200
Chapter-V Litho-structural and Tectonic Control on Drainage and Stream Profiles 201
Chapter V LITHO-STRUCTURAL AND TECTONIC CONTROL ON DRAINAGE AND STREAM PROFILES
5.1. Introduction: The most important finding that has emerged from the analysis of the morphometric and morphotectonic indices as well as the long profile characteristics presented in the previous chapters is that the middle domain of the Kaveri Basin, entrenched between the two prominent knickpoints of Shivasamudram and Hogenakkal Falls, most likely exhibits highest degree of tectonic control as compared to the upper and lower domains as well as the Palar and Ponnaiyar Basins. However, in the realm of tectonic geo morphology, one must also take into account various other identifiable features in the field which can provide additional information on the recent or past tectonic deformation or litho-structural control. These surficial features, often called as geomorphic markers provide a reference frame against which differential or active tectonic deformation is gauged (Burbank and Anderson, 2001). These readily recognizable landforms, surfaces or linear trends usually preserve unique records of various pulses of uplift in an area or region. Some of the most common surficial geomorphic markers which can be used as suitable indicators of tectonic activity and litho- structural control in an area include knickpoints, gorges, linear courses of rivers, outsized channels, barbed drainage patterns, paleo-channels and abandoned drainage courses, sharp bends, fluvial hanging valleys, river terraces, etc (Vaidyanadhan, 1971; Seeber and Gomitz, 1983; Burbank and Anderson, 2001; Kale and Gupta, 2001; Bishop et al., 2005; Ramasamy, 2006a; Wobus, et al., 2006; Crosby et al., 2007; Castillo et al., 2013 and the references therein). Here it is pertinent to mention that the rivers are the most sensitive and dynamic components of a landscape and swiftly respond to any change in the prevailing tectonic, climatic and base level conditions. Although the recovery period of the fluvial system varies, a drainage basin larger in areal extent and spread over diverse lithologies usually takes a long time to achieve a new equilibrium after the cessation of tectonic activity (Burbank and Anderson, 2001) or change in climatic or base level conditions. It is also important to 202
establish the age of these markers in order to ascertain whether these are associated with recent tectonism or are signatures of past phases of uphft. For example, a phase of tectonic upheaval will reflect itself with decrease in width-depth ratio of a river channel and hence the specific stream power is likely to increase. Needless to state this would imply greater erosion by the river and hence the sediment flux will undergo a major change. Similarly, the channel cross-section and consequently the specific stream power and sedimentary budget of the rivers will vary across rivers basins spread over diverse lithologies. Therefore, the fluvial hydraulic parameters such as the unit stream power and sediment budget have often been used as indicators to ascertain whether an area is under active tectonic forcing and/or litho- structural control (Milliman and Meade, 1983; Milliman and Syvitsky, 1992; Harbor, 1998; Burbank and Anderson, 2001; Sklar and Dietrich, 2001; Wohl and Achyuthan, 2002; Finnegan et al., 2005; Goode and Wohl, 2010; Pederson and Tressler, 2012; etc.). Apart from the fluvial evidence of tectonics; there are some geological signatures which have been used by a large number of researchers across the globe to understand the response of a river to lithology and tectonics. Prominent among them include control of lineaments and faults on drainage network (Subrahmanyam et al., 1995; Bhagat and Verma, 2006; Subrahmanyam et al., 2007; etc.), response to gravity anomalies (Braile et al., 1986; Singh et al., 2003; Ramasamy, 2006a); seismic epicentres (Valdiya, 2001; Secanell et al., 2004; Roy, 2006; Sitharam and Anbazhagan, 2007; Tiwari et al., 2009); etc. In this chapter, all the various geomorphic markers and other geological evidence observed in the basins under review are compiled, presented and discussed.
5.2. Methodology: In order to determine whether there is any control of lithology, structure and/or tectonics on the drainage and channel morphology of the rivers under consideration, the lineaments in the Kaveri, Palar and Ponnaiyar River Basins have been mapped from SRTM- DEM and its derivatives. A buffer of 500 m was created around the DEM-derived lineaments and all the streams that were falling in this zone were extracted and their lengths were measured. These lengths were compared with the total lengths of all the streams in the three 203
basins. Then the percentage of lineament control on the streams in the three river basins was obtained. As seismicity is often associated with the prevailing tectonic conditions in an area, records of seismic activity in the last two centuries were mapped from Valdiya (2001), Sitharam and Anbazhagan (2007) and Chopra et al., (2008). This map was overlain on the lineament and fault map of the studied basins in order to ascertain whether any association exists between them. The best geomorphic markers of tectonics are the readily recognizable landforms, surfaces or drainage trends. Under such circumstances, the exercise of identifying and defining the geometry of such markers has often been considered relevant in the realm of tectonic geo morphology (Burbank and Anderson, 2001). These surficial geomorphic markers may be visible in the field as well as on the satellite imageries or maps. Therefore, remote sensing studies as well as field investigations in the Kaveri, Palar and Ponnaiyar Basins were carried out in order to synthesize the information regarding all the geomorphic markers and morpho-tectonic features in addition to the drainage basin and network indices obtained from SRTM-DEM analysis (discussed in the previous chapters). Visual interpretation of satellite imageries and digital elevation models was carried out in order to identify various surficial markers such as knickpoints, gorges, drainage diversions, linear trends of streams, barbed drainage pattern, etc. Detailed maps of the river basins under consideration showing the various drainage anomalies were prepared in ArcGIS. Evidence of the presence or absence of tectonic uplift in an area is also reflected by the relationship between elevation and Bouguer Gravity Anomalies. Therefore, these data were also considered and analyzed. The Bouguer Gravity Anomaly Map was taken from Ramasamy (2006a). An attempt was made to understand the relationship between the gravity anomalies and elevation. Finally an attempt has been made to interpret the results obtained from above studies and provide an explanation for the presence of numerous anomalies in the area under review. Field work was carried out in the Kaveri Basin (Talakaveri to Srirangam) in order to search for depositional records and also to identify major geomorphic anomalies. Sedimentary deposits were located at several places along the Kaveri Channel and some of its tributaries. The older deposits occur at Siddapur, close to the source of Kaveri River, 204
upstream of Shivasamudram Falls in the Kollegal-Narsipura-Talakad area, in the Biligundulu area near Hogenakkal Falls, over the Tamil Nadu Plains and the delta deposits close to the mouth of the Kaveri River. Some of these deposits have been dated by earlier workers from late Holocene to late Pleistocene (Valdiya and Rajagopalan, 2000; Singh and Rajamani, 2001; Kale at al, 2010; Alappat, et al., 2010; Kale et al., 2014). The deposits occurring in the Kollegal-Narsipura-Talakad area (Valdiya and Rajagopalan, 2000) and at Biligundulu are significant from the point of view of tectonic history of the river. Therefore, they are discussed here, particularly the latter deposits, because they occur in the gorge section. Sediment samples were collected from these sites for textural and geochemical analysis as well as for luminescence dating. The Palar River presents an excellent example of an outsized channel on the Tamil Nadu Plains with the channel of the river being too wide as compared to the valley size and the upstream catchment area. In order to ascertain the channel morphological characteristics of the river, cross-sectional surveys were conducted at eleven sites on the river and data regarding channel cross-sections and channel geometry (width and depth) were generated using Electronic Distance Measurer (EDM) or measuring tape (Honnavar, 2008). Regression equation was estimated to evaluate the relationship between bankfull width and upstream drainage area for the Palar River. This was fiirther compared with the relationships of other rivers of the world in different climatic conditions given by Wolman and Gerson (1978). It is generally expected that the areas under active tectonic movement are characterized by higher specific stream powers and enhanced erosion. Therefore, the hydrological parameters of unit stream power, sediment load and the rate of denudation were also taken into consideration for the rivers under review and were compared with other Indian and world rivers. Other types of evidence were also studied to understand and establish the ongoing tectonic activity in the study area. This includes the tidal record and delta plan morphology. In order to ascertain whether the Kaveri Delta is prograding, contours were generated for the delta region from SRTM-DEM data and the contour pattern was interpreted. Tidal records available for Chennai for the last 60 years (Unnikrishnan et al., 2006) were analyzed to understand the long-term trends. The trend was determined by using 11-year moving average method. Finally, all the fluvial, hydrological, surficial and the geological signatures of 205
tectonics are synthesized in order to understand the htho-structural and tectonic control on the drainage network and stream profiles of the Kaveri, Palar and Ponnaiyar Rivers and their major tributaries.
5.3. Control of lineaments/fauIts on drainage network: Lineaments are distinct linear or slightly curvilinear geologic or geomorphic features, such as, faults, joints, dykes, shear zones, river segments, etc. (Selley et al., 2005). Geologic lineaments are considered to reflect subsurface phenomena, lithologic discontinuities, etc. A fault, on the other hand, is a surface or zone along which one side has displaced relative to the other in a direction parallel to the surface or zone (Goudie, 2004). The mapping of these features is generally considered very significant in the realm of tectonic geomorphology as these are commonly associated with various drainage anomalies such as drainage diversions, linear river courses, breaks in the long profile, etc.
5.3.1. Lineament and fault mapping: It is generally expected that the rivers follow the lineaments produced by fi'actures in the underlying rocks caused due to differential uplift and/or warping of the strata. Generally, straight or linear courses of the rivers are interpreted to be the result of the presence of lineaments. Lineament mapping and analysis have been used as an important tool in a large number of studies dealing with tectonics and crustal uplift (Bhagat and Verma, 2006; Ramasamy, 2006a; Subrahmanyam et al., 2007). Traditionally, the identification of lineaments has been based on visual interpretation of topographical contour maps, aerial photographs and satellite imageries. On the topographical maps and/or images/aerial photographs, lineaments are indicated by distinct patterns of topographic features, near straight stream segments, vegetation or soil tonal alignments, etc. The presence of drainage lines and man-made features often introduces a bias in the identification of lineaments because of the tendency to interpret straight stream segments as lineaments. Furthermore, the obscuring effects of vegetal cover or cultural features such as canals may also limit the utility of satellite imageries and aerial photographs. Therefore, many lineaments may not be real and reproducible geologic or geomorphic linear features. These problems could be 206
minimized by using DEM data and their derivatives because these digital elevation models inherently remove all the man-made features in the output image. Therefore, in the present study, lineaments have been identified with the help of SRTM-DEM data and their derivatives viz. slope, aspect, curvature and shaded relief maps (Fig. 5.1 A, B, C). The identified lineaments were then mapped (by on-screen digitization) in ArcGIS to obtain the lineament map of the Kaveri, Palar and Ponnaiyar River Basins. These were then verified by overlaying the mapped lineaments on the LANDSAT and IRS-P6 satellite imageries. The major faults in the Kaveri Basin were mapped after Valdiya (2001).
