change effects on the 600 and 500 mm a year but has since decreased to 500 basin, and the livelihoods of individuals residing along to 400 mm. Furthermore, in the Mahalapye and the basin (Petrie et al., 2015; Alemau et al., 2018). Bonwapitse sub-basins, the annual rainfall decreased However, these studies lacked the application of techni- from 450 mm in 1982 to approximately 380 mm ques utilising morphometry, remote sensing, and GIS, all between 1994 and 2018. Data from the Shashe sub- of which can address essential questions regarding the basin show that the total annual rainfall decreased hydrogeological behaviour of the basin in relation to from an estimated 530 mm in the 1970s to approxi- recent earth deformations (SARDC, 2002; Spaliviero mately 450 mm between 2008 and 2017. The data for et al., 2011; WMO, 2012). In southern Africa, morpho- the Lotsane sub-basin shows that the annual rainfall metric analysis has previously been used to characterise increased from 300 mm in 1988 to 400 mm in 2017. the history of the Cape Fold Belt region in South Africa, Furthermore, the annual average minimum and max- which has undergone reorganisation as a result of the imum temperature data showed a marginal increase of exhumation of the belt and the retreat of the great approximately 1.0–1.5°C between 1970 and 2019 escarpment in the Western Cape of South Africa (Moses, 2017, and references therein). The entire (Richardson et al., 2016). Alemaw (2018) examined Limpopo River Basin (LRB) lies in the eastern portion flood hazards in the LRB using the geospatial character- of southern Africa between 20°–26°S and 25°–35°E. istics of basin geomorphology. In this study, DEMs and The LRB straddles four nations (Figure 1): South the geomorphic instantaneous unit hydrograph (GIUH) Africa (47%), Botswana (17.7%), Zimbabwe (16%), were used to simulate hydrological responses to rainfall and Mozambique (19.3%) (Chilundu, 2008; Petrie in the basin. The GIUH depends on Horton’s morpho- et al., 2015). The LRB is spread across the above metric parameters and rainfall data. The results revealed nations, whilst its mainstream marks the international that fast-flowing floods in the LRB were generated in the boundary between South Africa and Botswana and upland region of the basin; thus, an early warning system between South Africa and Zimbabwe. The Limpopo was developed for the sub-basin. River flows between South Africa and Botswana and is The main objective of this study was, therefore to joined by more tributaries from Zimbabwe in the east use morphometric parameters extracted from an SRTM before entering Mozambique and draining into the digital elevation model (DEM) in a GIS environment to Indian Ocean. The LRB flows through several climatic understand the evolution of the Limpopo River basin in zones but generally lies within the Köppen climate Botswana through quantitative analysis. Such studies zone BSh (semi-arid, dry, hot). The Limpopo River are essential for basin management and prioritisation Basin primarily experiences an arid, hot, and dry cli- for soil and water conservation along the span of the mate except for the mouth of the river basin in basin. The Limpopo River Basin lies in eastern Mozambique, which experiences a tropical climate. Botswana and comprises six sub-basins, with rivers However, in most of Botswana and Zimbabwe, it lar- that drain to the east separated from rivers draining gely drains in arid environments (CAR, 2010). to the west (Figure 1). We computed parameters such as basin asymmetry, hypsometric integral, transverse Geological setting topographic symmetry factor, sinuosity index, stream length gradient, and longitudinal river profiles. Based The Limpopo River in Botswana trends NE – SW and on these parameters, we assessed the role of tectonics in cuts through several different geological formations, shaping the basin and described several features of the including lithological units related to the Kaapvaal and geomorphic development of the Limpopo Basin. Zimbabwe cratons and the Limpopo Belt, between the cratons (Chinoda et al., 2009; McCourt, 2004) (Figure 2a,b). The Limpopo Belt follows an east – Study area northeast trending granulite gneiss terrane that This study was carried out across six sub-basins of the forms a major geological structure developed in Limpopo River Basin in Botswana. The Limpopo Botswana, Zimbabwe, and South Africa. This is River drains a large area of six sub-basins, namely, believed to have resulted from a collision between Mahalapswe, Bonwapitse, Lotsane, Notwane, Zimbabwe and the Kaapvaal cratons at approximately Motloutse, and Shashe, all of which contain major 2.6–2.0 Ga (Blenkinsop & Rollinson, 1992; Van rivers and tributaries (Figure 1) (Table 1). The study Reenen et al., 1992). The centre of the belt consists of area was irregular and bounded by the latitude 20.448° strongly deformed granulite facies rocks derived from S to 25.419°S and longitude 25.111°E to 29.391°E, granite-greenstone that are similar to those found in defining a large watershed of 107 871 km . The annual the Zimbabwe Craton (Aldiss, 1991). Both the GEOLOGY, ECOLOGY, AND LANDSCAPES 3 Figure 1. Location map and DEM of the Limpopo River Basin (LRB) in southern Africa. A detailed DEM of the LRB subbasins in Botswana is shown on the right. Kaapvaal and Zimbabwe cratons consist of high-grade However, two major seismic events have occurred here, metamorphic rocks, including lithologies of the gran- initiated by two earthquakes in September and ite-greenstone terrains (Aldiss, 1991; Carney et al., October 1952 with ML Richter magnitudes of 6.1 and 1994; Ranganai et al., 2002) (Figure 2a,b). Stratified 6.7, respectively, in northern Botswana in the Okavango lithologies comprising amphibolite, tonalitic gneiss Rift Zone (ORZ). The Okavango Rift Zone (ORZ) is with metamorphosed basic/ultrabasic intrusions, and constructed by a series of horst and graben structures granitoid gneiss underlie much of this area (Figure 2a, in northwest Botswana at the southern tip of the East b). The southern portion of the LRB in South Africa is African Rift System (EARS) (Modisi et al., 2000; Modisi, underlain by mafic and intrusive ultramafic rocks, 2000). Rifting on the ORZ began after the emplacement including granite and felsic lavas (Chinoda et al., of the 179 Ma Karoo dyke swarm as the dykes are dis- 2009). The lower portion of the LRB in Mozambique placed by rifting (Le Gall et al., 2005). Several north- is an erosional plain formed from consolidated and easterly trending rift faults occur parallel to the unconsolidated sedimentary rocks, including argillite, Okavango Rift Zone, terminating in the Okavango fluviatile sandstones, and mudstones (SARDC, 2002). Delta in the south (Modisi et al., 2000; Modisi, 2000). Due to the different characteristics of the lithologies, Another region of active faulting is the Makgadikgadi their effect on the drainage patterns, including the Rift Zone, located to the southeast of the ORZ. The deformations experienced, can differ. Makgadikgadi Rift system occurs northwest of Sua Pan, with faults following the structural trend of the Paleoproterozoic Magondi Belt with faults that also strike Seismicity in the Limpopo River Basin (in NNE to NE (R. B. M. Mapeo et al., 2001; Eckardt et al., Botswana) 2015, and references therein). Nthaba et al. (2018) showed that between 1966 and 2012, when records Tectonically, a major part of the basin lies on the became available, 327 microseismic events were Limpopo Belt terrane, a primarily aseismic region. 