Journal of Indian Geomorphology Volume 8, 2020 l ISSN 2320-0731 Indian Institute of Geomorphologists (IGI) The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics and Ancestry

Vishwas S. Kale Formerly at Department of Geography, Savitribai Phule Pune University, Pune 411007 E-mail: [email protected]

Abstract: The Indian Peninsula dominantly displays an erosional landscape. Bedrock landforms and bedrock rivers are more common. The diversity in topography, drainage and landforms visible today has developed during the nearly 9000 km long northward journey of the since the breakup of the Gondwanaland. Some of the distinctive topographic and drainage network characteristics of this ancient landmass are described in this paper. An attempt has also been made to tentatively reconstruct the ancestry of the main-stem rivers on the basis of available geological and geomorphic data.Multiple lines of evidence suggest that many of the river systems of the Peninsula consist of segments of different ages, and the complete integration of the older and younger drainage has taken place during the Cenozoic. Recently acquired geochronological data strongly suggest that climate rather than tectonics is primarily responsible for the modern-day relief of this ancient landmass.

Introduction With an area of about 2.1 M km2, the from the Gondwanaland and onset of the Indian Peninsula is the largest and the oldest northward drift, the Indian landmass has been geomorphic province and morpho-tectonic chiefly fashioned by fluvial erosion, apart unit of India. Underlain by rocks of Archean from weathering and mass wasting processes. to Cretaceous-Eocene age and bordered by In spite of their modest catchment size (0.2 passive margins to the east and west, the to 3.2 × 105 km2), the major Peninsular fluvially sculpted landscape of the triangular- rivers, such as Godavari, Krishna, Mahanadi, shaped Peninsula bears the legacy of its long Narmada, Tapi, Kaveri, Pennar, Son, Mahi, denudational history that dates back to the Sabarmati, Subarnarekha, and others (Fig. 1), late Mesozoic times (Kale and Vaidyanadhan, have played a crucial role in the evolution of 2014). The spatial diversity in topography, the present prominent topographic features, drainage and landforms displayed by this such as the valleys, ridges, plateaus and ancient landmass is the product of the rifting escarpments. and drifting of the Indian Plate and Deccan In the last two to three decades, the volcanism as well as significant changes in availability of high resolution remote the regional climate and erosional base level sensing data, DEM derived morphometric (sea level) since the breakup of the Gondwana data and geochronological data of rocks, land in the late Jurassic. bedrock landforms and Quaternary deposits Rainfed rivers are the backbone of the have enabled earth scientists to make Peninsular landscape. Since its separation better observations and interpretations