7 7 '’2 0 'K 77"4()'F . 7 7 '2 0 ’K
12 ° 2 0 'N - •1 2 " 2 0 ’N 12^20'N H ^ I 2 ° 2 0 'N
I2 " N - I 2 ° N I2 "N H
11 ° 4 0 NH • IIWN IIWN' - i r 4 0 ’N
7 7 °2 0 'H 77'’40'H 7 7 ° 2 0 ’E 7 7 “4 0 'K 77”E 77"20'K 77°40’E
12'’20'NH “ I2 ° 2 0 ’N
< ■y.' I2^NH ■ }7'... •12‘=N
'mwrnwrns^' 11 ° 4 0 'N 4 I - 1 I ° 4 0 'N 0 50 km I 77°E 77°2Q'E 7 7 « 4 0 'E
Fig. 5.1. An example to demonstrate how the lineaments were mapped from the SRTM- DEM derivatives, such as shaded relief map (A), slope map (B) and profile cun’ature (C). Lineaments are marked by red lines. The area shown is the Hogenakkal Falls Region in the middle domain o f the Kaveri Basin. 207
It is evident from Fig. 5.2 that the density of lineaments is significantly higher in the middle domain of the Kaveri Basin. This is the expected outcome considering the fact that the geomorphometric indices discussed in the previous chapters have also pointed out that the middle domain of the basin displays the highest degree of structural and tectonic control on relief and drainage. Furthermore, the course of the Kaveri in this domain appears to be fault-controlled. Valdiya (2001) has identified four major NNE-SSW faults that cross the river in this domain viz. the Kollegal-Shivasamudram Fault (KSF), the Mekedatu Fault (MF), the Hogenakkal Fault (HF) and the Mettur Fault (MTF) (Fig. 5.2). The concentration of lineaments in the upper and lower domains of the Kaveri Basin as well as in the Palar and Ponnaiyar Basins is much lower than the middle domain. The Palar Basin has an area of high lineament concentration in a particular zone, especially in the area between the two prominent bends on the Palar and one of its tributaries, the Cheyyar i.e. the Vaniyambadi Bend (VB) and the Changam Bend (CB), respectively (Fig. 5.3).
80"F. J. Legend
— Lineaments
= Kaveri Main Channel
100 km _J
-T 76"E M)”R Fig. 5.2. Map of the Kaveri Basin showing the major lineaments and faults. Lineaments have been mapped from SRTM-DEM whereas the faults are after Valdiya (2001). KSF: Kollegal-Shivasamudram Fault, MF: Mekedatu Fault, HF: Hogenakkal Fault and MTF: Mettur Fault. 208
8()°E _l__
Legend
■ Lineaments
— Trunk Streams
I3°N - -I3°N
I2°nJ -I2 °N
40 km _1
78°E 79°E 80°E Fig. 5.3. Distribution of lineaments across the Palar and Ponnaiyar Basins. VB: Vaniyambadi Bend, CB: Changam Bend.
5.3.2. Control of lineaments and faults on stream courses: The next exercise involved understanding the role played by lineaments on the courses of major streams in this region. The ‘Buffer’ command in ArcGIS was used to understand the degree of lineament/fault control. A buffer zone of 500 m was created around the lineaments and faults in the Kaveri, Palar and Ponnaiyar River Basins and all the streams falling within this zone were extracted and measured for their lengths. These lengths were compared with the total lengths of all the streams in the three basins. Then the percentage of lineament- and fault-control on the streams in the three river basins was obtained. 209
Table 5.1. Control of lineaments and faults on stream flow
SI. No. Name of the Length of all Total length Percentage of river basin buffered streams of all streams lineament control on (km) (km) streams Kaveri 1955 10959 18
Palar 255 2273 11
Ponnaiyar 97 1738
It is evident from Table 5.1 as well as Figs. 5.4 and 5.5 that about 18% of all stream courses in tlie Kaveri Basin appear to be controlled by the lineaments and faults, whereas for the Palar and Ponnaiyar Basins, the corresponding values are 11% and 6%, respectively. In the middle domain of the Kaveri Basin, one can observe a number of streams which follow the lineaments/faults. In fact, the entire stretch of the Kaveri River in this domain between Shivasamudram and Hogenakkal Falls appear to be a fault-controlled reach. The upper and lower domains of the Kaveri Basin as well as the basins of Palar and Ponnaiyar show modest control of lineaments and faults. 76"E 80“E
Fig. 5.4. Kaveri Basin: Control o f lineaments and/or faults on the stream courses. 210
78°E 79“E XO-E
13°N- ■13°N
I2°N- I2°N
78'E 79°E SO'E Fig. 5.5. Control of lineaments on the stream courses in the Palar and Ponnaiyar Basins.
5.4. Earthquake epicenters: One of the most significant manifestations of recent tectonic activity is the occurrence of earthquakes. Normally earthquakes originate from places or zones where the accumulated stress is released in the form of movements and/or ruptures of the crustal segments (Khan, 2009). Numerous workers have related the occurrence of these seismic hazards with the prevailing tectonic conditions (Dewey, 1972; Reddy and Vijaya Rao, 2000). The Indian Shield including the Dharwar Craton has often been regarded as a relatively stable landmass devoid of any tectonic movements (Ramasamy, 2006a). The magnitude and frequency of earthquakes in the stable intra-plate regions is generally low in comparison with the plate boundaries. However, recent surge in the intra-plate earthquakes in the Shield has called for detailed geological and geophysical investigations in this part (Narain and Subrahmanyam, 1986; Valdiya, 2001; Khan, 2009; Ramasamy et al., 2013; Roy and Sethumadhav, 2014). 211
This distribution of seismicity in this region is probably associated with the tectonically- triggered reactivation of fractures and fissures. In order to get an idea about the association of the lineaments and faults with seismicity, the locations of historical and recent earthquake epicentres were plotted on the map of lineaments/faults and lithology (Figs. 5.6 and 5.7). It is clear that some of the recent and historical earthquakes in the middle domain of the Kaveri Basin are associated with the faults especially the Kollegal-Shivasamudram Fault (KSF). The lithological boundaries also appear to be an important factor in the distribution of major earthquakes in the Kaveri Basin because some of the epicentres in the lower domain of the basin are situated in proximity to the gneiss-alluvium and gneiss-charnockite boundaries. The upper domain of the Kaveri Basin as well as the Palar and Ponnaiyar Basins display low density of epicentres and do not reveal any specific trend or association in the distribution of earthquake epicentres. 76”E 80°E
L eg en d
Closepet Granite
m H I Cham(K'kite
Peninsular Gneiss Complex
Dharwar Schist Belt
Recent Alluvium
Lineaments
F au lb i 12"N' •12"N Kaveri Main Channel
10°N
Fig. 5.6. Map of the Kaveri Basin showing the location of historical and recent earthquakes of > 4.5 {solid circles) and < 4.5 (white circles) overlain on the geological map. Swann type of seismic activity (triangles) after Chopra et al, (2(X)8). KSF - Kollegal-Shivasamudram Fault, MF = Mekedatu Fault, HF = Hogenakkal Fault, MTF = Mettur Fault. Faults after Valdiya (2001) and earthquake epicentres after Valdiya (2001) and Sitharam and Anbazhagan (2007). Geological map after Shanna and Rajamani (2000). 212
7 8 ° E SO^E —I__
Legend
Lineam ents Trunk Streams
13°N - •I.VN
12°N' - 1 2 “N
Fig. 5.7. Map of the Polar and Ponnaiyar Basins showing the location of historical and recent earthquakes of > 4.5 (solid circles) and < 4.5 (white circles) overlain on the geological map. Earthquake epicentres after Valdiya (2001) and Sitharam and Anbazhagan (2007). Geological map after Sharma and Rajamani (2000). For inde.x of geological map, refer Fig. 5.6.
5,5. Relation between Gravity Anomalies and Elevation: Several earlier studies have shown an approximate correlation between Bouguer gravity anomalies and elevation (Qureshy, 1963; Subba Rao, 2002). The gravity anomaly of a place is derived via the comparison of the observed value of gravity with a previously calculated one. The calculated value is then corrected for altitude on the basis of an average crustal density (taking account of the varying distance from the centre of the earth - free air gravity anomaly) and finally for the topography to allow for the visible excesses and deficits of matter around the station (Summerfield, 1991). The then difference between the observed and calculated gravity values is called the Bouguer Gravity Anomaly, which is the most widely used measure of gravitational deviations on the earth’s surface (Summerfield, 1991). 213
Bouguer Anomalies are generally negative on land and positive over the oceans (Condie, 2003) due to the differences in thickness and density between the continental and oceanic crust. These are represented by means of isolines that represent gravity in milligals (mgals). Gravity studies have acquired prominence in the last few decades. Numerous geoscientists across the globe have used the gravity anomaly data for diverse fields such as mineral exploration and tectonics (Lyon-Caen and Molnar, 1985; Braile et al., 1986; Singh et al., 2003; Ramasamy, 2006a; Roy, 2006; Malleswari et al., 2012). In India, data on the gravity anomalies have been extensively used to understand and correlate with geology (Sreejith et al., 2011), tectonics (Lyon-Caen and Molnar, 1985; Narain and Subrahmanyam, 1986; Ram Babu and Lakshmi, 2004; Ramasamy, 2006a), nature and density of subcrustal material (Singh et al., 2003; Manglik, 2005; Malleswari et al., 2012) and seismicity (Roy, 2006; Tiwari et al., 2009).
5.5.1. Bouguer Gravity Anomalies Map: Ramasamy (2006a) has provided a map depicting the gravity anomalies for the Peninsular India. Therefore, this map was scanned and gravity contours at 10 mgals were digitized in ArcGIS. Then the Bouguer Gravity Anomaly distribution maps of the Kaveri, Palar and Ponnaiyar River Basins were obtained (Figs. 5.8 and 5.9). The gravity anomalies in the three basins under examination depict, more or less, similar pattern. It can be observed that, by and large, the basins display negative anomalies barring some patches along the coast. This is the expected outcome as the cratons and shield areas are usually associated with negative gravity anomalies (Condie, 2003). The upper reaches of all the rivers are characterized by gravity lows or high negative gravity anomalies and vice versa. In the Kaveri Basin the Bouguer Gravity Anomalies range from -130 mgals in the upper reaches which increases up to 0 mgal near the coast. The Palar and Ponnaiyar River Basins reveal relatively lesser range in the gravity anomaly values, from -80 to -10 mgals. 214
—I78°E__ Legend
Closepei Granite
ChaniiK'kjie
Peninsular Gneiss Complex
Dharuar Schisl Belt
Recent Alluvium
Bouguer Gravity Anomaly (mgals)
Kaveri Main Channel
76°E 78“E 80”E Fig. 5.8. Distribution o f Bouguer Gravity Anomalies (after Ranuisamy, 2006a) of the Kaveri Basin. Gravity contours represented by red lines at an interx’al o f JO mgals. Geological map after Sharma and Rajamani (2000).
