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GEOLOGY, ECOLOGY, AND LANDSCAPES 2020, VOL. 4, NO. 1, 11–22 INWASCON https://doi.org/10.1080/24749508.2019.1568130 RESEARCH ARTICLE Longitudinal proﬁles and geomorphic indices analysis on tectonic evidence of ﬂuvial form, process and landform deformation of Eastern Himalayan Rivers, India a b Suman Ayaz and Md Kutubuddin Dhali a b Former Student, Department of Geography, Presidency University, Kolkata, India; Department of Geography, Visva-Bharati, A Central University, Bolpur, India ABSTRACT ARTICLE HISTORY Received 19 October 2018 The drainage pattern and the morphology of the piedmont zone of the Himalayas are clear Accepted 1 January 2019 indicators of the active orogenic belt of most recent origin formed by the collision of Indian and Eurasian plate. The foothills of the Himalayas in West Bengal are zones of active tectonics drained KEYWORDS by the rivers belonging to the Brahmaputra system. The present study is conducted for the left Eastern Himalayan Rivers; bank tributaries and sub-tributaries of Tista which bear the imprint of active tectonics of the tectonic evidence; ﬂuvial region as they lie in the zone of Himalayan Frontal Fault, the most active thrust belt of Himalayas. form; ﬂuvial process; Data and subsequent ﬁeld experiences which showed that the region is constantly acted upon by landform deformation recent diastrophism. Various tectonic indices were calculated to evaluate the evidence of tectonism. This include hypsometric integral (HI), fractal dimension (FD), basin asymmetry factor (AF), basin shape index (Bs), stream-length gradient (SL), mountain front sinuosity (Smf) and valley-ﬂoor width to valley height ratio (Vf). The results of these indices are used to prepare an index of active tectonics with three classes which is represented in a map. Then the ﬁeld evidences of deformed landscape are matched with the areas showing high tectonic index values. 1. Introduction and sediment production. Local base level changes can The Himalayas is the most important orogenic belt in also modify this relationship, inﬂuencing the erosional the world. The collision of the Eurasian and Indian and depositional regime in the distal fan zone. Plates in the late Pleistocene caused subsidence of the However, the focus of this study is on the neotectonic lithosphere and thereby formed the current Himalayan imprints on the morphology and morphometry of the Foreland basin (Burbank & Anderson, 2001; Malik & rivers using diﬀerent proﬁles, indices, and the sur- Nakata, 2003;Nakata, 1989; Singh, Parkash, Mohindra, rounding landscape in the Quaternary alluvial belt Thomas, & Singhvi, 2001;Singh&Tandon, 2010; along the North Bengal mountain front. The dissection Suresh, Bagati, Kumar, & Thakur, 2007). This foreland by the younger streams altered the original morphology basin contains Main Frontal Thrust (Nakata, 1989). of coalescing alluvial fan in this piedmont zone. Forms, This Thrust was named the HFF (Himalayan Frontal process and related ﬂuvial landforms of Indian system Fault) by Nakata (1989). Thus the presence of this active and sub system of rivers are quite diﬀerent. Many of fault has made the region unstable after the formation Plateau rivers in India developed river bed potholes of HFF. The region is characterised by the presence of (Dhali & Biswas, 2017a, 2017b) by response to geologi- many substantial rivers and their innumerable branch cal control, their seasonal hydraulic behaviour and ero- of rivers. Tectonically imposed changes are noticed by sion zone are diﬀerent (Dhali & Sahana, 2017). Rivers the responses made in the adjoining morphology and Gish, Lish, Chel, Mal, Neora, and Murti drain the pre- morphometry of this ﬂuvial system. The rise and fall in sent sub-Himalayan piedmont zone of North Bengal the base proﬁle of the river can alter the course of the (Ayaz, Biswas, & Dhali, 2018) and their form- river, changes in channel geometry, the