Introduction

The Himalaya displays greatest relief on the Earth with most of the peaks rising 8000 m above the mean sea level. The formation of Himalaya resulted from the collision of Indian continental plate with the Eurasian plate around 65 Ma, and the suturing and welding of both the continents was completed by 55–50 Ma. The northward-moving Indian plate at the rate of 180–195 mm/year during Paleocene slowed down to 45 mm/year at 55 Ma, due to its collision with Eurasian continent. This collision zone was highly active during Paleocene to Middle Eocene (Valdiya 2010).

The rugged terrain characterized by thrusts, fault scarps, steep slopes, gorges, unpaired terraces, V-shaped valleys, and similar geomorphological features along with frequent occurrences of low to medium intensity earthquakes indicate towards tectonic activity being active even in Holocene.

The present study presents the results of the morphological evidences in relation to the relative tectonic activity in the Kosi River watershed extending between 29° 18′ N–79° 02′ E and 29° 51′ N–79° 51′ E in the Kumaun Lesser Himalaya. The Kosi catchment covers an area of 2135 km2 wherein the river flows N-S in the northern and southern parts of the watershed while in the middle part, it follows the E-W trend incising the bedrocks and forming broad valleys, strath as well as unpaired terraces. The Kosi River has produced an actively eroded landscape with bedrock incision evident at several locations. The present morphotectonic analysis using geomorphic parameters covers whole of the Kosi River basin and includes 28 sub-basins on the basis of flow directions and slope (Fig. 1).

Fig. 1
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Inset map of study area (Kosi watershed)

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Geology of the area

The tectono-stratigraphy of the Kumaun Lesser Himalaya has been characteristically a piggy-back type duplex structure (Gansser 1964; Saklani and Bahuguna 1983; Jhonson, 1986; Valdiya 1998, 2001a, b; Kothyari and Pant 2008). Several blocks of metamorphic rocks ranging from medium to high grade are being thrust over younger sedimentary formations in the study area. The thrusts present in the Kosi River basin from north to south are North Almora Thrust in north eastern part of basin, followed by South Almora Thrust (SAT), Ramgarh Thrust (RT), and Main Boundary Thrust (MBT) in the south separating Siwalik Group from Lesser Himalaya. Forming the part of the Lesser Himalaya, the rocks of the area are highly deformed. The sedimentary succession of the Rautgara and Gangolihat formations in the north are overlain by thrust sheet of Almora Group along the North Almora Thrust (Heim and Gansser 1939; Gansser 1964; Powar 1980; Valdiya 1980). The sedimentaries exposed in the north comprise cross-bedded quartzites, slates, and lithic arenites of the Rautgara Formation, overlain by calcareous sequence of Gangolihat Dolomites, predominantly along the Almora Thrust. The Gangolihat Dolomites comprise micrites, dolomitic limestone, and stromatolitic dolomites. The sedimentary succession is overlain by mesograde metamorphic rocks of the Almora Group along the Almora Thrust. The northern flank of the synformally folded Almora crystalline sheet is known as North Almora Thrust (NAT), and the southern limb is called South Almora Thrust (SAT).

Along the southern limb, the rocks of the Ramgarh Group, forming another thrust sheet overlying the Krol Belt, are exposed. The low-grade metamorphic rocks of Ramgarh Group mainly comprise phyllonite, quartzites, mylonite, and quartz porphyries. The Ramgarh Group is thrust over the rocks of the Krol Belt, along Ramgarh Thrust. The Krol Belt in the southern part in turn is thrust over the foreland Siwalik Belt along the Main Boundary Thrust (MBT).

The Krol Belt in the Betalghat Hills comprises Nagthat Formation of the Jaunsar Group at the base and is overlain by Mussoorie Group. The Mussoorie Group is constituted of Blaini—Infra Krol—Krol, and Tal formations in the area to the south and southeast of the area of investigation. The rocks show polyphased deformation and represent a complex tectonic setup. The distribution of different lithounits is shown in the geological map (Fig. 2.) The terrain is crisscrossed by a large number of transverse faults which are seismo-tectonically active (Pant and Paul 2007; Pathak et al. 2013).