Legend
Closep Chamotkite Peninsular Gneiv Quaternary Alluviun B o u g u er G ravity Anomaly (mgals) Trunk Streams 78'^E 79°E SO^E Fig. 5.9. Distribution o f Bouguer Gravity Anomalies o f Palar and Ponnaiyar Basins (after Ramasamy, 2006a). Gravity contours represented by red lines at an inter\'al of 10 mgals. Geological map after Sharma and Rajamani (2000). 215 The maps showing the distribution of gravity anomalies for the river basins under consideration show that the downstream reach of all the rivers is characterized by steep gravity gradient and a ‘gravity high’ is observed near the sea. The presence of local high in the gravity anomalies indicates possible mass excess at depth due to the presence of high- density materials or intrusive bodies (Malleswari, et al., 2012). In the downstream sections of the Kaveri, Palar and Ponnaiyar River Basins, the contours depicting the gravity anomalies are aligned in the NNE-SSW direction along the Eastern Ghat trend (Ram Babu and Lakshmi, 2004). Therefore, the gravity contours were overlain on the geological map of the basins (modified after Sharma and Rajamani, 2000) in order to understand the association between gravity anomaly and geology (Figs. 5.8 and 5.9). Here the three river basins reveal different results. In the Kaveri Basin, the zone of very steep gravity gradient (-90 to 0 mgal) is associated with the tri-junction of lithologies namely charnockite. Peninsular gneiss and Quaternary alluvium. The gravity gradient associated with the Palar and Ponnaiyar appears to be moderate as compared to the Kaveri. The upper reaches of the rivers reveal decreasing trend in the gravity values with a pronounced ‘gravity low’ in the headwaters of the Kaveri River. Here, the gravity anomaly decreases from -90 to -130 mgals in a steep gravity gradient. The Palar and Ponnaiyar Rivers also show a decreasing trend in the gravity values in the upper reaches but the gravity gradient is moderate. It is worthwhile to mention here that the upper reaches of these rivers fall within the Dharwar Crustal Province (DCP), which is predominantly composed of Archaean Rocks (Gunnell, 1998b). From the geological map of the Kaveri Basin it is clear that the well defined gravity low in the upstream section of the rivers is not confined to any one geological formation but is well spread over granites, gneisses, schists and charnockites as well. Thus, it indicates that geological formation is not the only factor responsible for the occurrence of the gravity low over this part. An important observation is that the general trend of the contours in this ‘gravity low’ is NNW-SSE. It is pertinent to mention here that the results obtained for the reach of the Kaveri River in its upper reaches over the DCP is in accordance with those derived for areas underlain by Palaeozoic orogeny which typically exhibit a Bouguer anomaly of -100 to -200 mgals (Condie, 2003). 21 6 5.5.2. Relationship between gravity anomalies and elevation: The next exercise involved estimating the relationship between the gravity anomalies and elevation. The process involved determination of elevations (from SRTM-DEM) of the intersections of the main channels of the Kaveri, Palar and the Ponnaiyar Rivers with the contours of gravity anomalies (Table 5.2 and Fig. 5.10). Table 5.2. Values of Correlation Coefficient of the bivariate relationship between Bouguer Gravity Anomalies and elevation SI. No. Rivers Correlation Coefficient (r) 1 Kaveri -0.69 2 Palar -0.70 3 Ponnaiyar -0.72 All correlations are statistically significant at 0.05 level of confidence It is evident that the three rivers under consideration reveal, more or less, similar patterns of relationship between elevation and gravity anomalies. The strong negative values of the correlation coefficient (r) imply that gravity highs are associated with areas of lower elevation whereas the higher elevation areas typically exhibit gravity lows (Fig. 5.10). 217 • Kaveri • Palar • Ponnaiyar Elevation (m) Fig. 5.10. Bivariate relationship between Bouguer Gravity Anomalies and elevation To understand the relationship between gravity anomalies and absolute relief, gravity zones bounded by 10 mgal isohnes were created. Five zones of gravity anomalies and seven classes of absolute relief were identified using the natural break algorithm in ArcGIS software. Then the grids of absolute relief were extracted in each gravity zone in ArcGIS. It is evident from Fig. 5.1 lA that for the Kaveri Basin, in 0 to -28 mgal zone, there is a decrease in frequency with an increase in elevation and this zone is completely absent in the higher elevation classes. Similarly, the gravity zone of -28 to -56 mgal zone is missing in the higher elevation classes (>1109 m). The gravity lows (-132 to -102 mgals, -102 to -80 mgals, -80 to -56 mgals) exhibit an increasing frequency with increase in elevation. The Palar Basin depicts similar picture for elevation (Fig. 5.1 IB). It can be observed that the highest gravity class of -47 to - 31 mgals is associated with lower elevation and appears to be completely missing in the higher elevation classes. The gravity low zone of -84 to -78 mgals is absent for lower elevation zone and shows a slight increase in the frequency with an increase in 218 elevation. For the remaining zones, the frequency shows a somewhat normal distribution with the mean classes varying for each zone. In the Ponnaiyar Basin (Fig. 5.11C), the gravity lows (-84 to -77 mgals) are associated with higher elevations whereas the lower elevation areas are characterized with gravity lows. Therefore, it is evident that higher elevation areas in the river basins under consideration display strong negative gravity anomalies. () - I 86 1 8f> - 466 466 - 7 I 4 7 1 4 - 868 868 - 1 1 I 109 - I 646 I 646 - 2629 AhNolute Relief (m) 50 ^ 30 20 lO 6 0 O - 2 62 2 6 2 - ‘♦.15 <435 - 6 0 I «S<)1 - ~ 7 * ^A - S>52 V52 - 1 1 77 I 177 - 1 632 AI>Kolute Kelief One interesting observation that can be made from the distribution of gravity anomalies over the river basins under consideration is that the headwaters of all the streams over the Mysore Plateau, a part of the Dharwar Crustal Province, are characterized by strong negative gravity anomalies. The well defined gravity low over the Plateau surface has been regarded to be the result of ancient collision tectonics (Qureshy, 1963). That the uplift along the Mysore Plateau is not geologically very recent is further substantiated by the low topographic relief of the area. An area of local gravity high or gravity maximum seems to be associated with the charnockite terrain of Biligirirangan-Mahadeswaramalai (BR-MM) Ranges in the middle domain of the Kaveri Basin (Fig. 5.8). Similarly, another gravity maximum is observed in the Nilgiri Mountains, which is also composed of charnockite rocks. Therefore, it is evident that the areas with high relief and charnockite terrains are generally associated with gravity maximunL Furthermore, the rock densities of the charnockite terrains do not differ much from the densities of the crystalline gneissic rocks of the Mysore Plateau (Qureshy, 1963). Therefore, it seems logical to negate lithology as a plausible explanation for the gravity variations between the crystalline gneissic terrains of the upper domain and the charnockite rocks of the middle domain. The most likely explanation, therefore, is that the areas of negative anomalies are associated with block uplift or upwarpmg. According to Subba Rao (2002), the Karnataka Plateau (including Mysore Plateau) is the results of domal upwarping and Valdiya (2001) has suggested that the BR-MM and Nilgiri Ranges are horst mountains. 5.6. Drainage Anomalies: Drainage anomalies refer to any striking deviation in the drainage network of an area which cannot be associated with normal fluvial mechanisms of erosion and deposition or normal drainage network development. It may be exhibited in a number of ways such as extremely high drainage densities, linear reaches of a river, elongated basins, misfit valleys, unusually sharp bends, rectangular and barbed drainage patterns, etc. These anomalies, referred to as geomorphic markers by Burbank and Anderson (2001), are often used as indicators of tectonic activity and/or litho-structural control. In this section, all the anomalies in drainage network that were encountered in the course of investigations in the Kaveri, Palar 220 and Ponnaiyar Basins were mapped and described from topographical maps and DEMs (Figs. 5.12 and 5.13). An attempt has also been made to explain their occurrence with the prevailing tectonic conditions in the river basins under consideration. 80‘ E Legend n Knick Points # Major bends ------Straight reaches I2°N- -12"N — ao”E Fig. 5.12. Map of the Kaveri Basin showing the major drainage anomalies. Prominent knickpoints at WI = Chuchunakatte, W2 = Kondagulii, W3 = Shivasamudram, W4 = Shimsha, W5 = Thattai Halla, W6 - Muguru, VV7 = Hogenakkal, WS = Pikkilimalai. Important bends at SP = Siddapur, B = Biddarakere, T = Talakad, CL = Closepet, AT = Attapadi, M = Mettur and KH = Kankanahalli. Major tributaries (marked in italics) H = Hemavathi, K = Kabbini, S - Shimsha, A = Arkavathi, BH = Bhavani and C = Chinnar are also shown. Note that downstream of the confluence of the Arkavathi River, no major tributary meets the Kaveri from the northern side. 5.6.1 Straight Reaches: It is a general tendency of a river to flow in a winding manner with some bends and/or meanders. But sometimes, due to the presence of faults/lineaments and litho- 221 boundaries, the streams follow these zones in a linear course. Therefore, the presence of straight reaches in the streams may often be attributed to structural and/or tectonic control. The lower domain of the Kaveri Basin does not exhibit any striking anomaly in the drainage network (Fig. 5.12). In the upper domain, a few straight reaches of the Kaveri River have been observed especially near Chuchunakatte, where the Hemavathi River joins the Kaveri. It is worthwhile to mention here that the site Chuchunakatte is also a major knickpoint in the course of the Kaveri River (Fig. 5.12). Another noteworthy point is that the Hemavathi River is characterized by a number of linear stretches especially in the headwaters. But the majority of linear reaches are confined to the middle domain of the Kaveri Basin, especially in the 92 km long gorge between the two prominent knickpoints of Shivasamudram and Hogenakkal. In fact, the entire stretch of the river between these two knickpoints is linear. A number of tributaries such as the Chinnar, Dodda Halla, Arkavathi, Shimsha and Nagavathi meet the Kaveri in this section in linear courses. In case of the Palar and Ponnaiyar Basins, such linear segments appear to be very limited. In the Palar Basin, linear reaches are particularly evident in the tributaries of Malattar and Kaundinya Nadi. The Palar River exhibits a remarkable straight course of about 20 km from Kangundi till Vaniyambadi, where it takes a sudden easterly turn (Fig. 5.13). The course of the streams along linear courses is interpreted as an indication of the fault- controlled drainage (Radhakrishna, 1952). 