4 O. MOSES ET AL. Figure 2. (a) a regional geological map of southeastern and central Botswana covering areas of the Notwane Subbasin of the LRB (source Mapeo & Tschirhart, 2022). (b) a regional geological map of southeastern and central Botswana covering areas of the Bonwapitse, Mahalapswe, Lotsane, Motloutse and Shashe Subbasins. Note the major structures that define the geological terrain of the Limpopo Belt (source Mapeo & Tschirhart, 2022). recorded, implying that this region of Botswana has low ancient faults within the Limpopo Belt, in a zone stress dominated by minor-to-moderate events. This bounded by two major shear zones/faults, the Phalala previous study showed that micro-seismic activities ran- and the Sunnyside Shear Zones (Figure 2b). Towards the ging from 1.3 to 5.7 in magnitude were distributed in the south, between the Notwane and Bonwapitse rivers, northern and southern parts of Botswana. In April 2017, Mulabisana et al. (2021) previously analysed InSAR an Mw 6.5 (moment magnitude) earthquake occurred in Sentinel-1 images to map an NW – trending active Central Botswana with the epicentre at Moiyabana fault that was in line with the location of the (25.134°E, 22.565°S; depth 22 ± 3 km). This earthquake Moiyabana Earthquake (Figure 2c). The Limpopo River was associated with 15 events with magnitudes up to Mw basin has a complex history of shifting and changing its 5 (Moorkamp et al., 2019; Mulabisana et al., 2021). These course, which is said to have been due to the splitting of events also attest to intraplate earthquake activity asso- the Gondwanaland supercontinent (Goudie, 2005). The ciated with reactivation along major NNW trending Limpopo River formed along a rift during the opening of the Mozambique channel (McCarthy, 2013). Here the triple junction failed, whilst only two branches formed Table 1. A table showing the subbasin of the Limpopo river in where Africa and South America had separated. Botswana and their major rivers. The East African rift system is also an example of LRB-BASIN RIVER LENGTH (km) AREA (km ) the evolution of drainage systems in the East African Shashe Thalogang 44 29612.2 Rift Valley system, which disrupted the upper tribu- Shashe 36.2 Tati 156.3 taries of the ancestral Limpopo River and diverted Ramokgwebane 43.0 some of this into the lower Zambezi River. For exam- Notwane Dikolakolare 81.4 17423.7 Sekhukhwane 34.4 ple, the Kwando River in northern Botswana was Notwane 21.5 diverted by faulting associated with the EARS into Metsimotlhaba 75.8 the lower Zambezi River (McCarthy, 2013). Evidence Taung 27.8 Motloutse Motloutse 288 19589.6 has shown that the southern African paleo drainage Moenyenane 46 system consisted of three major rivers: The Karoo and Thune 138 Makutlagaga 25 Kalahari rivers in the south-flowing to the Atlantic, Mhwana 35 and the Limpopo on the west coast flowing eastwards Mahalapye Mahalapye 74 13106 Taupye 77 to the Indian Ocean. Moore and Larkin (2001) sug- Bonwapitse Bonwapitse 132 13015 gested that during the Jurassic and Lower Cretaceous Serorome 129 Lotsane Lotsane 119 15124.5 periods, the Kafue and Luangwa Rivers were tribu- Susubela 76 taries of the upper Zambezi and flowed initially into GEOLOGY, ECOLOGY, AND LANDSCAPES 5 Figure 2. (c) a DEM image showing faults scarp of an NW trending fault. The fault is located north of the Dikolakolane and Sekhukwane rivers (Table 1). The fault crosses the A1 Road to the north of Botswana at 24.004°S, 26.376°E. All subbasins are largely located on the hanging wall of this fault (Notwane, Bonwapitse, Bonwapitse, Mahalapye, Lotsane, Motloutse and Shashe) (source Mulabisana et al., 2021). the Limpopo via the present Shashe River. The are designated as first-order, the confluence of two Limpopo River originally flowed towards the south- first-order channels gives channels segments easterly direction, implying a reversal of the flow of second-order, two second-order streams join to direction to the north-easterly direction (Moore, form a segment of third-order and so on. The areal 1999). or shape indices were computed using Horton’s, Strahler and Schumm’s laws. The indices estab- lished were the drainage density, drainage fre- Materials and methods quency, and elongation ratio. Horton’s laws and Strahler and Schumm’s methods were used to Morphometric analysis of the Limpopo River Basin establish the following indices: basin asymmetry, drainage system in Botswana required a delineation hypsometric integral, transverse topographic sym- of all existing rivers and streams and applied metry factor, sinuosity index, stream length gradi- a quantitative method to extract drainage networks ent, and longitudinal river profiles. Furthermore, and morphometric indices from the Shuttle Radar some indices were used to identify regions of activ- Topographic Mission (SRTM) Digital Elevation ity tectonics. The flow diagram below shows the Model (DEM) (90 m) (data available from https:// process of generating the morphometric and mor- earthexplorer.usgs.gov) using the Spatial Analyst photectonic parameters (Figure 3). tool of ArcGIS 10.5 software. The DEM was pre- processed using the fill tool to remove null data and sinks. Flow accumulation and direction were Basin asymmetry and sinuosity computed using DEM. Subsequently, streams and The basin asymmetry factor indicates deformation sub-basins were delineated. The drainage network caused by tectonic tilting that results in warping or was analysed through morphometry analysis flexing, thus leading to longer streams possessing (Horton’s laws and Strahler method) to extract a larger area on one side of the basin (Mathew, 2016). the morphometry indices and drainage structures. The basin asymmetry factor of the different sub-basins The morphometric parameters under linear and suggests that the basins were developed under active shape characteristics were computed using standard tectonic settings or that there was lithologic and struc- methods and formulae (Horton, 1932, 1945; Smith, tural control. This factor is used to assess the tilt and tilt 1950; Strahler, 1964) (Table 2). The linear para- direction of the basin. The Af is given by meters included stream order, length, length ratio, Ar and bifurcation ratio. The stream order is Af ¼ � 100;where Ar is the area of a basin on the At a hierarchical positional measurement of a stream right of the mainstream (looking downstream), whilst within its associated tributaries (Leopold et al., At is the total drainage basin area. The asymmetry 1964). The smallest, un-branched