river valleys processes are directly related to tectonic control. The may get constricted or widen, there can be increase in present morphology of the region is that of terraced the rate of deposition or changes in the stream energy landscape with secondary alluvial fans at the debouch- which may cause formation of ﬂuvial landforms like ing point of these rivers. These rivers have formed their alluvial fans, terraces, and bars. The changes in the base own micro fans which are being modiﬁed and their proﬁles of the rivers could be in response to tectonics or morphology is being changed due to neotectonic activ- climate. Due to tectonics the relief of the source area ities in the region (Ayaz et al., 2018; Mandal and Sarkar, could be altered. This inﬂuences the gradient of the fan 2016). All this change in the channel pattern and river CONTACT Md Kutubuddin Dhali firstname.lastname@example.org Department of Geography, Visva-Bharati, A Central University, West Bengal, India © 2019 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. 12 S. AYAZ AND M. K. DHALI diversions has been clearly observed in the Gish and and greater Himalayas, the Main Central Thrust (MCT) Lethi channels which are the feeder channels of the Gish separating the Greater Himalayas and Lesser Himalayas, alluvial fan. The presence of the terraces along the river the MBT (Main Boundary Thrust) is between the Lesser banks is an indicator of the active tectonism and the Himalayas and Siwaliks and the Himalayan Frontal deformation of the fan surface (Mandal and Sarkar, Thrust (HFT) separates Quaternaries from Siwaliks. 2016). Evidence of tectonic activities could be under- The three east-west trending south verging thrusts stood from the modiﬁcations in the drainage patterns, (MCT, MBT, and HFT) gets progressively younger formation of the east-west scarps and also folding of the towards the south. The MCT was active during the fan surface and the sediment strata making up the fan. early Miocene, MBT in the Pliocene, and HFT in the The formation of Matiali and Chalsa scarp is attributed Quarternary (Holocene) (Gansser, 1964;Kumar, 1993). to the movements of two north dipping blind thrusts. The present day convergence between India and Eurasia Thus the present study gives the evidence that morphol- is accommodated by the movement on the HFT. Late ogy of the region is tectonically controlled and there is quaternary movement in the eastern and western still the presence of active faults which is the reason Himalayas have been documented in several previous behind the dynamic nature of the landscape. The mor- works (Kumar, 1993; Malik & Nakata, 2003;Sarkar, phology of the present landscape is reﬂecting the eﬀects 1999, 2012). The faults present here can be divided into of these tectonic forces. Many modiﬁcations in the two major groups such as the East-West trending faults course of the rivers and the expansion of the alluvial along the mountain fronts and the NNE-SSW to NNW- fans have been noticed for the past 100 years. Thus this SSE trending faults along the river courses like Chel, Gish paper focuses on mapping such modiﬁcations and river, and Lethi rivers. All the channels like Lish, Gish, understanding the neotectonic imprints by the use of Pabriong, Ramthi, Neora, Mal, Murti, and Kurti are part various morphotectonic and morphometric indices. of Eastern Himalayan foothill zone which is eastern part of the river Teesta (Figure 1). The unconsolidated alluvial fan deposits have enveloped the evidences of these faults 2. Sites, geology, and tectonic setup (Bisaria, 1980). Along these faults two movements have been postulated (Bisaria, 1980). The ﬁrst phase of move- This Himalayas could be divided into ﬁve tectonic belts ment had taken place in the early Pleistocene that caused (longitudinally) from north to south; The Tibetan the upliftment of the foothill ranges and was responsible Himalayas, Greater Himalayas, Lesser Himalayas, and for the ﬁrst