5.6.2. Sharp bends and deflections: Any sudden or abnormal change in the prevailing direction of the course of a stream is referred here as a bend. Almost all the rivers in the world display numerous such bends in their flow paths. Most of these bends are insignificant, suggestive of only normal fluvial mechanism. However, some rivers take sharp bends as high as 90° which demand some attention. These sharp angled bends are often attributed to lithological and structural control on topography or to river capture. The Kaveri River displays numerous sharp bends which are clearly visible in Fig. 5.12. As observed in case of linear stream segments, the lower reach of the Kaveri River is devoid of any sharp bends. Also the tributaries which meet the Kaveri River in this reach are marked by conspicuous absence of high-angled bends. But the 222 situation changes as one moves upstream. From the figure it is clearly observed that the Kaveri River takes a W-shaped bend near Talakad and again it takes a 90° bend near Shivasamudram Falls. At Hogenakkal, the river changes its east-west orientation to a north- south one with a sharp bend. This continues till Mettur where it again reverts back to its original east-west orientation. Furthermore, the tributaries which meet the Kaveri in the middle domain, such as the Arkavathi, Chinnar, and Shimsha also flow through numerous such high-angled bends suggesting anomaly. Another striking feature observed is that many of these bends are associated with knickpoints. Tectonic or structural control cannot be ignored in these cases. In the upper domain of the Kaveri Basin, two prominent bends are seen. One such bend is displayed by the Kaveri near Siddapur, about 50 km downstream from its source at Talakaveri. The other bend is shown by the Hemavathi River near Biddarakere. However, none of the bends observed in this domain are associated with knick points and gorges. 78“E 79° E 80°E ___ I______I— __ I___ 13°N- -I3 °N Elevation (m) 0 - 286 286 - 544 I2“N - KI2°N 544 - 769 769 - 979 979 - 1,632 ------1------1------1------78“E 79°E 80°E Fig. 5.13. DEM of the Palar and Ponnaiyar River Basins showing the major bends (marked by solid circles) and straight reaches (denoted by bold red lines). 1 = The Palar River near Kangundi, 2 and 3 = Straight reaches of the Malattar and Kaundinya Nadi, respectively. VB - Vaniyambadi Bend, CB = Changam Bend. 223 Examination of the satellite imageries and topographical maps reveals that the Palar River encounters a very sharp bend in its course near the town of Vaniyambadi (Fig. 5.13) (henceforth referred to as the Vaniyambadi Bend). The bend is a typical elbow, associated with the phenomenon of river capture (Honnavar, 2008). Furthermore, the drainage areas on either side of the trunk stream are not uniform. It seems as though the right downstream side of the Palar River, below the Vaniyambadi Bend (VB) has lesser area than the left hand side. Similarly, the Cheyyar River, the largest tributary of the Palar River, encounters another such bend near the town of Changam (henceforth referred to as the Changam Bend) (Honnavar, 2008). The upstream area above the Changam Bend (CB) appears to be negligible. Normally, the parent stream, dictated by the regional slope, follows a, more or less, straight path and the tributaries join at an acute angle. Some tributaries may be orthogonal to the parent stream. The size of the tributaries increases downstream. The Palar River is an exception to this. The present upstream reach of the tributary (upstream of the Vaniyambadi Bend) should have been, in fact, a tributary if the drainage network of the Palar River was normal. Instead it is the trunk stream. Drainage to the west of the Vaniyambadi Bend appears to be completely missing. Careful scrutiny of the drainage network map of the Palar and Ponnaiyar Basins reveals that river capture and diversion of tributaries may be the possible reason for the apparent truncated nature of the drainage of the Palar and Cheyyar (Fig. 5.13). It appears that the drainage of Ponnaiyar River is protruding into the drainage area of the Palar River, between these two bends. Therefore, drainage reorganization in this part may be responsible for the capture of tributaries of Palar that used to flow southwest-southeast and join the Palar River. The two bends are in fact elbows, commonly associated with the phenomenon of river capture. This means that the Palar River has gained more area by river capture from the neighbouring Ponnaiyar River. This, in other words, implies that the anomalous network is due to truncation by river capture (Honnavar, 2008). The latter explanation seems likely if we consider other morphological evidence in the vicinity. Apart from the elbow, the occurrence of a distinct wind gap, and the drainage orientations of the tributaries of Ponnaiyar vis-^-vis Palar all support this hypothesis. Another supporting evidence comes from the differences in 224 the elevation of the two rivers. From the analysis of the DEM-derived long profiles (Fig. 5.14), it is clear that the lower reach of the Ponnaiyar River is higher than that of the Palar River. Needless to say this would imply higher elevation of the valley and the tributaries of the Ponnaiyar River. Such a situation would give more geomorphic advantage to the Palar River or its tributaries to cut headward and capture the tributaries of the Ponnaiyar. This seems most likely. Distance from the source (km) Fig. 5.14. The DEM-derived longitudinal profiles of the Palar and the Ponnaiyar Rivers. 5.6.3. River courses across topographic highs: Another major anomaly in the drainage network is that the Kaveri and Bhavani Rivers has their courses across topographic highs or the structural fabric of the terrain (Fig. 5.12). The main course of Kaveri River is across the roughly north-south trending Biligirirangan-Mahadeswaramalai (BR-MM) Ranges. Even in the source area, in the Coorg region, the Kaveri cuts across multiple NW-SE ranges (Radhakrishna, 1992). This suggests that the river is either an antecedent or superimposed river. 225 5.6.4. Asymmetric drainage network: Examination of the drainage network of the Kaveri Basin reveals a pecuHar arrangement in the distribution of tributaries, especially in the lower domain. It is clear that in the lower domain, downstream of Hogenakkal till its confluence with the Bay of Bengal, no major tributary comes from the north (Fig. 5.12). All the tributaries which meet the Kaveri in this lower domain are essentially short with areal extent not more than 2,000 km^. The situation is different in the south where larger tributaries such as the Bhavani, Amaravati and Noyyil meet the Kaveri downstream of Hogenakkal. It appears that the present courses of the tributaries from the north bear reminiscences of drainage reorganization due to river capture. In other words, the tributaries falling in the lower reach of the Kaveri from the north are curtailed at their sources due to river capture. An alternative explanation may be the fact that the tributaries in the lower domain of the Kaveri Basin coming from the north are restricted in their headward erosion due to the presence of the resistant charnockite rocks in the vicinity. Considering the fact that the resistant charnockite terrain is situated very close to the main channel of the Kaveri River from the north, it seems reasonable to infer that lithology might have played a part in the anomalous drainage arrangement of the Kaveri Basin. 5.6.5. The Palar River: Anomalies in the channel morphology: The Palar River is unique in the sense that it not only displays truncated drainage characteristics, but also an outsized channel. In a normal river, the width and depth increases downstream. The Palar River is no exception to this general rule. However, the rate at which the width of the channel increases downstream is extremely unusual and anomalous. The Palar Channel falls under the category of a misfit stream with the channel being too wide (Fig. 5.15) as compared to the valley size and the upstream catchment area. 226 Fig. 5.15. The outsized channel of the Palar near Mamandur, Tamil Nadu. The overall channel width is very large in comparison to the present active channel width. To ascertain whether the channel morphological characteristics of the Palar River are anomalous, it is imperative to compare the results obtained for the Palar with other rivers of the world in different climatic conditions. Channel cross-sections were generated using EDM - Electronic Distance Measurer or measuring tape in the field and the width and depth of the channel at 11 sites were estimated. Regression equation was calculated to evaluate the relationship between bankfull width and upstream drainage area for the Palar River. This was compared with the relationships of different rivers of the world in varying climatic conditions derived by Wolman and Gerson (1978). 227 The empirical relationship between cross-sectional width (w) and upstream drainage area (A) for the Palar River is shown in Fig. 5.16. Also the corresponding relationships obtained for other world rivers in different climatic regimes are shown. The following relationship was derived for the Palar River (Honnavar, 2008) w = 0.0091 A ‘ (r^ = 0.81) Eq. 5.1 For most world rivers the exponent for the increase of width is in the range of 0.1 - 0.5 (Wolman and Gerson, 1978). But the Palar River displays striking anomaly with the exponent being 1.21, an order of magnitude higher. This has been attributed to a major flood in the early 20'^ century (Honnavar, 2008) and does not appear to be related to tectonics or litho-structural control in the area, as similar anomalies are not displayed by other adjoining rivers basins. s i Draina);e Area (km^) Fig. 5.16. Relationship between drainage area (km^) and bankfull width (m) for rivers in different climatic regions (after Honnavar, 2008). w = width of the channel and A = upstream area in 1 = extremely arid regions, 2 = arid & semi-arid areas 3 = subhumid areas 4 = humid regions, Pw = width of the Palar channel, Pa = upstream drainage area for Palar Basin. Data for other world rivers after Wolman and Gerson (1978). 228 5.6.6. Barbed Drainage: An unusual characteristic associated with the Hogenakkal Falls area is the presence of a barbed drainage pattern just upstream of Hogenakkal. Barbed drainage is a peculiar type of drainage system that is usually associated with areas undergoing recent uplift and tilting (Kale and Gupta, 2001). In a normal drainage network, the junction angles of the tributaries are such that they always point downstream. However, in certain situations, the tributaries point upstream. This is called a barbed drainage pattern. Generally, the barbed pattern indicates geological control as manifested by reversal of drainage due to stream piracy or tilting of the surface due to tectonic movements. In the vicinity of the Hogenakkal Falls, a tributary named Bennatti Halla meets the Kaveri in a discordant junction near a village called Biligundulu, 13 km upstream of Hogenakkal Falls (Fig. 5.17B). The exact reason for the barbed nature of this stream could not be ascertained because of the absence of any clear evidence of stream piracy or tilt in the locally exposed rocks. 76^E TS'E e Elevation (m) 77”40'E ] 0 -5 1 2 L . 'jl 5 1 2 - 1.343 1.343-2,629 12"I5- - ! 2 “15'N Fig. 5.17. (A) DEM of the Kaveri Basin showing the location of the Hogenakkal Falls area. The area enclosed in the square is enlarged in (B) as a barbed drainage pattern, BG = Biligundulu, HF = Hogenakkal Falls. The direction of flow of the Kaveri River is denoted by arrows. 2 29 5.7. Lithological and structural control on the longitudinal profiles: It has been shown in Chapter III that although the longitudinal profiles of the major tributaries of the Kaveri, Palar and Ponnaiyar Rivers are broadly concave-upward, linearity, convexity and breaks in the profiles are present. Furthermore, the main channel of the Kaveri River displays two prominent breaks or knickpoints at Shivasamudram and Hogenakkal Falls. One cannot rule out the possibility of lithological control on the changes in channel bed elevation with distance from the source. Headward erosion is likely to get arrested due to the presence of more resistant rocks. Secondly, structural barriers made up of resistant lithologies act as local base levels to denudation (Gunnell, 1998a). In either case, the morphology and shape of the long profile get affected. Profile convexities and knickpoints have often been associated with lithological changes along the stream courses (Hack, 1973; Demoulin, 1998; Korup et al., 2005; Ferraris et al., 2012; Miller et al., 2013; Jacques et al, 2014; etc.). As the changes in the character of stream erosion reflect the interaction between surficial processes and geological substrate (Harbor and Gunnell, 2007), the long profiles of rivers often develop solely as a result of complex interplay between drainage and the underlying lithology. Therefore, it becomes necessary to isolate the effects of lithology by discussing the long profile parameters of the rivers in the light of lithological variations along the stream course (Demoulin, 1998). For river basins underlain by homogeneous lithology, any abrupt change in the longitudinal profile parameters may often indicate tectonic control (Miller et al., 2013; Jacques et al., 2014; etc). 