Fig. 2
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Geological map of the study area showing major thrusts and faults with stream order of the river. (after K.S. Valdiya 1980)

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Materials and methodology

Morphometry is defined as the quantitative analysis of landscape shape, size, elevation, and slope characteristics (Keller and Pinter 1996). Landscape evolves due to the combined effects of erosional-denudational processes as well as the relative tectonic upliftment or subsidence as a result of overburden removal. The geomorphology of an area is an imprint of the several geological processes active in carving the Earth’s surface, and the quantitative measurement of such features helps to compare different landforms providing an aid in deciphering the deformation and relative tectonic activity involved (Schumm et al. 2002).

For the present study, topographic maps of Survey of India (SOI), Dehradun, at scale of 1:50,000 were procured and used for digitizing various localities in the study area. Digital elevation model (DEM) of the indigenous satellite CARTOSAT-I with 2.5 m resolution was acquired from Bhuvan (ISRO’s GeoPortal) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Version 2 with 15 m resolution, procured from USGS, portal was used in ArcGIS 10.3 and Global Mapper 18 to derive the detailed drainage network and catchment boundaries of the Kosi watershed (Malik et al. 2014). The Terrain Analysis Using Digital Elevation Model (TauDEM) tool of ArcGIS developed by Tarboton and Department of Hydrology, University of Utah (1997), was used in the extraction of drainage network, sub-basins, and hydrologic analysis (Fig. 3).

Fig. 3
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CARTOSAT-I DEM showing elevation range and several thrusts and faults in the Kosi watershed

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The watershed delineation from DEM involved initial step of confirmation of hydrologically connected grids, i.e., every grid cell flowed into the next neighboring downstream cell of lowest elevation using D8 algorithm. The “pits” or holes present in data, representing null values, were filled by assigning average value of the neighboring grid cells. Further, the process was followed by calculation of the direction of flow from each grid cell to its downhill neighbor. This was achieved by examining the eight neighbors of the cell and determining the neighboring cell with steepest downhill descent. The flow directions hence obtained were coded as a value in the new flow direction grid. The flow-direction grid was used to sum the number of uphill cells that flow to any other cell. This summing was done for all cells within the grid to create a “flow-accumulation” grid in which each cell value represents the number of uphill cells flowing to it. Because a cell represents a geographic area (2.5 m × 2.5 m), the flow accumulation grid was multiplied by this area to calculate the total drainage area of the Kosi basin and its various sub-basins. The flow-direction and flow-accumulation grids were then used to delineate watershed and sub-watershed boundaries. These were then selected and converted to a polygon representing the watershed (Tarboton 1997, 1998; Farooq et al. 2015).

A total of 28 sub-basins were identified and demarcated in the Kosi watershed, and several sub-basins with less area than an average threshold were also undertaken due to abrupt changes in the river channel, coincidence with the knickpoints or were present along some previously studied faults as Ratighat-Khairna-Bamsyun fault, Jaurasi fault, Betalghat fault, etc. The following indices were calculated for all the 28 sub-basins of the Kosi watershed based on the formulae listed in Table 1, Fig. 4.

Table 1 Formulae adopted for calculating morphotectonic parameters
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Fig. 4
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Map of the Kosi watershed showing sub-basins with their entities

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Mountain front sinuosity

Mountain front sinuosity (Smf) is defined as the ratio of length of the mountain front at the foot of the mountain where prominent break in the slope occurs to the straight-line length of the mountain front (Bull and Mc Fadden, 1977; Keller and Pinter 1996). Tectonic forces being responsible for uplift in an area tend to produce straight mountain fronts along active faults. Continual erosion of the mountain fronts by rivers wherein tends to carve a sinuous pattern. Thus, in regions of active tectonics and uplift or along any fault, mountain front sinuosity (Smf) values are found to be low. Calculation of mountain front sinuosity (Smf) was done by preparing a slope map from high-resolution CARTOSAT-I DEM and demarcating the mountain fronts along the major river channels.

Asymmetry factor

Asymmetry factor (AF) proposed by Keller and Pinter (1996) is the ratio of area to the right of the basin Ar (facing downstream) to the total area of the basin At normalized by multiplying the ratio with 100. The asymmetry factor is sensitive to tilting perpendicular to the trunk of the stream (Keller and Pinter 1996; Kothyari 2014; Ali et al. 2015). Variation in stream lengths of tributaries on either side of the main channel suggests tilting where AF is > 50 <. Asymmetry factor proves helpful in ascertaining relative tectonic upliftment for larger areas.