phase of deposition of alluvial fans before the Siwaliks. These ﬁve belts are separated by major thrust Last Glaciation. The second phase of movement had belts, i.e., the south Tibetan detachment separates Tibetan Figure 1. Location of Eastern Himalayan foothill rivers, i.e., Lish, Pabriong, Mal, Murti, Lethi, Chel, Naora, Ramthi, Gish which non-perennial tributaries and sub-tributaries of River Teesta. GEOLOGY, ECOLOGY, AND LANDSCAPES 13 taken place after the deposition of the earliest alluvial fans 3.1. Stream-length gradient index (SLGI) and followed by the movements along the northerly The instantaneous changes in the river longitudinal trending faults. This second phase of movement respon- proﬁles are clear evidences of the structural and tec- siblefor theupliftmentofsomesegmentsofthe old tonic inﬂuence in the region resulting in changes in piedmont plain terraces. The terrace remnants on top of the channel dynamics. The SLGI as developed by Gorubathan hill and the Daling Hills are evidence of this Hack (1957) help to calculate these changes. This SL event. This second phase of neotectonic activity formed index to apprehend the inﬂuence of lithology and the NNW-SSE and NNE-SSW trending faults. Thus, the tectonics in the longitudinal proﬁles of the river geomorphology of this piedmont zone is mainly con- have been used by many researchers (Lee & Tsai, trolled by the quaternary tectonic activities. 2009) The formula explained by Hack (1957) is used The Gish Traverse Fault (GTF) is present between to calculate the SL Index of a segment of the river the Maynabari and Pathorjhora unit. The GTF is one between two points which is written as: of the famous active faults in this area (Mukul, Jade, Ansari, & Matin, 2014; Mullick, Riguzzi, & SL ¼ðÞ h1 h2 =½ lnðÞ d2 lnðÞ d1 ; (1) Mukhopadhyay, 2009). The activity of this fault has where SL = stream gradient index; h1 = height of the been observed through a network of Global Position ﬁrst point from the source; h2 = height of the second System stations. It was seen that the fault in extend- point from the source; d1 = source to ﬁrst point ing in east-west direction at the rate of 10.9 ± 1.6 mm distance; d2 = distance of the second point from the per year (Mullick et al., 2009). This is an indicator source; ln = natural log. that the neotectonic activities are quite signiﬁcant in this region and thereby inﬂuence the rate of sedimen- tation and thus aﬀect the morphology of the fans in 3.2. Normalised stream-length gradient index the region. The Chel and Lethi were once guided in (NSL) the direction of the orientation of this fault (Bisaria, It is very common and useful method for analysis of 1980). The presence of this fault is marked by the recent tectonic activity of ﬂuvial system (Seeber & presence of a scarp along the eastern margin of Gornitz, 1983). Mathematically, the statement can Bagrakota which is 10–15 m high. Tectonic activities be represented as: may aﬀect the slope of the river and the local base level to a large extent. Thus changes in the slopes of NSL ¼ SL=k; (2) the rivers, may aﬀect the channel pattern of the river where NSL = Normalized Gradient Index for the and can also cause diversions in the river given part of length, SL = Stream gradient index for (Karssenberg & Bridge, 2008). Both the Chalsa and the given part of length, k = Slope of the idealized Matiali scarps are present across the prime channels Hack’s graded proﬁle. The Segments having NSL ≥ 2 such as Murti and Kurti rivers. The MBT is repre- are considered as notably steeper while it is NSL ≥ 10 sented by the Matiali Fault and HFT by the Chalsa are identiﬁed as immensely steep reaches. The results Fault (Goswami, Mukhopadhyay, & Poddar, 2012; of NSL < 2 imply gentle gradient. Nakata, 1989). These are the two major lineaments along with Neora and Murti Lineament along which is followed by the two rivers; Murti river and Neora 3.3. Analysis of hack proﬁle river. The metamorphic rocks of