5.7.1. Major knickpoints and gorges: Knickpoints are generally referred to any break in the longitudinal profiles of the rivers. Prominent knickpoints are suggestive of either significant changes in the substrate properties or spatial and temporal changes in the rock uplift rate. They often coincide with thrust zones suggesting present or past tectonic events (Seeber and Gomitz, 1983). One interesting point associated with the knickpoints is that they are not stable and propagate upstream by headward erosion carving out a deep gorge downstream. Therefore, identification and mapping of these features is regarded as an important exercise in geomorphology as their presence may be associated with one or more episodes of uplift in 230 the area or litho-structural control. Fig. 5.18 gives the distribution of the major knickpoints and gorges that are observed in the Kaveri Basin. 76'>E SO'-E ___l _ Legend Closepet Granite Chamockite Peninsular Gneiss Dharwar Sciiist Quaternary Alluvium Knickpoints 12°N- -|2 ° N Gorges Kaveri Main Channel —I-- —I-- —r 76°E 78°E 80°E Fig. 5.18. Distribution of knickpoints and gorges in the Kaveri Basin overlain on the geological map. Prominent knickpoints at W1 = Chucimnakatte, W2 = Kondagulu, W3 = Shivasamudram, W4 = Shimsha, W5 - Muguru, W6 - Thattai Hal la, W7 = Hogenakkal, VVS = Pikkilimalai. Geological map after Sharma and Rajamani (2000). Eight prominent knickpoints are observed in this basin. Three of them are located on the course of the Kaveri River whereas one knickpoint is found each in the case of the Kabbini, Arkavathi, Thattai Halla, Chinnar and the Shimsha Rivers. Apart from the knickpoints at Kondagulu and Chuchunakatte (Fig. 5.19A) on the Kabbini and Kaveri Rivers, respectively, all other knickpoints are situated in the middle domain of the Kaveri Basin. The most prominent of these knickpoints are Shivasamudram (91-m fall) and Hogenakkal (40-m fall), both on the course of the Kaveri River. The 91-m Shivasamudram waterfall (Fig. 5.19B) is situated at the point where the Kaveri River descends from the high elevation 231 Mysore Plateau into the Kaveri Gorge. The 40-m Hogenakkal Falls (5.19D) is located about 90 km downstream of the Shivasamudram Falls. In the vicinity of the Shivasamudram Falls the river crosses the Kollegal-Shivasamudram Fault (Valdiya, 2001) before entering the fault-controlled reach up to the Hogenakkal Falls. In this high-gradient reach, the river is deeply entrenched into bedrock and coincides with course across the block mountains of Biligirirangan and Mahadeswaramalai Ranges. The width of the rocky channel is variable, but is narrowest near Mekedatu (Fig. 5.19C); about 40 km downstream of the Shivasamudram Falls. Here the river displays channel-in-channel characteristics. B Fig. 5.19. Photographs of the waterfalls at A) Chuchunakatte B) Shivasamudram. C) View of the bedrock gorge downstream of Shivasamudram Falls near Mekedatu. D) The picturesque Hogenakkal Falls on the Kaveri River The Shimsha River which meets the Kaveri in the middle domain displays a prominent 76-m knickpoint just before its confluence. Similarly, the Arkavathi River which joins the Kaveri River before the latter enters the gorge at Mekedatu exhibits a 40-m fall at 2 32 Muguru. The Chinnar River is marked by the presence of a break at Pikkilimalai, about 40 km upstream of its confluence with the Kaveri River near the Hogenakkal Falls. All the above-mentioned knickpoints are observed in the middle domain of the Kaveri Basin. The breaks in the longitudinal profiles of these rivers are indicative of the fact that the stream profiles are not in equilibrium along the full length of their courses. 5.7.2. Hanging Valleys: In a normal fluvial system, it is generally expected that the longitudinal profiles of the parent stream and its tributaries will have steep upper segments and gentle lower segments. But sometimes over-steepening of the lower segments of the tributaries may be seen close to their junctions with the trunk stream. Such over-steepened lower segments of the tributary streams are referred to as hanging valleys (Wobus et al., 2006). Hanging tributaries occur where the erosion rates are rapid (such as in tectonically active areas) and in these cases, trunk stream erosion exceeds erosion by the tributaries (Crosby et al., 2007). Several tributaries draining into the Kaveri in the middle domain have developed narrow gorges near their mouths and a waterfall is usually located at the gorge head. Some of the prominent examples are Dodda Halla (30 m fall), Arkavathi (40 m fall), Shimsha (90 m fall), etc. All these streams have developed over-steepened sections in their lower reaches (Fig. 5.20). The presence of such hanging valleys indicates rapid erosion of the Kaveri River in its middle reaches and the inability of these tributaries to keep pace with the rates of incision by the Kaveri main channel. An important observation that can be made from Fig. 5.20 is that the Shimsha and Arkavathi which meet the Kaveri in the course between Shivasamudram Falls and Mekedatu Gorge have their lower steeper segment near the confluence. In comparison, although the Chinnar River drains into the Kaveri near Hogenakkal Falls, the oversteepened reach is situated slightly upstream of the confluence (Fig. 5.20). It appears that the Kaveri Gorge between Shivasamudram Falls and the Hogenakkal Falls is characterized by subtle differences in the incision rates. In the reach between Shivasamudram and Mekedatu, the erosion rates are highly elevated. Downstream of Mekedatu till the lower end of the gorge at Hogenakkal, the erosion rates of the Kaveri are relatively low. 233 c •■s© >a Distance from the source(km) o •ss > Distance from source (km) Distance from source (km) Fig. 5.20. Longitudinal profiles of the Arkavathi, Chinnar and Shimsha Rivers showing over-steepened lower segments or hanging valleys (denoted by hollow ovals). 234 5.7.3. Control of Uthology and structure on stream profiles: The Kaveri, Palar and Ponnaiyar Basins flow across areas of varying lithology and structure. In order to isolate the effect of lithology and tectonics, the long profiles of the Kaveri and Palar Rivers were sub-divided on the basis of the underlying lithology (Figs. 5.21 and 5.22 ). The Ponnaiyar River was not included in the analysis because the long profile of this river does not display any noticeable abnormality. The primary objective of this analysis was to ascertain whether the knickpoints and drainage anomalies in the longitudinal profiles coincide with a specific rock type or at a transition from one lithology to another. From Fig. 5.21 it is observed that in the upper reach the course of the Kaveri River is mostly over charnockites and Peninsular gneisses. A solitary knickpoint at Chuchunakatte is observed in the upper reach. Although apparently the knickpoint does not show any noticeable association with any lithological boundary (Fig. 5.21), the exposure of the schist rocks (called Dharwar Schist Belts) in the course of the Kaveri River about 40 km downstream appears to have some link with this knickpoint. This knickpoint is believed to be lithological in origin where the river cuts across a narrow band of schists (Radhakrishna, 1952). The middle reach of the Kaveri Basin drains across different lithologies. The domain starts with the Shivasamudram Falls at the lithological boundary between Peninsular gneiss and charnockite rocks. Then the river flows over the areas underlain by the charnockite rocks with the exposure of Closepet Granite in the vicinity of Mekedatu, about 40 km downstream of the Shivasamudram Falls. It has been already mentioned earlier that the width of the Kaveri Channel at Mekedatu is the narrowest (Kale et al., 2014). It appears that the presence of these resistant granitic intrusions which are regarded to be a part of the Dharwar Batholith (Sharma, 2009) might be responsible for the remarkable narrowing of the Kaveri Channel at Mekedatu. Downstream of this site, the river flows again over the charnockite rocks before taking a descent of 40 m at Hogenakkal. The lower reach of the river is mostly over late Quaternary alluvial deposits. 235 CHARNOCKITB PENINSULAR GNBISS 1 1 DHARWAR SCHIST 1 1 C L O SB PbT G R A N H H I 1 1 QUATERNARY ALLUVIUM • KNICKPOINTS ▲ SHARPBENDS 200 400 500 600 700 800 900 Distance from the source (km) Fig. 5.21. Association between knickpoints, drainage deflections and lithology in the longitudinal profile of the Kaveri River. Knickpoints at CK = Chuchunakatte, SM = Shivasam udram and H G = Hogenakkal, MK = Mekedatu Gorge. lO O O 400 D tetancc (Icm ) Fig. 5.22. Association between knickpoints, drainage deflections and lithology in the longitudinal profile of the Palar River. VB = Vaniyambadi Bend 236 The Palar River does not display any prominent knickpoint in its longitudinal profile. The solitary bend at Vaniyambadi is located on the Peninsular gneiss complex (Fig. 5.22). As this bend is associated with river capture, the role of lithology appears to be insignificant. 5.8. Distribution of unit stream power: Stream power is defined as the rate of expenditure of potential energy per unit length of the stream (Baker and Costa, 1987). It is directly proportional to the channel slope and river discharge implying that the power of the stream to perform geomorphic work gets enhanced with an increase in channel slope and discharge. Unit stream power or specific stream power is the stream power per unit area of the bed and is generally expressed in Watts/m^. The unit stream power of a cross-section in a river is expressed as (Baker and Costa, 1987): CO = pgQS/w Eq. 5.2 where, p = density of water i.e. 1000 kgW , g = acceleration due to gravity i.e. i.e. 9.8m/s^, Q = discharge in m^/s, S = channel gradient and w = width of the channel in m (Whittaker, et al., 2007). The product of the density of water (p) and the acceleration due to gravity (g) is called the specific weight of water (y). For all practical purposes, the value of y is generally taken to be 9807. So, Eq. 5.2 can be rewritten as (Baker and Costa, 1987; Wohl and Legleiter, 2003): (0 = yQSAV Eq. 5.3 In many earlier studies, the unit stream power is taken as a proxy for variation in the channel incision rate in areas affected by tectonics (Harbor, 1998; Wohl and Ikeda, 1998; Wohl and Legleiter, 2003; Finnegan et al., 2005; Whittaker et al., 2007). Changes in the long term tectonic conditions invariably result in changes in stream gradient and stream energy which leads to adjustments in channel geometry and hence the shear stress and unit stream power gets redistributed across the reach (Whittaker et al, 2007). It is pertinent to mention here that bedrock channel incision rate is proportional to the unit stream power (Snyder et al., 2000). In tectonically quiescent areas underlain by, more or less, uniform lithology it is expected that the stream power generally decreases downstream due to reduction of channel 237 slope and increase in channel width. Localized deposition is associated with areas of declining unit stream power because the ability of the river to perform the geomorphic works of erosion and transportation reduces. Table 5.3. Unit stream power of different CWC sites in the Kaveri Basin ID CWC Site River d Q (0 1 Kudige Kaveri 82.5 2265 270 2 Chuchunakatte Kaveri 143.7 1456 385 3 T. Narsipur Kaveri' 242.0 2325 67 4 Kollegal Kaveri 277.0 6038 67 5 Biligundulu Kaveri 380.5 6688 308 6 Hogenakkal Kaveri 393.5 251 336 7 Sevanur Kaveri 467.0 78 8 8 Urachikottai Kaveri 482.2 5854 437 9 Kodumudi Kaveri 539.8 6585 133 10 Musiri Kaveri 620.1 7690 28 11 Saklespur Hemavathi 35.4 613 69 12 M.H.Halli Hemavathi 97.4 2172 80 13 Muthankera Kabbini 36.0 2050 248 14 K.M.Vadi L.Tirtha 78.7 681 89 15 Bendgahalli Suvarnavati 68.6 37 28 16 T.K.Halli Shimsha 191.7 1219 372 17 T. Bekuppe Arkavathi 123.6 425 239 18 Kudlur Palar 39.9 78 35 19 Thoppur Nagavathi 31.7 61 61 20 Thevur Sarabhanga 74.1 63 52 21 Thengumarahada Bhavani 67.1 653 1886 22 Nellithurai Bhavani 74.9 1348 832 23 Savandapur Bhavani 185.1 1446 169 24 E.B.Mangalam Noyyil 159.1 175.3 30 25 N.M. Patti Amaravati 189.7 5571 445 d = distance from the source (km), Q = discharge unit stream power in W/m‘ The measurement of unit stream power is a difficult task because of the non availability of data on discharge and channel geometry. In India, the Central Water Commission (CWC) has provided the maximum observed discharge (Q) data during the period of 1993-94 to 2009-10 for some sites in the Kaveri Basin. Here it is important to 238 mention that the mean discharge would have been more rehable in the estimation of the unit stream power. But this information Is not available due to the prevailing water disputes between the intra-basin states of Tamil Nadu and Karnataka. In the absence of mean discharge data, the maximum observed discharge has been taken as a proxy to calculate the unit stream power. Data on channel cross section of these sites are made available to the public by the Water Resource Information System (WRIS) portal of the website of the National Remote Sensing Center (NRSC) (http://www. india-wris.nrsc.gov.in). The channel cross sectional data were downloaded from this website and the channel width was determined. Channel slope data were obtained from SRTM-DEM data. Substituting the values of W, Q, S in Equation 5.3, the unit stream power for each site was obtained (Table 5.3). 