Channel sinuosity

Channel sinuosity (S) is defined as the ratio of curvilinear length of the channel to the Euclidean length (straight-line length) of the river valley. Channel sinuosity is a key parameter in understanding the role of active tectonics. Practically, no river follows a straight course unless guided by some kind of deformation or relative tectonic activity. Along active faults where erosion rates are high due to the pulverized rock material, river tends to erode its valley linearly forming strath terraces. Channel sinuosity values tend to be low in areas experiencing relatively high tectonic influence. Based on channel sinuosity index, Mueller 1968) and Morisawa (1985) proposed a classification of channel pattern as straight (< 1.05), sinuous (> 1.05), braided (> 1.3), meandering (> 1.5), and anastomosing (> 2.0).

Hypsometric integral

Strahler (1952) proposed a dimensionless parameter hypsometric integral as a tool for measuring landscape evolution. Hypsometric analysis (or area-altitude analysis) is the study of the distribution of horizontal cross-sectional area of a landmass with respect to elevation. Hypsometric analysis has been used to interpret the relative amount of deformation or degradation due to hydrological and erosional processes that have taken place in a watershed during an appreciable part of geological time period. Hypsometric integral proves beneficial in calculating and comparing various sub-basins in a watershed having different area-elevation values irrespective of scale.

Hypsometric integral and its relationship to degree of dissection allow it to be used as an indicator of a landscape’s stage in cycle of erosion. High HI values indicate that most of the topography is high relative to the mean representing a youthful topographic stage. Low HI values represent more evenly dissected drainage basins, indicating a mature stage of development (Keller and Pinter 1996; Pike and Wilson 1971; Ali et al. 2015).

Basin elongation ratio

Basin elongation ratio is a significant parameter for analysis of basin shape and is defined as the ratio of a circle with the same area as that of the basin to the maximum basin length. Basin elongation ratio suggests more elongated to moderately elongated sub-basins that are less flood prone than the oval sub-basins which are highly prone to flash floods and landslides during cloud burst events (Kothyari 2014; Farooq et al. 2015).

Results and discussions

The statistical summary of the obtained values is presented in Table 2. On the basis of the morphotectonic parameters calculated and interpreted, it may concluded that the Kosi River basin lies in the active tectonic regime, but several sub-basins within watershed along faults and thrusts are relatively more active than others.

Table 2 Morphometric indices calculated for the Kosi River basin depicting comparative analysis of sub-basins susceptive to tectonic activity.
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Mountain front sinuosity-calculated basin wise indicates that the basins along thrusts and faults are tectonically active. Straight mountain fronts with most values ranging between 1.0 and 1.5 adhere to this conclusion. In the Kosi watershed, several sub-basins such as Someshwar (1.09), Karkai (1.07), Takula (1.07), Majhera (1.04), Garampani (1.06), and Betalghat (1.19) that lie along thrusts and faults are found to be active in the present study. Comparative analysis of the mountain front sinuosity values calculated in the Kosi River basin is shown in (Fig. 5). The transect along the Someshwar-Gananath near the NAT is tectonically very active. Likewise, the areas Bamsyun-Bhujan-Khairna located near to RT are also very active. The southernmost tectonic line MBT passing from Mohan-Kotabagh also depicts low mountain front sinuosity values inferring to relatively more tectonically active.

Fig. 5
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Map showing mountain front sinuosity in sub-basins of Kosi watershed

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Asymmetry factor proves helpful in ascertaining relative tectonic upliftment for larger areas. The upliftment is indicated in several sub-basins while others show tilting in response to the tectonic stress along South Almora Thrust (SAT), Ramgarh Thrust (RT), and Main Boundary Thrust (MBT). Asymmetry factor analysis suggests uplift in eastern and southern parts of the basin and southwest tilting along the North Almora Thrust (NAT), South Almora Thrust (SAT), Ramgarh Thrust (RT), Ratighat–Garampani Fault (RGF), and Jaurasi Fault (JF) (Fig.6).