Darjeeling gneiss, For morphometric study the scanned Topographical the Daling group of rocks (Schist and Quartzites), the sheets and the SRTM 30 m DEM were used. The Buxa series and also the sedimentary rocks of elevation of each contour crossing the six main tri- Gondwanas, and Siwaliks form the diﬀerent tectonic butaries of River Tista, i.e Neora, Gish, Chel, Lish, units of Darjeeling Himalayas (Banerjee, 1955; Mal, and Murti along with their sub-tributaries were Acharya, 1971). obtained and the distance from the source were also calculated to obtain the longitudinal proﬁles of the rivers by using the editor tool in ArcGIS 10.2.2. 3. Data and methods Rivers that are in equilibrium have a concave shape The previous literatures were done to formulate the which represents a decline in the slope of the channel research design and the objectives of our research. downstream. If the river overﬂows through an area Required data were also collected by the diﬀerent topo- marked by active tectonism then it displays a convex graphical maps such as78B/9, 78B/13, 78 A/12, 78 A/16 long proﬁle. For ideal rivers, if the long proﬁles are from GSI. The satellite images of the study area (Landsat plotted in a semi-logarithmic graph paper then they 8) and the SRTM 30 m DEM were downloaded from are represented as a straight line (Hack, 1973). This USGS Earth Explorer. proﬁle is also called the “Hack Proﬁle” (Hack, 1973). 14 S. AYAZ AND M. K. DHALI ﬂoor but low value of “Vf” is very narrow, active H ¼ C K ln ðÞ L : (3) tectonic related uplifting results. K ¼ H H = ln L In L : (4) i j i j 3.6. Hypsometric curve and hypsometric integral K is the average stream length gradient index (SL (HI) Index). C is the regression intercept. As in logarith- mic scale, zero cannot be plotted; the source of all the Hypsometric curve and hypsometric integral is rivers is taken as 0.02 km. a common method for use to explain the stage of land- forms in a particular river basin or any types of land- forms. Hypsometric cure mainly showing the diﬀerence 3.4. Long proﬁle length and relief normalisation of erosional landforms and stages of evolution of those landforms (Strahler, 1952). Percentage of relief and The variations in the basin relief and size are reﬂected cumulative percentage of area refer by way of hypso- in the long proﬁles of the river. Thus in order to metric integral (Pike & Wilson, 1971) which is varying reduce the eﬀects of the basin size and relief the 0–1 range. When the value of hypsometric integral is long proﬁle length and relief are normalised. The closed to 0, it is highly eroded region and near 1 is elevations and distances were divided by the head showing opposite nature of character. (maximum basin relief) and the total stream length no X X respectively to normalize the proﬁles (Lee & Tsai, H:I: ¼ ðÞ xiðÞ:yiðÞ þ 1 , ðÞ xiðÞ þ 1 :yiðÞ =2: 2009). Thus the presence of breaks in the river proﬁle is indicative of strong structural inﬂuence in the river (10) course. The normalised long proﬁle model is based on four simple mathematical functions; 4. Results and discussions The linear function y ¼ ax þ b: (5) The tectonic deformations are imperceptible to human bx The exponential function y ¼ ae : (6) eye. Deformation of the earth surface due to tectonic activities takes place for thousands of years. Rivers are The logarithmic function y ¼ aln x þ b: (7) very sensitive to these deformations even if it takes place very slowly. Thus river system analysis are an important tool for studying the tectonic geomorphology as they The power regression model y ¼ ax : (8) are capable of adjusting to the deformations that takes The R value determines the best ﬁt. The curve with the place over periods of centuries to decades (Keller & highest R value is the best ﬁt curve. The previous litera- Pinter, 1996, 2002). Thus the sensitivity of the six left tures indicate that when channel grain size is greater than bank tributaries of Teesta and four