76°E 78°E 80°E Table 5.3. 2 39 Among the 26 CWC sites, 10 sites are situated on the Kaveri River (Fig. 5.23). In the upper reach, the sites are Kudige (ID: 1), T. Narsipur (ID: 2), Chuchunakatte (ID: 3) and Kollegal (ID: 4). The sites of BiUgundulu (ID: 5) and Hogenakkal (ID: 6) are located in the Kaveri Gorge in the middle reach of the river, whereas the other four sites viz. Sevanur (ID: 7), Urachikottai (ID: 8), Kodumudi (ID: 9) and Musiri (ID: 10) and are located downstream of the Hogenakkal Falls in the lower reach. Other gauging sites are situated on the tributaries of the Kaveri such as the Hemavathi, Arkavathi, Shimsha and Bhavani. The unit stream power of all the sites in the basin shows a wide range - from 8W/m^ at Sevanur site in the lower reach of the Kaveri River to 1886 w W at Thengumarahada, situated in the upper reach of the Bhavani River (Table 5.3). HX) - I E 10 1 lO(XX) 1()(X)(X) KKXKXX) Distance from the source (m) Fig. 5.24. Relationship between the unit stream power and distance from the source for the CWC Sites on the Kaveri River Wohl and Ikeda (1998) have pointed out that the most important factor affecting unit stream power is the channel gradient and that the variations in the unit stream power across different stations on a river are caused by corresponding changes in the channel gradient. 240 Normally, a river reveals a downstream decrease in channel gradient and an increase in channel width resulting in decrease in the unit stream power. Therefore, it is of no surprise that the unit stream power of the Kaveri River, in general, decreases downstream (Fig. 5.24). However, there are significant variations along its course (Fig. 5.25A). 500 E t 400 - 300 ^ 200 100 • 0 ■SS’ B mtf 1 1.000 - = I s. 3 b -g 0.100 i t S 0.010 0.001 <>- Fig. 5.25. Variation o f (A) unit stream power and (B) unit stream power per unit discharge across different CWC sites on the Kaveri River. The highest value of 437 W/m^ was obtained for the Urachikottai Site. Interestingly, this site is located in the lower reach of the Kaveri River just downstream of its confluence with the Bhavani River. It appears that the confluence of the two rivers suddenly increases the discharge at this site. High values are also noted at Chuchunakatte (385 W/m^), Biligundulu (308 W/m^) and Hogenakkal (287 W/m^). It has been mentioned in Section 5.7.1 and Fig. 5.18 that Chuchunakatte and Hogenakkal are two prominent knickpoints on the Kaveri in its upper and middle reach, respectively. Biligundulu is located close to the Hogenakkal Falls. This accounts for higher unit stream power at these sites. At other sites located in the upper and lower reaches of the Kaveri River, the values of unit stream power are relatively low. The Shivasamudram Site does not appear in the list because there is no CWC station at this site. 241 One important observation that could be made from Fig. 5.25 (A) is that the unit stream power gets elevated with an increase in the discharge. It is known that the discharge of a river increases downstream of its confluence with a tributary due to inflow of more water. Therefore, the effect of discharge has been neutralized by calculating the unit stream power per unit discharge across the above sites on the Kaveri River (Fig. 5.25 B). It is evident that the value derived for the Hogenakkal Site (1.14) is higher by at least an order of magnitude than the second-placed site i.e. Chuchunakatte (0.26). Other sites exhibit still lower values of unit stream power per unit discharge, including the Urachikottai Site. Wohl and Achyuthan (2002) have shown that in actively migrating knickzones the values of stream power differ by one to three orders of magnitude than the adjacent reaches. The 40 m Hogenakkal Falls on the Kaveri River is, therefore, an area of knickzone incision. The CWC sites on other tributaries of the Kaveri Basin also display wide variation in the unit stream power. The highest stream power values are concentrated in the headwaters of the Bhavani River (Fig. 5.26). The unit stream power of two sites on this river viz. Thengumarahada and Nellithurai is very high i.e. 1886 and 832 W/m^, respectively. It is pertinent to mention here that Thengumarahada is located adjacent to the Kollegal- Shivasamudram Fault (Fig. 5.26) which has been described to be active by Valdiya (2001). Also, the headwaters of the Bhavani River fall within the Bhavani-Moyar Shear Zone which is a zone of mobility in the stable landmass of the Indian Shield (Shanna, 2009). This might explain steeper channel slopes and higher values of unit stream power at these sites. 242 76°E irE 80"E i ,1 , ■ __U. Legend Unit stream power (W/sq. m) • 8 - 1 6 9 # 170- 832 ^ 833 - 1886 Faulls Kaveri Main Channel 12"N- -I2 ”N ------1------n------r— Fig. 5.26. Mdtp"of the Kaveri Basin showing the uMf stream power of different CWC sitW- and the location of major active faults. KSF = Kollegal-Shivasamudram Fault, MF = Mekedatu Fault, HF = Hogenakkal Fault, MTF - Mettur Fault. Location of faults after Valdiya (2001). The CWC sites have been numbered as per Table 5.3. Another notable observation that could be made from Fig. 5.26 is that in the case of the sites in the middle domain of the Kaveri Basin the values of unit stream power are elevated. Here the sites such as Biligundulu (308 W/m^) and Hogenakkal (336 W/m^) on the Kaveri River as well as T.K. Halli (372 W/m^) on the Shimsha River and T. Bekuppe (239 W/m^) on the Arkavathi River display much higher values of specific stream power as compared to other CWC sites within the basin. Both these tributaries (Shimsha and Arkavathi) meet the Kaveri in the high-gradient middle domain where it crosses a series of faults that have been considered as active (Valdiya, 2001). Moreover, these two sites 243 (T.K.Halli and T. Bekuppe) are located in the downstream reaches of these tributaries. It has been shown in Fig. 5.20 that the lower reaches of these tributaries are quite steep and they meet the Kaveri in its reach between Shivasamudram and Hogenakkal as hanging tributaries. This explains the high values of unit stream power at these sites. Thus, it is quite evident that the middle domain of the Kaveri Basin and the headwaters of the Bhavani River, are characterized by comparatively higher values of unit stream power. In some other studies on unit stream power, the values obtained were highly elevated for regions affected by recent tectonic activity. For example, Whittaker et al. (2007) have obtained values of about 35000 w W in streams passing through a network of active faults in the Central Italian Apennines. In the non-uplifting regions, the corresponding results are in the range of 600 - 1300 w W . In Peninsular India, Wohl and Achyuthan (2002) have worked on an actively migrating knickzone in an ephemeral stream to obtain a unit stream power of about 5000 W W . In case of the Kaveri Basin, the highest value obtained is only 1886 W/m^ which is modest as compared to the values obtained by previous workers for actively uplifting regions. Therefore, it may be inferred that although the middle reach of the Kaveri River and the headwaters of the Bhavani River display greater values of unit stream power than other reaches, the possibility of strong tectonic uplift in this domain is low. The movement along the major faults such as the Kollegal-Shivasamudram Fault, Mekedatu Fault, Hogenakkal Fault and Mettur Fault may not be geologically very recent as postulated by Valdiya (2001). 5.9. Other evidence of litho-stnictural and tectonic control: A fluvial system may respond to one or more pulses of uplift in several ways. Consequently, the response of the rivers to tectonic movements may be reflected in several manners such as high rates of sedimentary fluxes and rates of denudation, preservation of sedimentary archives along the river banks, etc. In this section, all the available evidence of fluvial response to the changes in the prevailing tectonic conditions have been considered in order to ascertain the intensity of fluvial incision going on in the Kaveri Basin. Although it has been well established that the canyon-like valley formed by the Kaveri River in its middle domain is most likely an outcome of a phase of tectonic uplift, attempts to decipher 244 the age of the canyon has not been made till now. Therefore, sedimentary records in the Kaveri Canyon have been investigated and analysed with respect to their chronostratigraphy. Further, it is believed that the delta formed by the Kaveri River is an example of prograding delta (Sambasiva Rao, 1982; Ramasamy et al., 2006) which has witnessed neotectonic uphft along N-S and ENE-WSW trending active faults. So, the contour pattern of this delta area was carefully studied and supplemented by tidal data in order to verify the most prevalent view that neotectonic movements are continuing in this deltaic part of the Kaveri Basin. 5.9.1. Sedimentary Fluxes in rivers: Evidence that could possibly provide some indication of the intensity of the ongoing fluvial erosion in response to tectonic uplift is the annua! sediment flux. The estimates of sediment load and sediment yield of the fluvial systems serve as an indirect source of information for any sort of tectonic activity which may be going on in the catchment (Sinha et al., 2012). Subramaniam (1993) has suggested that the spatial variability in sediment loads among and within the river basins may be due to lithological, structural and geomorphic controls on the rivers. It is expected that the greatest sediment loads are associated with rivers draining the areas affected by recent tectonism. The Himalayan Rivers, for example, provide excellent examples of such rivers (Subramaniam, 1993; Burbank and Anderson, 2001). In contrast, the rivers draining shield or cratonic areas of relative tectonic stability are characterized by low sediment loads (Sinha et al., 2012). Indian rivers, in general, exhibit sharp distinction in sediment supply on the basis of the morphotectonic differences between the Peninsula and the Extra-Peninsula. Whereas the Ganga River System draining the Himalaya Mountains, characterized by uplift-driven tectonism, display a high sediment yield of about 2390 tons/km^; but the Peninsular River Systems, draining the Archaean rocks of the relatively stable part of the Indian Shield show modest sediment yield of only 216 tons/km^ (Panda et al., 2011). In this section, the annual sediment load data of the Kaveri are compared with those obtained for other rivers in the Peninsular India so as to understand the effect of tectonics on the fluvial regime of the rivers. The datasets for the analysis have been derived ft'om the published reports of the Central Water Commission (CWC 2007, 2012). Site-specific information on sediment load is available in the Integrated Hydrological Data 245 Book published by the CWC. Panda et al., (2011) have provided data on sediment load of major rivers of the Peninsular India. The data have been represented in Fig. 5.27. 