fingertip streams factor should be 50 or closer in a stable environment, 6 O. MOSES ET AL. thus indicating no, or only a small amount of tilting of 2002; Pavano et al., 2016; Pérez-Peña et al., 2009; the main drainage line. Basin sinuosity deals with the Pérez‐Peña et al., 2009). Several previous studies channel pattern of a drainage basin. In general, its value have demonstrated that hypsometric integrals can varies from 1 to 4 or more. Rivers with a sinuosity of 1.5 provide information regarding lithological resis- are called sinuous, whilst those above 1.5 are called tance and tectonic uplift, both of which assess meandering (Ahmed & Rao, 2016). the disequilibrium balance between erosion and tectonic forces in each basin (Hurtrez et al., 1999; Lifton & Chase, 1992). The shape of the Basin transverse topographic symmetry factor hypsometric curves and integral values provide (TTSF) valuable information regarding the erosional stage of the basin and the tectonic, climatic, and This is another quantitative index used to evalu- lithological factors controlling landform develop- ate basin asymmetry (Keller & Pinter, 2002). The ment (Huang & Niemann, 2006; Moglen et al., absence of tectonics would cause the main river 1998; Willgoose & Hancock, 1998). Strahler to flow evenly from both sides, forming (1952a, 1952b) classified drainage basins according a perfectly symmetric basin with a TTSF value to their hypsometric integral (HI) and curve (HC), of 0. TTSF is defined by TTSF = Da/Dd, where where HCs are convex upwards with high HIs > Da is the distance from the midline of the drai- 0.6 are of the youthful stage, the S-shaped HCs nage basin to the midline of the active meander and HIs between 0.35 < HI < 0.6 are in the matur- belt whilst Dd is the distance from the basin ity stage, and concave upward HCs and low HIs < midline to the divide. The basin midline is the 0.35 are in the old stages. symmetry line of the basin, with values closer to 1 indicating basin tilt. Longitudinal profile analyses and lineaments Hypsometric integrals A longitudinal river profile is a powerful tool The hypsometric curve and integral are accepted required to draw relevant conclusions regarding indicators of active tectonics in terrains under- the geological setup and structural and tectonic going denudation and indicate erosional processes deformations of a watershed (Singh & Awasthi, and patterns along a river profile (Mathew, 2016). 2010; Dar et al., 2014; Pavano et al., 2016; Roy Hypsometric analysis of drainage basins has been & Sahu, 2015; Snyder et al., 2000). Longitudinal applied to areas experiencing rapid tectonic activ- profiles in nature are depicted by a concave ity and allows for comparisons between different upward curve that is generally not smooth. sub-basins (Hurtrez et al., 1999; Keller & Pinter, Table 2. A table showing morphometry and morphotectonic indices used in this study and their formulas. Morphometric/ morphotectonic parameter Formula Reference Linear parameters Stream order Hierarchical rank Strahler (1964) Stream length Length of the stream Strahler (1964) Stream number Nu= N1 +N2 + . . . . Nn Horton (1945) Lu Mean stream length Nu Strahler (1964) Lsm ¼ Lu Stream length ratio Horton (1945) RL ¼ Lu 1 Nu Bifurcation ratio Schumm (1956) Rb ¼ Nuþ1 Mean bifurcation ratio Rbm = Average of bifurcation ratios of all orders Strahler (1964) Areal parameters Lu Drainage Density Dd ¼ Horton (1945) Nu Stream frequency Horton (1945) Df ¼ Form factor R ¼ Horton (1945) f 2 Elongation ratio Re= √A/π/L Drainage texture Dt = Nμ/P Where, Nμ = No. of streams in a given order and P = Perimeter (Kms) Horton (1945) Relief parameters Relief Hb ¼ H h Schumm (1956) Ruggedness number Rn=Dd*Hb Schumm (1956) Hypsometric integral Hi ¼ ðh h Þ=ðh h ÞHi Strahler (1952b) mean min max min Morphotectonic parameters Ar Basin Asymmetry factor Keller and Af ¼ � 100 At Nichola (1996) Transverse Topographic T = Keller and symmetry Factor Da/Dd Nichola (1996) where, Da is the distance from the midline of the drainage basin to the midline of the active meander belt and Dd is the distance from the basin midline to the basin divide Sinuosity index SI=AL/SL, AL =actual channel length between source and mouth, SL =straight line length Schumm (1956) GEOLOGY, ECOLOGY, AND LANDSCAPES 7 Figure 3. A schematic flow diagram for the methodology used for the extraction of morphometric and morphotectonic parameters from the STRM DEM for the Limpopo River subbasins in Botswana. According to the concave-upward power function, Bonwapitse, and Notwane Rivers. The objective the long profile deviation of a steady-state river of the field survey was to identify the knick- from this ideal shape can be used to infer active points along the river profiles that were inter- tectonic deformation in the basin (Hack, 1957; preted as river terraces. River terraces are rem- Roy & Sahu, 2015). This is affected by several nants of abandoned floodplains, riverbeds, or factors, including discharge, amount of river valley floors formed during previous erosion, load, bed and bank material, topographic relief, deposition, or both, along river valleys. They depth and depth width, and river gradient. The express the previous river channel and floodplain profiles may also include convexities due to the levels that correspond to valley aggradation and local steepening of the channel from resistive rock river incision (Maddy, 1997). The formation of strata, larger loads, and tectonic activity. The river terraces requires a sustained change in base lineaments were identified using DEM and exist- level that can be achieved by uplift, sea-level ing geological maps. Using the spatial analyst tool changes, and climate changes (Bridgeland & in ArcGIS 10.5, a hill shade was created, which Westaway, 2008). Furthermore, river terraces pos- enhanced linear features on the DEM; the linear sess a flat surface called the tread, a steep slope features were then traced and saved as a new separating it from adjacent flood plains or ter- shapefile. The geological maps were used to vali- races, and an erosional base called strath date the structures, whilst the extracted linea- (Schirmer, 2020). ments included faults, dykes, and joint systems. Results and analyses Field survey The drainage basin morphometric parameters of A field survey was conducted to verify the drai- the Limpopo River basin in Botswana have not nage characteristics obtained from the digital ele- previously been used to understand the basin’s vation model analysis along the Mahalapye, hydrological and morphological characteristics as 8 O. MOSES ET AL. well as structural controls if any (Lifton & Chase, (Table 3). The main truck segments of the four 1992; Rai et al., 2017; Prakash et al., 2017, 2019). sub-basins (Bonwapitse, Lotsane, Motloutse, and The results of different hydrological and morpho- Notwane Rivers) are fourth-order, whilst Shashe is logical characteristics derived from different mor- of order five. In total, 1145 streams are present in phometric parameters are discussed