sub-tributaries in the capacity of the river for transportation, the long the mountain front of North Bengal has been analysed proﬁle shows a low degree of concavity and hence using the morphometric and Morphotectonic para- abetterlinearfunction ﬁt(Lee&Tsai, 2009). When meters. This has helped to understand the recent tec- there is balance in erosion and resistance as postulated tonics of the region from where passes the MCT of by Hack (1973), the channel sediment grain size will Himalayas. decrease downstream and hence the long proﬁle ﬁts more suitably for the logarithmic function. This is the 4.1. Semi-logarithmic proﬁle of the rivers and “Graded Proﬁle” of the river. With further increase in average SL index proﬁle concavity, the power function becomes more appropriate. Thus, the evolution sequence should be The semi-logarithmic proﬁles or the Hack Proﬁles linear –– exponential – logarithmic ––power. (Hack, 1973) of the rivers are an important indicator of the deformation in the region. If there is some lineament or fault lying in the region then the rivers 3.5. Valley-ﬂoor width to height ratio (Vf) will make adjustments in its proﬁle. Thus there will The present scientiﬁc explanation was put forward by be deviation in the ideal proﬁle of the river which is Bull (1977). It is expressed as; a straight line in a semi-logarithmic scale. Rivers in many tectonically active regions have been studied Vf ¼ 2Vfw=½ ðÞ Eld EscþðÞ Erd Esc ; (9) which show changes in their anomalies. Brookﬁeld where Vfw is showing the valley width, Eld is altitude (1998) showed convex Hack Proﬁles of major rivers of the left bank of valley, Erd is indicated altitude of of South and south-east Asia caused by the tectonic the right bank of valley, and Esc is provided altitude processes during the Cenozoic because of the colli- of the channel. High value of Valley-ﬂoor width to sion of Indian and Eurasian plates. Convex-upward height ratio (Vf) indicated highly ﬂat and wide valley- long proﬁles of the rivers are observed in areas of GEOLOGY, ECOLOGY, AND LANDSCAPES 15 general uplift as seen in South Carolina Coastal plain. and power regression models to the elevation- Concave proﬁles for the rivers in the Kaveri river distance data. Thus a best ﬁt model is used to under- basin have been observed, which is an indicator of stand the form-process relationship. The long proﬁle some form of disturbance in the region (Kale, 2009). show low degree of concavity and trends towards Hack proﬁles are convex in high uplifted areas, and a straight line and better ﬁt the linear function, almost straight or slightly concave in low uplifted when the grain size of the river is greater than its areas. In Taiwan in the Central foothill region, transportation capacity (Lee & Tsai, 2009). The long Hack’s proﬁle of the rivers are convex indicating proﬁle ﬁts the exponential function when the deposi- early stage of the rivers adjusting to fault movements tion and transportation rate of the channel attains while in the south-western foothills the proﬁles are dynamic equilibrium. When the channel becomes convex–concave indicating later stage of adjustments graded, i.e., the sediment grain size decreases down- to fault movements (Chen, Sung, Chen, & Jean, stream, then long proﬁle ﬁts more suitably for loga- 2006). The segments of the rivers above the equili- rithmic function. When the discharge and load brium have high grade energy promoting erosion and suspension are large then the long proﬁle ﬁts the down cutting while those below equilibrium have low power function. Thus the evolution sequence for the grade energy and are promoting deposition (Kale, long proﬁles of the channels should be linear- 2009). In the present study, it has been observed exponential-logarithmic-power (Lee & Tsai, 2009). that almost all the rivers have convex proﬁles indicat- Thus the proﬁles which exhibit linear to exponential ing early stage of adjustment to fault movements, model implies