Fig. 6
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Map showing uplift of sub-basins more prominently along thrusts and faults in Kosi watershed

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Channel sinuosity is an indicator of the youthfulness of a river. The channel pattern and course of the Kosi River manifests a strong tectonic control. Based on the sinuosity, the values of sub-basin 23 of Kosi River show mature stage of river channel i.e., meandering pattern probably due to wide deformation zone of MBT and RT. The channel pattern and course of the Kosi River manifests strong tectonic control (Fig. 7).

Fig. 7
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Map showing channel sinuosity index calculated for sub-basins in Kosi watershed. Several tributaries and the main channel follow straight course along faults and thrusts

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Hypsometric integral values obtained sub-basin wise lie mostly in range 0.30 to 0.45 indicating the higher erosional rates, and hence, removal of sedimentary cover may cause isostatic uplifting in the Kosi River basin. Along the NAT, there is moderate to high erosion while the SAT in general shows less erosion. The Ramgarh thrust shows largely more erosion. It is along MBT; the Kosi River shows deflections and the transect shows moderate to high erosion (Fig. 8).

Fig. 8
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Map showing comparative HI values range in various sub-basins of Kosi watershed

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Basin elongation ratio finally calculated suggests more elongated to moderately elongated sub-basins that are less flood prone than the oval sub-basins which are highly prone to flash floods and landslides during cloud burst events. In the Kosi River catchment sub-basin 19 (Bamsyun), 21 (Garampani) were found to be elongated due to local faults, and 23 (Betalghat) is exceptionally longer and covers an area approximately 421 km2 as it lies in between Ramgarh and Main Boundary thrusts (MBT). The ratio range between 0.6 and 1.0 depicts a wide variety of climatic and geologic types (Schumn 1956). Elongation ratio index has been used for classification of varying slopes of watershed into circular (0.9–0.10), oval (0.8–0.9), less elongated (0.7–0.8), elongated (0.5–0.7), and more elongated (< 0.5). Elongated sub-basins in the Kosi watershed are less prone to flash floods due to ease in downstream discharge while oval to moderately elongated basins are quite prone to flooding. From the map shown in Fig. 9, it can be deciphered that sub-basins to the NE of SAT are moderately elongated to oval while in between RT and MBT; due to thrusting and shearing of the rocks, the basins are elongated having broad valleys in Simalkhan, Rataura, and Betalghat area along the Kosi River.

Fig. 9
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Map showing elongated, moderately elongated, and almost oval sub-basins in Kosi watershed

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Five levels of terraces T1-T5 are present in the right bank of the present river channel in association to steep slopes on other bank in Kakrighat area (Fig. 10a). The Nathuakhan formation of the Ramgarh Group is uplifted, and the change in river course is well observed near the Suyalbari area where a paleochannel present, confirms to the shifting of river due to relative tectonic activity (Fig. 10b). Four levels of terraces T1-T4 adjacent to steep gradient on other bank of river channel are present in the Naugaon area, where T1 lies over the Ramgarh Group of rocks being strath terrace that manifests the active incision of the river stream in bed rocks (Fig. 10c). Cross-bedded multi-storeyed fining upward deposits present in the Quaternary profiles suggests the fluvial depositional environment in the past and tilting of the deposits to the NW adheres to the relative upliftment in S-SE direction. The presence of broad valley in the Betalghat area, having five levels of unpaired terraces and the presence of several landslides in a series present on the opposite side facing terraces, may be due to the neotectonic activity along the Ramgarh thrust passing from the area (Fig. 10e). The huge fan with a prominent break in gradient, along with several smaller slope failures and triangular facets in the Simalkhan village, confirms to the active tectonic influence (Fig. 10f). The geomorphological indices calculated and the evidences obtained undeniably indicate the neotectonic ascendancy in the area.

Fig. 10
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a Five levels of terraces in association to steep slopes in the Kakrighat area. b Uplifted paleochannel of the Kosi River near Kakrighat. c Four levels of terrace development near Naugaon village with active incision of river stream in bedrocks. d Tilting in Quatrernary deposits along Khairna-Almora roadcut. e Broad river valley near Betalghat with five levels of terraces and steep slope on the other side in the Ramgarh thrust zone. f Huge fan-shaped landslide deposit with break in gradual slope in Simalkhan village

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