1000 1 L. e 5 100 - oc ■o 03 EV .i 10 ^ ■s 1/1 Mahi Brahmani Mahanadi Narmada Tapi Godavari Krishna Kaveri River Basins of Peninsular India Fig. 5.27. Annual sediment load of the major rivers of the Indian Peninsula. Source o f data: Panda et al., (2011). From the figure, it is evident that the sediment load of the Kaveri River is extremely low as compared to other rivers of Peninsular India. Milliman and Syvitsky (1992) have shown a strong positive correlation between sediment load and the basin area for a large number of river basins in the world varying in terms of lithology, elevation and chmate. This is because as the area of the river basin increases, the volume of the parent material that will be eroded gets enhanced thereby increasing the sediment load. Therefore, in order to determine whether the variation in the sediment loads of the rivers in Peninsular India is due to subtle to major differences in their catchment area characteristics, scatter plot between the mean annual sediment load and basin area was constructed.(Fig. 5.28). 24 6 Basin Area {10*sq. km) Fig. 5.28. Bivariate relationship between basin area and mean annual sediment load for the rivers in Peninsular India. Data after Panda et ai, (2011). From Fig. 5.28 it is clear that for the rivers in the Indian Peninsula, the sediment load tends to increase with basin area notwithstanding a low explained variance of only 16%. In this plot, the Kaveri can be seen as an outlier, much below the line of best fit. This means that the river is extremely underloaded with sediments with respect to its basin area. Even the rivers such as the Mahi, Brahmani and Tapi, which are smaller in areal extent, have higher sediment load. As multiple medium and large dams are present on all other Indian Peninsular Rivers (Narmada, Tapi, Mahanadi, Godavari and Krishna), the possibility of remarkably low sediment delivery rate of the Kaveri River solely due to sequestration of sediments behind large dams can be ruled out (Kale et al., 2014). 247 It has been pointed out earlier that the rivers draining cratonic and shield areas display lower values of sediment load. The Kaveri River, flowing over the Archaean-Proterozoic rocks of the Indian Shield is no exception. Therefore, it is expected that all the rivers draining the major cratons of the world would be similar in their sediment load characteristics. Milliman and Syvitsky (1992) have provided information on sediment loads of almost all the major rivers in the world. These data have been used to compare the sedimentary flux of the Kaveri River with those of the rivers draining the major cratonic areas such as the African Shield, Siberian Shield and the Canadian Shield. The results are displayed in Fig. 5.29. It is observed that for the rivers flowing over a cratonic surface, there is a weak positive correlation (explained variance of 13%) between sediment load and basin area. In case of cratonic rivers, although the sediment load tends to increase with basin area, the rate of increase is much lower. The fact that all the cratons of the world are composed of hard crystalline rocks of Archaean-Proterozoic Period can be a possible explanation for this. Fig. 5.29 further illustrates that even within the cratonic rivers; the Kaveri is extremely underloaded with sediments as compared to its counterparts. This again points out to the lack of erosion in the areas drained by this river. One possible explanation may be the presence of charnockite rocks over a major part of the basin which are regarded to be highly erosion- resistant (Gunnell, 1998a). Consequently, the sediment delivery rate is remarkably low. 248 1 0 0 1 G t Bc 2 I 10 1 73 B3 9iet • Cratonic rivers of the world O Kaveri 0.01 0.1 10 Basin Area (10‘km^) Fig. 5.29. Relationship between sediment load and basin area for rivers draining the shields and cratons. Data for the cratonic rivers after Milliman and Syvitsky (1992) and for the Kaveri River after Panda et al, (2011). The next exercise involved getting an idea about the downstream changes in the sediment loads along the Kaveri River. The Central Water Commission (CWC) has provided information on the annual sediment load at different sites in the Kaveri Basin. Out of them, six are situated on the Kaveri River viz. Kudige and Kollegal (upstream of Shivasamudram Falls), Biligundulu (in the gorge between Shivasamudram and Hogenakkal Falls) and Kodumudi, Musiri and Urachikottai (downstream of Hogenakkal Falls). The annual sediment load for all these sites are available from 1993- 94 to 2009-10 (CWC 2007, 2012). Average annual sediment load for all these sites was calculated and plotted to see the longitudinal variation (Fig. 5.30). 249 KAVERI GORGE 0,5 n Biligundulu - 0.4 ♦ Musiri Urachikottai 0.2 - Kollegal c 0.1 ♦ Kodumudi Kudige 0 20000 40000 60000 80000 Upstream Catchment Area (km^) Fig. 5.30. Plot of mean annual sediment load and upstream basin area for six gauging stations on the Kaveri River. Mean sediment load based on data from 1993-94 to 2009 10. The names of the gauging stations are also given. Data source: CWC (2007, 2012). The shaded area represents the Kaveri Gorge Section. The dashed line is the linear regression line (r^ = 0.39). It is clear from Fig. 5.30 that the average sediment load at the Biligundulu Site, located within the Kaveri Gorge section is higher than the upper and lower reaches. This is expected considering the fact that the gorge displays characteristics such as high channel gradient, presence of knickpoints, entrenched courses of rivers, hanging valleys, etc. However, considering the numerous evidence of tectonics which is noticed in the Kaveri Gorge, one would expect a manifold increase in the sediment load within the gorge. But this is not the case. In fact there is not even an order of magnitude increase in the sediment load between the upper (Kollegal) and downstream (Kodumudi) gauging stations. Therefore, it is reasonable to assume that although erosion by the Kaveri River in the gorge section is higher than other reaches, the increase is not significant enough to be associated with remarkably high tectonic uplift in the middle domain. 2 5 0 5.9.2. Rates of denudation: The sediment loads transported by a river may be used as a measure of the denudation rates undergone by it. The mean denudation rates for a number of rivers in India and elsewhere have been calculated by employing the following formula (Singh, et al., 2008): Rate of denudation = Total Sediment Load (tonnes) / (Area of watersheds*Specific gravity of the parent material) From Fig. 5.31, it is clear that the rates of denudation for the rivers draining the Himalaya are at least an order of magnitude higher than the rivers of the Peninsular India. It is a well established fact that the Himalaya Mountains have undergone multiple phases of tectonic uplift within the Quaternary (Seeber and Gomitz, 1983, Einsele et al., 1996; Finnegan et al., 2005, Singh et al., 2008 and the references therein). Furthermore, the denudation rate for the cratonic Kaveri River is extremely low (-40 tons km'^ yr '). Another important observation from Fig. 5.31 is that the denudation rates of Kaveri and the Congo River in Central Africa are comparable. Similar to the Kaveri, the Congo is also a cratonic river over the African Shield (Sinha et al., 2012). Therefore, the very modest denudation rate of the Kaveri River indicates very low probability of strong tectonic activity within the basin. 251 1500 RIVERS OF PENINSULAR HIMALAYAN RIVERS OTHER WORLD RIVERS INDIA Ji 1 u E 1000 a a 2 c .2 CQ 500 ■ ■§ ae Fig. 5.31. Denudation rates for different rivers of the world. Data source: Singh et al, (2008). 5.9.3. Fluvial Records in the Kaveri Gorge: The fluvial records provide information about the response of the fluvial systems to many instances of climatic and tectonic changes on different spatio-temporal time scales. The fluvial archives preserved along the banks of a number of rivers in the Indian Shield have attracted a large number of researchers to understand their occurrences with respect to the prevailing climatic and tectonic conditions (Valdiya and Rajagopalan, 2000; Singh and Rajamani, 2001; Kale et al., 2003; etc.). The morphometric and morphotectonic indices as well as the characteristics of the longitudinal profiles reveal that the entrenched course of the Kaveri River between Shivasamudram and Hogenakkal Falls (which has been referred to as the Kaveri Gorge) displays high degree of structural and tectonic control. During the course of field 252 investigations, old alluvial deposits were located near the Biligundulu Village, exposed along the banks of Bennatti Halla. lf>°E 77“E 79°E 80»E Fig. 5.32. Map of the Kaveri Basin showing the location of the Biligunduhi Site (8). C = Chinnar River, H - Hogenakkal Falls, N - Nilgiri Hills, BR-MM = Biligirirangan- Mahadeswaramalai Hills The site has also preserved some slackwater deposits on the banks of the Bennatti Halla (Jaiswal, et al., 2009). The area of the Kaveri catchment up to this site is about 36,700 km^ and the maximum discharge on record at Biligundulu is 7,0(X) mVs recorded in 1991 (Kale et al., 2014). The occurrence of older alluvial deposits at this site demands some attention because this site falls in the middle domain of the Kaveri Basin in the gorge between Shivasamudram and Hogenakkal. The highly calcretized and indurated Biligundulu alluvial deposits (BLU-ALU) (Fig. 5.33 A and C) range in thickness from 3 to >5 m. The top of the deposits occur about 6-7 m above the post monsoon water level of the Kaveri River. This site is also marked by the 253 exposure of cemented gravel and conglomerates close to the bed of the Bennatti Halla Stream (5.33 B). - W S 'y -^ V . Fig. 5.33. (A) Photograph of the older alhivial deposits exposed on the right bank of Bennatti Halla near Biligundulu. The break indicated by hammer represents a sandy unit that has been eroded (B) Close-up view of the cemented fluvial gravel present on the left bank of the Bennatti Halla Stream. Pen for scale and (C) Indurated character of the deposits at Biligundulu. Hammer for scale. 254 In order to ascertain whether the alluvial deposits observed at Biligundulu are locally derived material or are riverine sediments brought by the Kaveri from upper reaches, sediment samples were collected for textural, geochemical and chronological characteristics. 5.9.3. a. Textural and geochemical properties of the alluvial deposits: Grain size analysis was carried out in the Department of Geography, University of Pune using a Micromeritics Sedigraph Particle Size Analyzer. The textural data obtained from the Sedigraph were plotted on a probability graph paper to compute the median grain size and the percentage of clay in the samples. Elemental composition was obtained by X- Ray Fluorescence (XRF) analysis in the Department of Geology, University of Pune. The geochemical data obtained from XRF was used to estimate the Chemical Index of Alteration (CIA) and Ruxton Ratio (RR) of the samples following Nesbitt and Young (1982) and Ruxton (1968), respectively. The results of the alluvial samples were compared with the textural and geochemical data available for fresh and slightly weathered charnockites from the Uttamali Area (located between Hogenakkal and Biligundulu) as well as for the Kaveri flood plain sediments occurring downstream (Sharma and Rajamani, 2001; Singh and Rajamani, 2001). The Biligundulu deposits show noteworthy similarity with the riverine deposits of the Kaveri River occurring over the flood plains (Fig. 5.34 A and B). This implies that the sedimentary deposits preserved at Biligundulu have been transported from upstream because if the deposits were locally-derived weathered/hillslope wash material, they would have shown distinct signatures similar to that of slightly weathered or unweathered charnockite rocks occurring locally (Fig. 5.34 B). 