less concavity in their proﬁles thus are while some rivers show convex–concave proﬁles indi- evidences of disturbances in the course of the rivers cating later stage of adjustments to fault movements and implications of recent tectonic movements. The (Figure 2). This is an implication of disturbance in task of ﬁtting the best ﬁt model has been undertaken the areas and completely convex proﬁles of Rivers for the left bank tributaries and sub-tributaries of like Lish, Pabriong, Mal, Murti, and Kurti are indi- Teesta River. All the channels like Lish, Gish, cators of adjustments made by the rivers due to Pabriong, Ramthi, Neora, Mal, Murti, and Kurti exhi- recent tectonic movements. bit exponential model except Lethi. Though Lethi River, being a zone of active fault exhibit logarithmic best ﬁt curve mainly because of small length of its 4.2 Modelling long proﬁles course (Figure 3; Table 1). However there is complete absence of power regression models which is a clear In order to understand the forms of long proﬁles, it is indicator of recent disturbances in the region. better to ﬁt simple, logarithmic, linear, exponential, Figure 2. Semi-logarithmic or the Hack’s Proﬁle of the diﬀerent tributary rivers (after Ayaz et al., 2018). The rivers which have convex proﬁles indicate early stage of adjustment to fault movements and convex-concave proﬁles indicating later stage of adjustments to fault movements. 16 S. AYAZ AND M. K. DHALI Figure 3. Normalised long proﬁles of left bank tributaries of Teesta i.e Lish, Pabriong, Mal, Murti, Lethi, Chel, Naora, Ramthi, Gish. The proﬁles which exhibit linear to exponential model implies less concavity in their proﬁles thus are evidences of disturbances in the course of the rivers and implications of recent tectonic movements. Table 1. Fitting of best ﬁt model to the left bank tributaries of Teesta like Lish, Pabriong, Mal, Murti, Lethi, Chel, Naora, Ramthi, Gish rivers. Coeﬃcient of determination (R ) River Basin Linear Exponential Logarithmic Power Lish River 0.8289 0.9769 0.9066 0.7516 Pabriong Khola 0.961 0.9821 0.8011 0.7127 Gish River 0.5417 0.8633 0.8025 0.7044 Lethi Nadi 0.7228 0.8785 0.9772 0.9108 Ramthi Khola 0.7661 0.9186 0.9083 0.7987 Chel River 0.7731 0.923 0.8876 0.7544 Mal River 0.9195 0.9849 0.8094 0.7025 Neora River 0.8222 0.9429 0.7245 0.6118 Murti River 0.6962 0.8996 0.8451 0.7415 Kurti River 0.8063 0.9216 0.9063 0.7972 4.3. Segment-wise stream gradient index (SGL showing positive values of SL downstream (Figure index) 4). Neora and Murti Rivers are showing very irregular plots of SL values. Thus in order to understand the Stream length gradient index is a clear indicator of rate of change in the SL values, a simple trend line the control of lithology and tectonics in the long was ﬁtted to the scatter plot between rate of change of proﬁle of the river. This index was proposed by SL index and relief ratio (Figure 5). The statistical Hack (1957). Many studies have been conducted mechanics of the ﬁgure is indicative of the fact that recently to understand the impact of lithology and the river basin have high rate of changes in Stream tectonics in the proﬁles of major rivers (Bishop, Gradient Indices. Thus the result clearly depicts that Hoey, Jansen, & Artza, 2005; Goldrick & Bishop, all the rivers are ﬂowing in the zone of active tec- 1995; Lee & Tsai, 2009; Seeber & Gornitz, 1983). tonics and their proﬁles are controlled by the under- Normally, for homogeneous terrain, the SL Index of lying faults. a river reduces downstream. This is mainly the result of decrease in channel slope as one move downstream away from the source. But most of the rivers in the 4.4. Valley-ﬂoor width to height ratio (Vf) study area are showing a rise in the values of SL at the From selective eight sites and their output results downstream section and thus are indicators of areas clearly indicated active tectonic evidence of this undergoing uplift (Kale, 2009). Rivers Lish, Pabriong