below. the basin and define the various catchments. Linear morphometry indices Stream length Stream order The stream length distances within the basin were measured along the various channels from the The stream order is a hierarchical positional mea- river source to an outlet or given point. The indi- surement of a stream within its associated tribu- vidual stream lengths in all orders were added to taries (Leopold et al., 1964; Rai et al., 2017). The determine the total stream length for the six smallest, un-branched fingertip streams are desig- basins, which are 1465, 1276.50, 919.99, 2102.20, nated as first-order, the confluence of two first- 1843, and 3434.40 km, respectively (Table 3). The order channels gives channels segments of second individual stream lengths decreased with increas- order, two second-order streams join to form ing order. The graphical representation of the a segment of third order and so on. Four of the logarithm of stream length versus stream order six sub-basins (Notwane, Bonwapitse, Lotsane, and for Notwane, Bonwapitse, and Lotsane (Figure 5) Motloutse) in the study area are fourth-order exhibits a certain amount of deviation from basins; Shashe is a fifth-order basin, and the a linear trend, thus suggesting variation in lithol- Mahalapswe sub-basin is a third-order (Figure 4). ogy and topography. In contrast, the Mahalapswe The number of streams decreases with increasing and Shashe basins display a linear pattern, thus order, with the first-order basins generally posses- indicating that they are incised through sing a significantly higher number of streams Figure 4. DEM of the Limpopo Subbasin showing the stream orders for the watershed of the study area. GEOLOGY, ECOLOGY, AND LANDSCAPES 9 Table 3. A table showing the stream orders, number of streams, stream length and stream length ratios in the six subbasins. Sub-basin Stream Order No of Streams Stream Length (Km) Mean stream Length Stream Ratio Total stream Length(Km) Notwane 1 146 973.8 6.7 2 20 482.9 24.1 0.5 3 13 144.6 11.1 0.3 4 7 241.7 34.5 1.67 1843 Bonwapitse 1 114 809.5 7.1 2 21 416.1 19.8 0.51 3 7 97.6 13.9 0.23 4 2 141.8 70.9 1.46 1465 Mahalapswe 1 76 498 6.6 2 20 291 14.6 0.58 3 3 130.9 43.6 0.45 919.99 Lotsane 1 99 656 6.6 2 17 393.3 23.1 0.59 3 7 101.5 14.5 0.26 4 1 125.7 125.7 1.23 1276.5 Motloutse 1 175 1093.9 6.3 2 16 475.7 29.7 0.43 3 7 202.2 28.9 0.43 4 3 330.4 110.1 1.63 2102.2 Shashe 1 202 1512.3 7.5 2 95 1120.9 11.8 0.74 3 62 484.2 7.8 0.43 4 8 127.3 15.9 0.26 5 24 189.7 7.9 1.5 3434.4 homogeneous underlying material, perhaps com- 1.67, suggesting a late youth to maturity stage of geo- prising weathered basement rocks. morphic development. Stream length ratio Bifurcation ratio The mean stream length in Botswana ranges from 6.0 This factor describes the ratio between the number of to 125 m and increases with increasing stream orders. streams in one order and the next and has been calcu- The mainstream trunk has the highest mean stream lated for the six basins (Table 2). The mean bifurcation length, ranging from 15.9 for the Shashe sub-basin to ratio in the six sub-basins ranges from 2.9 to 5.3. 125.7 for the Lotsane sub-basin (Table 3). The stream A high bifurcation ratio indicates a short concentra- length ratio ranges between 0.5 and 1.7; these ratios tion time and a high probability of flooding. The mean are inconsistent, likely reflecting differences in slope bifurcation ratio is 5.16, which is larger than the stan- and topography (Table 3). The stream length ratio dard ratio (Table 4). The bifurcation ratio varies between the third and fourth order is significantly between 2 in flat and rolling surfaces to 4 or 5 in higher than the others. The values range from 0.43 to mountainous or highly dissected drainage basins NOTWANE SUBBASIN MOTLOUTSE SUBBASIN SHASHE SUBBASIN 3 4 4 3 3 y = -0.2845x + 3.0236 2 y = -0.1931x + 3.1181 y = -0.2339x + 3.1388 R² = 0.9828 2 2 R² = 0.6503 R² = 0.7018 1 1 0 0 0 1 2 3 4 1 2 3 4 1 2 3 4 STREAM ORDER STREAM ORDER STREAM ORDER a) b) c) MAHALAPSWE SUBBASIN LOTSANE SUBBASIN BONWAPITSE SUBBASIN 3 3 4 y = -0.2741x + 3.0646 2 2 y = -0.2899x + 3.142 R² = 0.8251 y = -0.2901x + 3.0063 R² = 0.7851 1 1 R² = 0.9874 0 0 0 1 2 3 1 2 3 4 1 2 3 4 STREAM ORDER STREAM ORDER STREAM ORDER d) e) f) Figure 5. Plots showing the relationship between the logarithmic of stream length and stream order. The graphs (a, b, c, d, e and f) show a relationship between stream length and stream; the stream length increases with increasing stream order. LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH LOG OF STREAM LENGTH 10 O. MOSES ET AL. Table 4. A table showing the geomorphic indices of the six subbasins in the study area. Notwane Bonwapitse Mahalapswe Lotsane Motloutse Shashe Geomorphic Indices Basin 1 Basin 2 Basin 3 Basin 4 Basin 5 Basin 6 Basin Asymmetry 43.23 (high) 32.07 (high) 10.7 (moderate) 27.08 (moderate) 60.08 (high) 18.75 (moderate) Sinuosity Index 1.39 0.81 0.61 1.28 1.33 1.21 Bifurcation Ratio (mean) 3.6 (moderate) 3.9 (moderate) 5.3 (moderate) 5.1 (moderate) 5.2 (moderate) 2.9 (low) Transverse Topographic Symmetry Factor 0.27 (low) 0.48 (moderate 0.12 0.34 (low) 0.37 (low) 0.47 (moderate) Hysometric Intergral 0.49 (high) 0.49 (high) 0.5 (high) 0.5 (high) 0.5 (high) 0.49 (high) Basin Elonagtion ratio 0.623 (high) 0.763 (moderate) 0.742 (moderate) 0.472 (high) 0.423 (high) 0.409 (high) Hysometric Curves Convex(high) Convex (high) Comvex (high) Convex (high) Convex (high) Concave (high) (Verstappen, 1983). The minimum bifurcation ratio density in these sub-basins is low and likely reflects indicates whether it is a flatter or rolling drainage a low dissected catchment with a relatively slow basin. The study watershed is highly susceptible to hydrogeological response to precipitation events. The flooding due to the high bifurcation ratio. The stream frequency provides detailed information Mahalapswe sub-basin exhibits the highest mean regarding a basin’s permeability, infiltration capacity, bifurcation ratio of 5.3, possibly due to the observation and relief. Stream frequency depends on rainfall in the that it possesses the lowest number of streams. The area, relief, and erosive resistivity of the underlying Shashe sub-basin shows the lowest mean bifurcation rocks (Mahala, 2020). The stream frequency values in ratio of 2.9 (Table 3). The mean bifurcation values sub-basins are low and range from 0.008 to 0.012, indicate structural disturbance in the basin, which suggesting a low run-off component in the sub- could reflect the hilly terrain with less permeable basins alongside a high infiltration likelihood. The rocks in the basin. drainage texture values in sub-basins range from 0.085 to 0.201; in this respect, all the basins are coarse- textured (Table 5). Although the infiltration capacity Textural morphometry indices has not yet been derived for any of the sub-basins, judging by the low drainage density and the low Drainage area, density, stream frequency, and stream frequency, the infiltration numbers would drainage texture likely