255 6 N0> 3 0 10 20 30 40 50 Percentage of clay Ruxton Ratio (RR) Fig. 5.34. (A) Plot of the percentage of clay and the mean grain size on the phi scale (B) Plot of Ruxton Ratio (SiOz/AhOs) versus chemical index of alteration (CIA). The solid circles represent the ancient alluvial deposits found in Biligundulu whereas the hollow circles and hollow triangles are used to denote the deposits occurring at flood plain and delta, respectively (data after Singh and Rajamani, 2001). The hollow squares represent the samples of Uttamali composed of fresh to slight weathered chamockite rocks (data after Sharma and Rajamani, 2001). Ruxton Ratio was computed after Ruxton (1968) and CIA after Nesbitt and Young (1982). 25 6 5.9.3. b. Optically Simulated Luminescence (OSL) dating: In order to determine the age of the alluvial deposits at Biligundulu, three samples were collected in galvanized iron pipes for optically simulated luminescence (OSL) dating. Two samples (BLU-ALU-2 and BLU-ALU-3) were respectively collected from the top and bottom of a section (Fig. 5.33A) exposed about 700 m upstream of the confluence. The third sample (BLU-ALU-1) was collected very close to the mouth of the Bennatti Halla, -1.4 m from the top of the exposed section. These samples were dated at the Department of Geosciences, National Taiwan University, Taipei, Taiwan (Kale et al., 2014). The estimated OSL ages are given in Table 5.4. Table 5.4. OSL ages of the Biligundulu sediment samples Samples Radioactive element concentration Dose rate De (Gy) Age U (ppm) Th (ppm) K(%) (Gy/ka) (ka) BLU-ALU-1 0.91 +0.01 8.14 + 0.05 1.26 1.83 + 0.2 78.1 + 1.8 43 ±5 BLU-ALU-2 0.42+0.01 4.49 ± 0.02 0.8 1.14 + 0.1 35.1 +0.9 31 ±3 BLU-ALU-3 0.58+0.01 7.59 + 0.04 0.83 1.41 +0.1 53.8 + 1.6 38 ±3 (after Kale et al., 2014) It is clear that the old alluvial sediments observed on the banks of the Bennatti Halla at Biligundulu were deposited about 30,000 to 40,000 years ago. This period coincides with the late Pleistocene Epoch. This appears to be the oldest deposits occurring in the Upper Kaveri Basin in general and the Kaveri Gorge in particular. Valdiya and Rajagopalan (2000) have dated deposits (-26,000 ± 825 to 11,800 ± 825 yr BP) around Talakad by using *'*C and Ramasamy et al. (2006) have some Holocene ages from the lower Kaveri Basin (delta region). The occurrence of the deposits very close to the present channel of the Kaveri River suggests that the river has not incised and lowered its bed level significantly during the last 30-40 ka. This has important implications for the neotectonic activity in the region. If the area is neotectonically active, as suggested by many workers (Valdiya, 1998; Valdiya, 2001; Ramasamy, 2006a) then the Kaveri River would have responded by downcutting. The absence of any striking evidence of noteworthy incision implies that tectonically-driven uplift during the last 30 - 40 ka has been insignificant in this part of the Kaveri Gorge. 257 5.9.5. Delta Plan Morphology: The Kaveri Delta falls in the category of a wave-dominated delta with its apex located east of Tiruchirapally and covering area of about 4000 km^ (Ramasamy et al., 2006). The seaward outline of the delta is unusually straight indicating dominance of waves and tides. This delta is one of very low gradient of 1 in 4,400 (44cm/lkm) (Sambasiva Rao, 1982) The most common view regarding the morphology of the delta is that it has been affected by fluvial and tectonic events in the past (Ramasamy, 2006b; Ramasamy et al., 2006). This delta has been regarded to be prograding unevenly resulting in its asymmetrical shape (Sambasiva Rao, 1982; Ramasamy et al., 2006). The present rate of progradation of the Kaveri River is about 10 km per 1000 years (Sambasiva Rao, 1982). In the current study, the contours of the deltaic part of the Kaveri Basin were generated at an interval of 20 m from SRTM-DEM in ArcGIS. The contour pattern does not display seaward convexity of the contours as expected in case of prograding delta (Fig. 5.35). In fact the contour pattern indicates that the Kaveri channel and its tributaries within the delta region are incising and the incision is proceeding upstream. The lineament map derived from the SRTM-DEM dataset was overlain on the contour map. A number of N-S and ENE-WSW lineaments are observed (Fig. 5.35). Some of the incised streams are following the lineaments. 258 78°30'E 79°0'E 79°30’E 11 °30'N- -n ° 3 0 'N I l'O N - - i r o 'N I0°30'N- I0°30'N 78°30’E 79°0’E 79°30'E Fig. 5.35. Map of the deltaic part of the Kaveri Basin showing the arrangement of contours (brown lines). The contours have been generated from SRTM data at an interval of 20 m. The bold black dotted lines represent the SRTM-DEM derived lineaments. The main conclusion is that there is very little evidence to suggest that the delta is prograding. This could be explained in term of very low sediment load by the Kaveri River, as described in section 5.9.1. 5.9.6. Tidal Gauge records: Another parameter commonly used to infer about tectonic movement in the coastal areas is the eustatic sea-level changes. The temporal changes in the level of sea may be attributed to both climatic and tectonic reasons (Unnikrishnan, et al., 2006). For an area undergoing uplift, the sea level curve is expected to display a falling trend. The reverse is the 259 case for an area undergoing subsidence. Various workers have used the fluctuations in the sea level as an evidence of active tectonics (Watts and Thorne, 1984; Kiden et al., 2002). In India, Unnikrishnan et al., (2006) have analysed the mean tidal gauge data for four stations from 1939 to 2006. The stations include Mumbai, Chennai, Kochi and Visakhapatnam. Among them the tidal gauge data at Chennai is significant in the context of this study because of its proximity to the river basins under consideration. The trend of the tidal data has been estimated by the 11-yr moving average method (Fig. 5.36). It is clear that the tidal records do not exhibit any specific long-term trend. It appears that the sea level at Chennai has witnessed a phase of rise from 1940 to 1965 after which it shows a decline till the 1980s. After remaining stable till 1998, the sea level has again started to increase, which has continued till 2006. More or less, similar trend is displayed by the tidal records at Mumbai located on the west coast of India (Unnikrishnan et al., 2006). Thus, the tidal gauge data at Chennai do not provide any evidence to infer that the lower parts of the Kaveri, Palar and Ponnaiyar River Basins are being affected by recent tectonic activity or change in erosional base level (sea level). 6800 -Sea level Change ■ 11 year Moving Averagi 6750 1939 1945 1951 1957 1963 1969 1975 1981 1987 1993 1999 2005 Years Fig. 5.36. Annual mean sea level variations based on tidal data for Chennai. The black line represents 11-year moving average (Basic data source: Unnikrishnan et al., 2006). 26 0 5.10. Key points from this chapter: The following key and noteworthy points can be listed on the basis of the analysis of various lithological and structural controls on the drainage network and stream profiles of the Kaveri, Palar and Ponnaiyar Rivers and their tributaries; • The Kaveri Basin in its middle domain is densely criss-crossed by lineaments of various lengths. Fault controlled topography is particularly evident in this domain, confined between the Kollegal-Shivasamudram Fault and Hogenakkal Fault. In the upper and lower domains, the lineament density is very low. About 18% of all the streams in the Kaveri Basin, with the majority being in the middle domain, are controlled by lineaments. Lineament control is very modest in the Palar and Ponnaiyar Basins. • Some of the recent and historical earthquakes in the middle domain of the Kaveri Basin are associated with faults, especially the Kollegal-Shivasamudram Fault. This indicates that these faults are active. The geological boundaries, particularly the charnockite-gneiss boundary and gneiss-alluvium boundary also have an association with the earthquakes. Elsewhere earthquakes do not show any specific association with lithology and/or structure. • The Kaveri River in the middle reach between the knickpoints of Shivasamudram and Hogenakkal Falls exhibits a number of drainage and channel anomalies, such as knickpoints, gorges, sharp bends, linear stream segments, hanging valleys, etc. Fault and lineament control on drainage network and channel morphology is particularly well reflected by the presence of these anomalous drainage network and channel characteristics. • The Palar River and its major southern tributary, the Cheyyar exhibit sharp bends or elbows at Vaniyambadi and Changam, respectively. Drainage reorganization due to river capture in this area is indicated. 261 Several tributaries draining into the Kaveri in its middle domain such as Dodda Halla, Arkavathi, Shimsha and Chinnar display narrow gorges and over-steepened courses near their mouths. The presence of such hanging valleys is the consequence of the inability of the tributary valleys to keep pace with the incision rate of the trunk stream. Hanging tributaries are generally reported from tectonically active areas. Two prominent knickpoints are observed in the course of the Kaveri River at Shivasamudram and Hogenakkal. The course of the river between these two knickpoints is extremely steep and the river channel is very narrow, especially near Mekedatu, which displays channel-in-channel characteristics. A noteworthy break of the Kaveri River is also present at Chuchunakatte over the Mysore Plateau ~ 150 km upstream of Shivasamudram. It appears that lithology, to some extent, has also contributed to the development of the knickpoints and gorges on the Kaveri River. For example, the knickpoints at Chuchunakatte and Shivasamudram appear to be lithologically-controlled. Furthermore, at Mekedatu where the width of the Kaveri Gorge is the narrowest, the river has to incise through the resistant Closepet granites. A good negative relationship exists between Bouguer gravity anomalies and elevation. This implies that higher elevation areas typically exhibit gravity lows (i.e. negative gravity anomaly) and vice versa. The gravity patterns in the charnockite terrains of BR-MM and Nilgiri Mountains provide robust evidence of block or domal uplift. The middle domain of the Kaveri Basin appears to be a zone of active incision as revealed by high values of unit stream power on the Kaveri as well as many of its tributaries such as the Arkavathi, Shimsha, etc. The highest stream powers are associated with the headwaters of the Bhavani River. But the values of unit stream power of these sites do not match with those obtained for rivers draining actively uplifting areas, such as in Himalaya, obtained by previous workers. 262 The annual sediment load (as well as the denudation rates) of the Kaveri River is significantly lower than other peninsular Indian rivers as well as many rivers draining cratons and shield surfaces in other parts of the world. Therefore, it is clear that the river is extremely underloaded with sediments. The sediment load also does not increase significantly within the gorge section of the Kaveri River. The Biligundulu Site has preserved highly indurated and calcretized alluvial deposits on the banks of Bennatti Halla, The presence of the deposits very close to the Kaveri channel bed level suggests that the Kaveri River has not incised and lowered its bed significantly during the last 30-40 ka. The contour pattern in the deltaic part of the Kaveri Basin does not suggest that the Kaveri Delta is prograding. The tidal data at Chennai do not provide any evidence to suggest that there is any relative change in sea level or erosional base level due to tectonic activity.