The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 31 Figure 1. Map of the Indian Peninsula showing the major relief features and rivers. The map also shows the location of noteworthy geomorphic features mentioned in the text. about regional denudation rates, landscape Geology and geological history of the evolution and denudation chronology on Indian Peninsula different spatio-temporal scales that were not The Indian landmass attained its possible previously. Some of the noteworthy present outline between 130 and 65 Ma findings about the Indian Peninsula and its ago (Chatterjee and Bajpai, 2016), but rivers that have emerged from recent as well as the basement rocks and major geological some previous studies are briefly summarised structures are much older (Valdiya, 2016). here.The two main objectives of this review Even though the Indian Peninsula formed a are: (a) to outline the salient features of the part of the Gondwana super-continent from Peninsular rivers, and (b) to tentatively Cambrian until its breakup in Mesozoic reconstruct the ancestry of the Peninsular times, it constituted part of three older super- rivers on the basis of available geological, continental configurations, namely the Ur, geomorphic and geochronological data. the Columbia andthe Rodinia (Meert et al., In order to appreciate the distinctive 2010).Geologically, the Indian Peninsula, geomorphic characteristics of the Peninsular described as the Indian Shield, is composed rivers, it is imperative to have a general idea of several Archean cratonic blocks bordered about the litho-structural setup of the Peninsula by rifts and Proterozoic orogenic (fold) belts. as well as the geological and climatic history The Bundelkhand-Aravalli, Singhbhum, spanning the last approximately 200 million Bastar, and Dharwad cratons (Fig. 2) contain years (~200 Ma in the standard notation). In some of the oldest granitic rocks ranging in the following section the geological setup and age from 3 to 4 billion years (Valdiya, 2016). the geological history are briefly described. The cratons include intra-cratonic basins 32 journal of indian geomorphology: volume 8, 2020 Figure 2. The main geological terranes of the Indian Peninsula. The map is based on the Geological Map of India published by the Geological Survey of India (GSI) filled with Proterozoic sediments (areas of Until the end of the Jurassic Period (~145 sandstones/limestones, etc. in Fig. 2). The Ma), Precambrian rocks formed the land cratons are separated by rifts or grabens, such surface over much of the Peninsula. Only as the Son-Narmada-Tapi (SONATA) rift Gondwana basins (Fig. 2) contained Mesozoic zone, the Mahanadi graben, and the Godavari sediments. However, this scenario changed rift (Fig. 2) and in some cases the cratons are during the Cretaceous following voluminous flanked by Proterozoic fold (orogenic) belts, outpouring of lava around 113–118 Ma in such as the Aravalli, the Satpuda, and the eastern India (Rajmahal Traps) and around 65 Eastern Ghat (Fig. 1). The Palghat-Kaveri Ma in western and represented Shear Zone forms the boundary between the by the Deccan Traps (Valdiya, 2016).The Dharwad Craton and the Southern Granulite Deccan lava flows flooded and buried the Terrain (Fig. 2). While the SONATA and the ancient rocks and the Gondwana landscape Palghat-Kaveri Shear Zone are much older over an area of ~1.5M km2. The split between (Precambrian), the roughly W–E and NW–SE India and Seychelles immediately after the oriented grabens of Damodar, Mahanadi and end of Deccan volcanism created the present Godavari attained their final form during the western margin of India. The Western Ghat rifting of the Gondwana landmass in the early Escarpment is the product of the erosion and Permian (Valdiya, 2016). In comparison, the inland retreat of this 65 Ma old continental formation of the Cambay rift is considered edge (Kale, 2010). to be coeval with the split between India During the terminal Cretaceous-Paleogene, and Seychelles, during terminal Cretaceous- when the Indian Plate was drifting northward Palaeogene (Biswas, 1999). as a huge island, partially covered by Deccan The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 33 Figure 3. Major geological, geomorphic and climatic events during the last 200 Ma. Global climatic events are mainly based on Huber et al. (2018), Westerhold et al. (2020) and other sources. PETM = Paleocene-Eocene Thermal Maximum. The temporal changes in the rate of drift of the Indian Plate from 200 Ma to the present estimated by Yoshida and Santosh (2018). Major weathering events (yellow boxes) after Jean et al. (2020).

Traps, and at unusually high velocities of et al., 2012). Recent analyses of high-fidelity ~18 cm yr–1 (Yoshida and Santosh, 2018), the palaeoclimatic records reveal that warmhouse Earth’s climate was extremely warm (Fig. 3) and hothouse states have prevailed during and the eustatic sea level was higher by several Palaeocene and Eocene (~66 to 34 Ma), and tens of meters (Zachos et al., 2001; Gasson hothouse conditions (temperatures > 10°C