be extremely low, suggesting a higher infiltration The Shashe sub-basin possesses the largest drainage potential within the sub-basins. area of 29,612.2 km ; however, of this, only 10,790.2 km is located in Botswana, whilst the remaining is in Zimbabwe. The next largest is the Notwane sub-basin Elongation ratio which possesses a moderate drainage area of 17,423.7 km . The Bonwapitse sub-basin has the smallest drai- The elongation ratio of watersheds is defined as the nage area of 13,015.3 km (Table 5). The drainage ratio of the diameter of a circle of the same area as the density could be related to the state of dissection of watershed to the maximum watershed length. The the catchment areas as well as the response to preci- numerical value varies from 0 (for highly elongated pitation events. Regarding behaviour, high magni- basins) to 1 (in the circular basin). Meanwhile, the tudes of drainage density indicate that the catchment elongation ratio values are close to 1.0 for regions of is highly dissected and responds quickly to precipita- tremendously low relief and are between 0.6 and 0.8 tion events. A low drainage density would indicate the for regions of strong relief and steep ground slope. In opposite scenario. The drainage density for the six this study, it was suggested that the Lotsane, Shashe, sub-basins ranges from 0.099–0.122, thus indicating and Motloutse sub-basins are more elongated (<0.5), that there is approximately 90 to 120 m of stream while the Notwane sub-basin is elongated (0.623). The length for each square kilometre area and is of the low category. The results suggest that the drainage Mahalapswe and Bonwapitse sub-basins are less Table 5. Table showing textural characteristics of the Limpopo River Subbasins Notwane Bonwapitse Mahalapswe Lotsane Motloutse Shashe Areal Indices Subbasin 1 Subbasin 2 Subbasin 3 Subbasin 4 Subbasin 5 Subbasin 6 Drainage area (Km ) 17423.7 13015.3 13106 15124.5 19589.6 29612.2 Drainage Density (Km/Km ) 0.113 0.122 0.106 0.101 0.108 0.099 Drainage Texture 0.201 0.194 0.164 0.14 0.194 0.085 Form Factor 0.305 0.456 0.433 0.175 0.14 0.131 Relief (m) 646 516 620 832 722 872 Stream Frequency 0.011 0.012 0.011 0.009 0.01 0.008 Ruggedness Number 0.072 0.063 0.031 0.084 0.078 0.042 GEOLOGY, ECOLOGY, AND LANDSCAPES 11 elongated (0.7–0.8). These results suggested that the with AF > 50 are tilted to the left. The results showed that the Bonwapitse, Lotsane, Mahalapswe, sub-basins are in areas with high relief and steep and Shashe sub-basins are tilted to the right of the ground slopes and will be associated with high pick drainage basin. In contrast, the Motloutse sub-basin discharges. is tilted to the left of the drainage basin. The asym- metry factor for the Notwane sub-basin (Table 4) Form factor and relief suggests minimum tilting (Figure 7). Meanwhile, the analysis of lineaments (faults and dykes) shows The form factor of the sub-basins in the study area that most lineaments trend NW – SE. In contrast, ranges from 0.131 to 0.456, indicating that the catch- the Notwane basin has major dykes and faults that ment is moderately circular and somewhat elongated. trend E – W, with the main streams being super- The Bonwapitse and Mahalapswe sub-basins possess imposed on the same general trends as the linea- form factors of 0.433 and 0.456, respectively, thus ments within the rose diagrams, suggesting an indicating that the sub-basins are nearly circular elon- influence of tectonics on river development gated basins, implying low hydrograph peaks for (Figure 8a,b). The tilting in the sub-basins may be a longer duration or extended time (Table 5). In com- due to the active tectonics associated with recent earthquakes (Moorkamp et al., 2019; Mulabisana parison, the Shashe sub-basin will have lower pick et al., 2021). All sub-basins lie on the hanging wall flows of long duration, whereas the Mahalapswe and of a normal fault which is an extension of the Bonwapitse will have high peak flows of short dura- Moijabana earthquake fault (Mulabisana et al., tion. In general, the relief of the sub-basins increases in 2021) (Figure 2c). a northward direction from 646 m in the Notwane to 872 m in the Shashe sub-basin. The high relief also suggests the presence of faster runoff that may result Basin sinuosity in more peaked basin discharge and greater erosive The stream sinuosity index is a proxy for assessing power than in other basins. tectonic activity and describes the relationship between channel length and valley length. If the sinu- osity index of a stream is 1.3 or more, the stream is Ruggedness number considered meandering; a straight stream has The ruggedness number combines the slope steepness a sinuosity index of 1 and sinuous streams have and length, indicating the extent of the instability of indices between 1.05 and 1.3. The sinuosity results the land surface (Strahler, 1957). The estimated rug- show that Lotsane (1.28), Shashe (1.21), Motloutse gedness number value for the sub-basins in the study (1.33), and Notwane (1.39) are more sinuous than area ranges from 0.031 for Mahalapswe to 0.084 for the Bonwapitse (0.81) and Mahalapswe (0.61) rivers the Lotsane sub-basin. The ruggedness numbers for and can be considered semi-meandering. If we used the sub-basins are typically low, indicating that the the sinuosity index results as a proxy for neotectonics, sub-basin is mature and at its maximum denudation other than the Bonwapitse (0.81) and Mahalapye riv- erosive stage. The sub-basin regions are typically flat ers (0.61), most sub-basins would fall under the mod- with gentle slopes and possess small hills with flat tops erate to high tectonic activity zone category. at approximately 1200 m above sea level. The sub- basins typically possess low slopes in the range of 0– 4.0̊ (Figure 6). Medium slopes are observed within Transverse topographic symmetry factor (TTSF) sub-basins that have hilly areas, such as in the The Transverse Topographic Symmetry factor (TTSF) Shoshong and Tswapong hills, and the hilly terranes provides information about the tilt direction in a basin of southern Botswana (Carney et al., 1994; Ermanovics and is also a proxy for tectonic activity in the area. The & Skinner, 1980; Ermanovics et al., 1978; Mapeo et al., truly symmetric basins exhibit a transverse topo- 2004). graphic symmetry factor (TTSF) = 0 and TTSF > 0; generally, for asymmetric basins, the TTSF increases Tectonic geomorphology (morphotectonic as the asymmetry increases to values closer to 1. The indices) Limpopo sub-basins in Botswana possess TTSF values in the range of 0.12–0.48; TTSF > 0.38 suggest high Basin asymmetry active tectonics in the Bonwapitse and Shashe (0.47) The basin asymmetry factor computed for the sub- and Bonwapitse (0.48), whereas the Notwane (0.27), basins ranges from 18.75 to 60.08, indicating that Motloutse (0.37), and Bonwapitse (0.34) are asym- the sub-basins are tilted in different directions, metric and belong to the moderate tectonic zones basins with AF < 50 are tilted to the right; those (Table 4). Furthermore, the