region. Rivers Lethi, Gish, Ramthi Khola, Chel, Khola, Mal, Ramthi Khola, Lethi, and Chel are GEOLOGY, ECOLOGY, AND LANDSCAPES 17 Figure 4. Segment-wise stream gradient index of diﬀerent rivers such as Lish, Pabriong, Mal, Murti, Lethi, Chel, Naora, Ramthi, Gish. Ramthi Khola, Lethi Nadi and Chel are showing positive values of SL downstream, Neora and Murti Rivers are showing extremely irregular plots of SL values. -20 -40 -60 0 20 40 60 80 100 120 140 160 180 y = 0.2771x - 22.75 Relief Ratio R² = 0.1506 Figure 5. Trend line indicating high rate of change of SL index plotted against relief ratio of selective rivers like Ramthi Khola, Lethi Nadi and Chel Neora and Murti. and Neora are showing low values of valley-ﬂoor 4.5. Hypsometric curve and hypsometric integral width to height ratio (Vf), Mal and Murti are (HI) provided almost moderate value of valley-ﬂoor The diﬀerent stages of the evolution of the erosional width to height ratio and exceptionally river Lish landform are understood by the hypsometric analysis indicated high value of valley-ﬂoor width to height (Strahler, 1952). The degree of diﬀerent part of basin (Figure 6, Table 2). Rivers Lethi, Gish, Ramthi or basin and relative landform age can be analysed Khola, Chel, Neora, Mal, and Murti are very active through the hypsometric value. The value of hypso- according to Vf values because low valley-ﬂoor metric integral close to 1 is quietly eroded and near to width to height ratio indicated active tectonic evi- 0 are highly eroded regions (Schumm, 1956; Strahler, dence (Bull, 1977;Sarkar, 1999, 1999; Mandal and 1952). Hypsometric curve is very useful tool to Sarkar, 2016; Ayaz et al., 2018). explain the age of a landscape. The hypsometric Rate of change of SL Index 18 S. AYAZ AND M. K. DHALI Figure 6. Valley-ﬂoor ratio of Eastern Himalayan Rivers (Neora, Gish, Lethi, Ramthi, Murti, Lish, Mal and Chel). The values are indicative of active tectonics along the river valleys (After Ayaz et al., 2018). Table 2. Calculation of valley-ﬂoor width to height for streams ﬂanking over foothill rivers, i.e., Lish, Pabriong, Mal, Murti, Lethi, Chel, Naora, Ramthi, Gish. Streams Vfw (m) ELd (m) Esc (m) (Ed- Esc) (m) ERd (m) (Erd-Esc) (m) 2 Vfw/[(Eld- Esc) + (Erd-Esc)] Gish River 80 500 200 300 700 500 0.200 Lethi River 10 420 300 120 460 160 0.071 Ramthi River 40 400 200 200 260 60 0.310 Murti River 150 900 700 200 1200 500 0.430 Neora River 450 900 450 450 650 200 0.266 Chel River 200 920 740 180 1000 80 0.290 Mal River 100 850 680 170 950 270 0.450 Lish River 800 850 500 350 1200 700 1.520 integral value which is obtained from the calculation mountain front of eastern India. Braided channels, of hypsometric curve with the help of elevation and mid-channel bars, straight channels are other results area data indicates whether the region is in youth, by inﬂuence of the recent tectonics in this region mature or old stage. Alluvial fans formed by the (Figures 8 and 9). Morphotectonic events over the rivers are signs of tectonic disturbances in the region. ﬂuvial from, process and landform deformation are Thus in this study the hypsometry values of the very alive; there closed integration makes an outstand- alluvial fans formed by the concerned rivers have ing geomorphic footprint and speciality (Figure 10). been calculated instead of the whole basin. Major four alluvial fans like Lish alluvial fan, Gish alluvial 5. Conclusion fan, Chel-Mal alluvial fan and Matiali alluvial fan are showing their maturity by this technique. Lish alluvial The drainage system, pattern, and the ﬂuvial associate fan, Gish alluvial fan, Chel-Mal alluvial fan are indi- morphology of the piedmont zone of the Eastern cated 0.60, 0.64, and 0.69 hypsometric integral values Himalayas are aﬀected by the active tectonics. Rivers (Figure 7) which is youth to mature stage of their Lish, Pabriong, Mal, Murti, and Kurti show recent evolution. tectonic movements because of convex semi- This region is