Mahalapswe Basin (0.12) 12 O. MOSES ET AL. Figure 6. A DEM map showing the slope characteristics of Botswana’s six subbasins of the Limpopo River basin. (a) Shashe, (b) Notwane, (c) Motloutse, (d) Mahalapswe, (e) Bonwapitse, and (f) Lotsane. is symmetrical and lies within a low active tectonic substratum, whilst many pick-up points along the zone. profile suggest the presence of knick-point zones. The Lotsane sub-basin also exhibits a rough profile curve with two sharp distinctive spikes at the 25,000 m Hypsometric integrals mark at an elevation of approximately 868 m and 850 The hypsometric curves and HIs indices were acquired m above sea level (Figure 10). The Mahalapswe sub- for the six basins and range between 0.49 and 0.50 basin has a much smoother profile curve than those of (Table 4), within 0.35 < HI < 0.60, which indicates that the Bonwapitse and Lotsane sub-basins. However, the sub-basins are in the mature stage. Considering the there is a sharp drop in the gradient (representing individual HCs, the Notwane and Mahalapswe sub- >100 m change in elevation) from just above 40,000 basins exhibit convex HCs, suggesting that they are in m to approximately the 34,000 m mark, with this the youthful stage and that the river experiences lateral being immediately followed by a pick-up point that erosion (Figure 9). The channels are V-shaped, wider, could potentially be a knick point (Figure 10). The and possess gentle slopes. In contrast, the Bonwapitse, Motloutse sub-basin exhibits a rough profile with ele- Lotsane, Motloutse, and Shashe sub-basins all possess vations ranging between 1000 and 550 m above sea S-shaped HCs, thus indicating a maturity state charac- level, which could be due to a rapid change in slope terised by U-shaped channels and more expansive val- resulting from the rough topography. A sharp spike is leys. This implies that they exhibit significant discharges also observed at the 210,000 m mark, which may be compared with those of young rivers. a knick-point (Figure 10). The Notwane sub-basins exhibit a steady decrease in the gradient, whilst the elevation ranges between 840 and 940 m. The long- Longitudinal profile analyses itudinal profile is rough between 0 and 30,000 m and All six sub-basins exhibit a concave upward longitu- between the 80,000 and 120,000 m marks, thus indi- dinal profile; however, the smoothness of the curves cating rough topography. The Shashe River profile differs. The Bonwapitse sub-basin profile is a rough elevation ranges between 550 and 900 m above sea curve that suggests the presence of a resistive level. There are breaks in the Shashe longitudinal GEOLOGY, ECOLOGY, AND LANDSCAPES 13 Figure 7. Maps showing the asymmetry factor for the subbasins. The red line shows the basin midline, which is theoretically where the river trunk should be; deviation from this line suggests shifting the river channel due to tilting. (a) Bonwapitse, (b) Motloutse, (c) Mahalapswe, (d) Notwane, (e) Lotsane, and (f) Shashe. profile, whilst the numerous spikes observed in the Bonwapitse and Notwane River profiles profile graph indicate rough topography (Figure 10). Terrace deposits in the Bonwapitse sub-basin were observed on the left bank facing upstream at locations −23.49072°S and 26.77469°E. The deposits are 110 m Field observation-pilot survey of knick-points thick from the current riverbed and consist of coarse on three river profiles red sandstone, pebble, angular, and irregular pebbles and cobbles, including quartz, jaspilite, and iron for- Mahalapswe river profile mation clasts cemented by calcrete. A well-exposed The knick-points observed in Figure 10 were investigated terrace deposit is exposed within the current channel in the field for the presence of river terraces and the along the Notwane River at 23.93094°S and 26.73955° development of new terraces. During field observations, E. The terrace deposits possess a thickness of 153 m it was observed that river terraces existed, as suggested by above the current riverbed and consist of cobbles and the profile analysis. There are clear benches that are now pebbles held by calcrete developed above weathered overgrown with tall grass. A three-step flat surface exhi- dolerite (Figure 11c). These deposits extend along the biting bench-like features was observed on the left side Notwane River and are incised by the river into the facing downstream. The terrace deposits were identified weathered dolerite. Google Earth images also show at 23.22861°S and 26.892028°E, comprising three- several meander scars along the Notwane River, indi- stepped flat surfaces with thick grass-covered soils. The cative of a well-developed meandering river system thickness of the recent deposits ranges from 1.7 m to 1 m, with oxbow lakes in the past. The meander scars whilst these deposits are comprised of unconsolidated represent neck cut-off, suggesting that the Notwane dark grey to grey soils (Figure 11a-c). Older terrace River previously had regular discharge with stable deposits incised by the river below consist of conglom- channels. Meanwhile, other river activity features erate pebbles and cobbles cemented by calcrete. show several river migration instances (Figure 12a-c). 14 O. MOSES ET AL. Figure 8. (a) a map showing lineaments (faults and dykes) on which the main rivers are superimposed within the subbasins. Discussion a fifth-order basin, whilst the Mahalapswe sub-basin is of the third order. It was observed that sub-basins Drainage morphometry- (morphometry indices) possessing larger areas also possess high-order The results of the stream order analysis revealed that streams. For example, at 29,612.2 km , the Shashe sub- the first-order streams are typically associated with basin is larger than the other basins, while the high-relief regions that tend to exhibit shorter lengths. Bonwapitse sub-basin covers the smallest area of A high number of segments in the lower order indi- 13,015.3 km . Meanwhile, the Mahalapswe sub-basin cates high erosion characteristics. Mahala (2020) pre- exhibits a lower number of lower-order streams, indi- viously observed that rivers possessing an increased cative of a mature topography. Mahala (2020) argued number of lower-order streams (I, II, and III) exhibit that the lower number of streams in higher relief areas higher sediment yields and receive a large amount of may indicate lower water regimes and water stress water, thus creating a large water flux in the lower conditions. The Shashe sub-basin possesses the largest plains. The highest orders are observed in low relief basin area, the highest stream order, and the highest areas (flat land), which typically account for the main stream number; this is in agreement with Strahler trunk of the river and possess longer lengths. (1957) that the stream orders increase with an increase The high mean stream length values (6.0–125) for in the basin area. the sub-basins indicate high stream orders. Four sub- The analysis of the bifurcation ratios in the sub- basins are fourth-order basins (Bonwapitse, Lotsane, basins reveals considerable variation in the values of the different stream orders due to variations in Motloutse, and Notwane). The Shashe sub-basin is GEOLOGY, ECOLOGY, AND LANDSCAPES 15 Figure 8. (b). Rose diagrams of lineaments and streams (i) Faults and dykes, ii) Rivers and streams. geological and lithological controls (Mahala, 2020; the mean stream length values between the orders Strahler, 1952a, 1952b). The Lotsane, Mahalapswe, result from slow changes that may lead to rapid and Motloutse rivers exhibited mean bifurcation ratios changes in flow characteristics. greater than 5.0, suggesting that these regions have The stream length ratio is higher in the high-order experienced structural disturbances that may have streams (Bonwapitse, Lotsane, Mahalapswe, influenced the present drainage patterns. The Motloutse, and Notwane rivers), whilst the stream Bonwapitse and Notwane sub-basins possess bifurca- length ratio between orders III and IV exhibits the tion ratios within the range of 3.6 to 5.3, indicating that highest stream ratio (1.23–1.67), while the lowest the drainage basin pattern may have been affected exists between orders III and II (0.23–0.45). For the mildly by tectonic deformation. The Shashe sub-basin Shashe sub-basin, the stream length ratio between also has a bifurcation ratio of less than 3, thus suggest- orders IV and V is the highest (1.50), while the lowest ing that it exhibits the lowest structural control among is between orders III and IV (0.26). These changes in all sub-basins. The high bifurcation ratio for the high- the stream length ratio indicate an early stage of geo- order streams indicates that the basin receives a large morphic development. amount of water in the high areas of the stream head- The low drainage densities (0.099–0.122) are attrib- waters. Furthermore, a low bifurcation ratio signifies uted to highly permeable subsoils that lead to low a high probability of flooding in some parts of the basin. surface run-off and soil erosion. Half of the sub- All six sub-basins (Bonwapitse, Lotsane, basins in Botswana are underlain by aerosols that Mahalapswe, Motloutse, Notwane, and Shashe) exhi- possess a loam-like and sandy texture (>70% sand), bit low mean stream length for the first order, thus indicating that they are highly permeable. Luvisols indicating young morphological development and underly much of the northern part of Botswana high erosion potential. The higher-order sub-basin (Shashe and Motloutse sub-basins) and possess (III, IV, and V) streams possess a high mean stream a coarse-grained sandy loam with a coarse-grained length since they have completed their channel length- sandy clay texture that makes them permeable and ening. These characteristics indicate that the sub- well-drained (Bangira & Manyevere, 2009). The basins (Bonwapitse, Lotsane, Mahalapswe, Motloutse, results suggest a lower surface run-off potential in Notwane, and Shashe) are in the early stages of geo- these regions, thus indicating the possibility of high morphic development. The inconsistent differences in infiltration in the area. 16 O. MOSES ET AL. Figure 9. Hypsometric curves of the six subbasins. The curves were generated from 90 m SRTM DEM released in 2013. The low stream frequency (0.009–0.012) in the circular. Sub-basins possessing this characteristic same region may have been due to less permeable feature generally experience a high peak flow of underlying rocks that can lead to less percolation. shorter duration, thus making them prone to This suggests that the area possesses a poor drai- flood hazards (Bali et al., 2012; Farhan et al., nage network. This value also indicates a very 2016,b). For example, catchments of drainage coarse drainage texture, as confirmed by the low basins in Ras en Raqbin, in Jordan, exhibited drainage densities for the sub-basins described a form factor of 0.4 and were affected by the above. In a study, Avinash et al. (2014) estab- May 2014 flash floods in the Aqaba area in lished that the Mulki-Pavanje in Karnataka, a manner consistent with this characteristic. Bali India, exhibited a very coarse texture in areas et al. (2012) previously demonstrated that sub- with high precipitation, leading to the develop- basins in the Pindari glacial-fluvial basin in ment of high groundwater potentiality. The find- India could be described by morphometric fea- ings in these studies were similar to those in the tures that include high peak flows over a short different sub-basins of Limpopo in Botswana, time. It has been shown that some sub-basins which are characterised by generally low relief possessing high form factors (0.6–0.72) can be areas (Table 5) and except for deficient rainfall. affected by flash floods due to the bursting of The elongation ratio (0.409–0.763) suggests that the supraglacial and englacial water bodies all six sub-basins are elongated. More elongated (Farhan & Anaba, 2016; Farhan et al., 2016, sub-basins may possess active tectonic influences 2017). related to high relief and steep slopes (Bali et al., The Mahalapswe and Bonwapitse sub-basins 2012; Mathew, 2016; Rawat et al., 2013). The form exhibit the same high form factor features, 0.433 factors for Mahalapswe and Bonwapitse are nearly and 0.456, respectively, whilst settlements along GEOLOGY, ECOLOGY, AND LANDSCAPES 17 Figure 10. Longitudinal profiles of Bonwapitse, Motloutse, Notwane and Shashe sub-basins. Morphotectonic Indices-Tectonic this basin, such as the Mahalapswe and Shoshong, Geomorphology are identified as flood-prone areas (Tsheko, 2004). For example, in December 2019, this area was The basin asymmetry factor is an important para- affected by floods that impacted the transporta- meter of basin drainage and determines the exis- tion system, causing a train derailment (Gaofise, tence of tectonic tilting in the basin, whilst the 2019; Kealeboga, 2020). Historical data indicates results indicate tilting within the sub-basins. several train derailments between the Bonwapitse Specifically, the Motloutse River is tilted to the River and the Mahalapswe River dating back to left, and the other five sub-basins are tilted to November 1987 (Shaw, 1988) that occurred due to the right. The basin asymmetry values (18.75– flooding. Shaw (1988) previously claimed that 60.08) suggested that the sub-basin had formed floods in this area have a recurring period of 20 under an active tectonic setting. The area has years and are significant due to their ability to undergone an uplift along the OKZ axis between cause structural damage (Shaw, 1988). The the Late Cretaceous and Early Tertiary stages Lotsane, Motloutse, Notwane, and Shashe sub- (Moore & Larkin, 2001; Moore, 1999), and this basins exhibit form factors of 0.175, 0.305, 0.131, may have contributed to the tilt of the river within and 0.140, respectively, suggesting that the sub- the different sub-basins. Overall, the region is basins are elongated. Information from the litera- aseismic, but recently, earthquake events along ture indicates that elongated basins exhibit low ancient faults have suggested that the region is peak discharge for a longer duration, thus making undergoing intraplate tectonic deformation it easy to manage flood flows in such areas (Nthaba et al., 2018; Moorkamp et al., 2019; (Farhan & Anaba, 2016; Farhan et al., 2016). Mulabiasana et al., 2021). 