characterised by chain of alluvial fans logarithmic proﬁle. It is clearly indicating early stage formed by the rivers when they debouches from the of adjustment to fault movements. Long proﬁles mountains to the plains and thus line of rivers have which display linear to exponential model imply less formed fan-in-fan topography in the Himalayan concavity in their proﬁles thus are evidences of GEOLOGY, ECOLOGY, AND LANDSCAPES 19 Figure 7. Hypsometric curve and integral of diﬀerent alluvial fans of Eastern Himalayan foothill region (after Ayaz et al., 2018) namely Lish alluvial fan, Gish alluvial fan, Chel-Mal alluvial fan, Metiali fan. Figure 8. Heterogeneous ﬂuvial forms of Eastern Himalayan piedmont zone, India. The ﬂuvial landforms observed like Channel bars, straight channel form, channel avulsion, alluvial fans braided channels are evidences of recent tectonics. Present study not concentred on hydrological components 20 S. AYAZ AND M. K. DHALI Figure 9. Photographs showing ﬁeld evidences of Fluvial form, process and landform deformation of Eastern Himalayan Rivers which is strongly related with recent tectonics: 1. vertical incision of river valley, 2. synformal axis near Neora River and straight channel, 3. vertical drop of Lish River which results in the development of alluvial fan composed of new alluvium, 4. mid- channel bars in lower part of Chel River are common features of North Bengal Rivers. Recent tectonic Morphological behavior of Hydrological activities of fluvial selective rivers of the study components system and subsystems sites variation of the rivers River bed slope, bed Lithology, Geology of the elevation, bed load, Wetted perimeter, hydraulic study area (major faults and suspended load, Mode of radius, Stream power, orientation) sediment collection specific stream power, Shear stress, South Kalijhora Thrust (SKT), Main Boundary Thrust (MBT), Ramgarh Thrust [North Kalijhora Thrust Change of aggradation and (NKT), Main Central Thrust (MCT) degradation behaviour Seasonal variation of fluvial hydraulics and change of fluvial form, Matiali scarp, synformal axis near process and landform Neora River, Chalsa scarp are major Alluvial fan deposition, mid events of recent tectonic activities channel bars, braided channel formation, high meander Straight channel observe, orientation Change of aggradation and of channel, new alluvium, river degradation behaviour, incision , confluence angle change Modification of flow avulsion and channel alluvial fan, mid avulsion of Himalayan channel bars, braided piedmont rivers channel formation, high meander Recent tectonic activities of fluvial form, process and landform deformation of Eastern Himalayan foothill rivers Figure 10. Tree diagram summarising the mechanism that has resulted in the present morphology and morphometry of the region tectonics evidences and their response over the ﬂuvial system and subsystems such as river incision, alluvial fan, straight channel, mid- channel bars, uneven deformation of sediment aggradation and degradation were integrated part of ﬂuvial form, process and landforms in this regions GEOLOGY, ECOLOGY, AND LANDSCAPES 21 disturbances in the course of the rivers and implica- Chen, Y. C., Sung, Q., Chen, C. N., & Jean, J. S. (2006). Variations in tectonic activities of the central and south- tions of recent tectonic movements. Rivers Lish, Gish, western foothills, Taiwan, inferred from river Hack Pabriong, Ramthi, Neora, Mal, Murti, and Kurti exhi- proﬁles. Terrestrial, Atmospheric and Oceanic Sciences, bit exponential model. Similarly, Lish, Pabriong 17, 563–578. Khola, Mal, Ramthi Khola, Lethi Nadi, and Chel are Dhali, K., & Sahana, M. (2017). 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Geology Ecology and Landscapes – Taylor & Francis
Published: Jan 2, 2020
Keywords: Eastern Himalayan Rivers; tectonic evidence; fluvial form; fluvial process; landform deformation
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