18 O. MOSES ET AL. Figure 11. Field photographs showing river terrace deposits identified in the cross-sectional profiles of the subbasins. (a) Mahalapswe, (Bonwapitse and (c) Notwane. The transverse topographic symmetry factor (Morgan, 1976; Rodríguez-ItRodri-Guez-Iturbe & (0.112–0.479) indicates the influence of new tectonic Escobar, 1982). Morgan (1976) observed that a high activities within the basins. HIs analyses revealed drainage density is required for a drainage network a range between 0.49 and 0.5, suggesting that the sub- with moderate to regular rainfall. Rodri-Guez-Iturbe basins are in their youth development stage, thus and Escobar (1982) suggested that the analysis of implying domination of vertical erosion. The long- drainage densities should remain general due to the itudinal profiles results for all the sub-basins variation in the variables influencing drainage density. (Bonwapitse, Lotsane, Mahalapswe, Motloutse, The drainage densities of the sub-basins are espe- −1 Notwane, and Shashe) exhibit rough topography, cially low (<0.25 km ), which differs from what is which is in agreement with the results of the drainage expected from the literature, which reports that semi- density and texture analysed for these sub-basins. arid areas possess high drainage densities. Carlston Cross-sectional profiles for the Bonwapitse, (1963) investigated 15 drainage basins in different Motloutse, Mahalapswe, and Notwane sub-basins mono-lithological areas in the Eastern United States indicated the presence of terraces, thus suggesting and observed no correlation between high drainage possible incision in the area. The Mahalapswe sub- density and increased rainfall intensity and run-off. It basin exhibits differential erosion, as indicated by the was found that drainage densities correlated with run- difference in the bank elevation (Figure 11). off in certain regions, a characteristic influence of cli- matic conditions. High temperatures also led to high Drainage density is a morphometric index that is drainage density because of their ability to increase the closely linked to climate change. This increases with evaporation rate, thus causing a decrease in vegetation decreasing precipitation, being high in semi-arid areas cover and leading to high erodibility (Eccker, 1984). with sparse vegetation and low in humid areas with The differences in drainage density of the current sub- dense vegetation (Rodri-Guez-Iturbe & Escobar, basins from those expected based on previous studies 1982). Semi-arid regions are known for their low may indicate that the drainage densities are not vegetation and soil cover, increasing drainage density GEOLOGY, ECOLOGY, AND LANDSCAPES 19 Figure 12. Google Earth images of the Lotsane, Notwand and Motloutse rivers showing various features, including meander scars and river migration. influenced by climatic conditions alone but by other (a) The sub-basins in eastern Botswana are drained variables such as tectonics, rock type, and relief. by fourth and fifth-order streams with high water flux in regions that have highly perme- able soils with a coarse drainage texture. The Conclusions slopes are low, suggesting water flows with less potential energy increase the possibility of high Based on Shuttle Radar Topographic Mission (SRTM) infiltration and groundwater recharge. DEM, we conducted a morphometric study on the (b) The mean stream length ratios show that all the Limpopo River Basin in Botswana to understand the sub-basins are in their early stages of geo- role of the underlying geology and tectonics on its morphic development, whereas an indication geomorphic and hydrogeological development. of mature topography is found in others. The Consequently, we arrived at the following conclusions: 20 O. MOSES ET AL. form factors allow us to designate the of Geology, Ecology and Landscapes for their valuable and constructive comments on the earlier draft of this Mahalapswe and Bonwapitse sub-basins as manuscript. flood-prone and requiring management. The HCs indicate that some river systems within the region are in their youthful stage, charac- Disclosure statement terised by wider V-shaped channels with gentle slopes. In contrast, others have S-shaped HCs, No potential conflict of interest was reported by the author(s). which are characteristic of a mature state with U-shaped channels and more expansive valleys. (c) Some sub-basins show a high frequency of Data availability statement NW – SE trending lineaments, with minor All data sets are available on request from the corresponding E – W trending lineaments that coincide with author (MapeoRBM). The data are not publicly available the drainage pattern orientations. This indi- because they are part of ongoing research yet to be wound cates that the drainage systems are largely down. structurally controlled. The Asymmetry Factor and TTSF analysis show that the sub-basins are References tilted due to tectonic activities. The tectonic activities started in the Jurassic and Lower Ahmed, F., & Rao, K. S. (2016). 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Journal

Geology Ecology and Landscapes – Taylor & Francis

Published: Jun 21, 2023

Keywords: Morphometry; geospatial analyses; active tectonics; drainage basin; Limpopo River Basin; Botswana

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Morphometric properties of river basins as indicators of relative tectonic activity – Problems of data handling and interpretation
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Evolution of drainage systems and slopes in badlands at Perth Amboy, New Jersey
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Standards for grading textures of erosional topography
Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California
Participatory approach for integrated Basin planning with focus on disaster risk reduction: The case of the Limpopo River
Dynamic Basis of Geomorphology
Hypsometric (area-altitude) analysis of erosional topography
Quantitative analysis of watershed geomorphology
Quantitative Geomorphology of Drainage Basin and Channel Network
Rainfall reliability, drought, and flood vulnerability in Botswana
River patterns and their meaning
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Limpopo River Basin: A Proposal to Improve the Flood Forecasting and Early Warning System

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Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

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Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

Evolution of the Limpopo River Basin in Botswana based on morphometric and morphotectonic features from selected rivers using GIS techniques

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