34 journal of indian geomorphology: volume 8, 2020 warmer than present) were experienced swift flowing west coast rivers drain into the during early Eocene between 56 to 47 Ma Arabian Sea. (Westerhold et al., 2020). The analyses further indicate that the present coolhouse Structurally controlled drainage state began in the early Oligocene and lasted Rifts or grabens are structural forms that until the terminal Pliocene (~34 to 3.3 Ma). act as major drainage corridors on geological The Quaternary period (the last 2.6 Ma) has time scales. The intra-cratonic grabens, filled been dominated by icehouse conditions (Fig. with sediments of the Gondwana period, 3) with alternating glacial-interglacial periods exist as linear belts and it is along these driven by cyclic changes in the Earth’s elongated depressions that the present river orbital parameters or the Milankovitch valleys of Pranhita-Godavari, Damodar, and cycles. The hydro-climatic characteristics of Mahanadi occur. Similarly, the Narmada, the peninsular rivers were affected by these Tapi, Mahi, and Sabarmati occupy ancient global changes in the climate as well as the structural depressions represented by the latitudinal position of the Indian Plate during Cambay graben and the SONATA rift zone. the drift. These rivers have the thickest (1–5 km) Palaeogene to Quaternary deposits in the Salient features of the peninsular rivers Indian Peninsula. Furthermore, the control of The Indian Peninsula displays an Neoproterozoic Palghat-Kaveri Shear Zone erosional landscape dominated by bedrock on the lower course of Kaveri is also evident. landforms and partially to deeply weathered rocks (Kale, 2014). Bedrock landforms Transverse drainage and bedrock rivers are more common Several Peninsular rivers, such as the Son, than the depositional features and alluvial Godavari, Krishna, Pennar, and Kaveri flow rivers. Many of the landscape features of across hill ranges in their middle or lower the Peninsula have developed their present domains. Some of the most spectacular gorges characters during the nearly 9000 km long are observed in these stretches, such as the northward journey of the Indian Plate since Gandikota gorge on Pennar (Plate 1A), the the break-up of the Gondwana land. Some Srisalium gorge on Krishna, the Papikonda of the distinctive topographic and drainage gorge on Godavari, and the Mekedatu gorge on network characteristics are described below. Kaveri (Fig. 1). River Som, the highest order right-bank tributary of the Mahi river, cuts Asymmetric drainage across the Aravalli ranges. Four mechanisms The drainage network of the Peninsula is have been suggested in the geomorphic strikingly asymmetric. The large rivers all literature to explain the transverse drainage- flow to the east (Fig. 1). Due to the eastward antecedence, superimposition, stream piracy slope of the Deccan Plateau and roughly W–E and overflow (Larson et al., 2017). The first and NW–SE orientation of the grabens, rivers two mechanisms provide likely explanation originating in the Western Ghat, the Maikala for some of the rivers mentioned above. range, and the Chhotanagpur Plateau flow The Godavari, Krishna and Pennar rivers eastward and drain into the Bay of Bengal. In are cutting across the Eastern Ghat, and the comparison, the small-sized west- or south- Kaveri flows through the Biligirirangan Hill west flowing rivers such as the Narmada, ranges, and appear to be antecedent in nature. Tapi, Mahi, and Sabarmati occupy structural The Som river flowing across the Aravalli depressions (rift valleys) and the short and ranges also displays all the characteristics

The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 35 of antecedent rivers. However, geological highly elongated basin shape and has no evidence indicating that the rivers predate the matching tributary. Except Tawa, all other Eastern Ghat or Biligirirangan Hill ranges tributaries of Narmada are very small in size, or Aravalli Hills is lacking. While the ages length and stream order. Son is another river of the rocks forming these hill ranges are that occupies the SONATA zone, but flows in broadly known, the exact geological ages of the SW-NE direction to join the Ganga. The the hill ranges (topographic form) per se are upper Son valley displays the characteristics not known. If stream piracy or river capture of a beheaded or head-truncated valley. via head ward erosion was responsible for the Furthermore, the Deccan Traps rivers, such transverse river courses, presently there is as the Godavari, Krishna, Bhima and most very little sedimentological or morphological of their major tributaries originating in the evidence in support of this hypothesis about Western Ghat, display all the features of river capture. Therefore, proving antecedence under-fit streams and beheaded or head- or superimposition unambiguously remains truncated valleys. These rivers occupy a difficult proposition. Only in the case of unusually large valleys right from their source the Papikonda gorge, there is a satisfactory (Kale and Shejwalkar, 2008). These rivers

Plate 1A. The Gandikota gorge on Pennar river. The Plate 1B. Narmada river bank section showing the coarse- almost horizontally bedded rocks exposed on the gorge walls grained alluvial fan deposits near Tilkwada, downstream of the are quartzites of Meso-proterozoic age. (Photo: V.S. Kale) Dhadgaon gorge. The horizontally bedded units, containing well-rounded pebbles and cobbles, were dated by OSL to about 90 ka (Chamyal and Juyal, 2008). (Photo: V.S. Kale). geological explanation. The lower Godavari have acquired their under-fit characteristics occupies the early Permian rift, and this after the loss in the headwater area due graben cuts across the Eastern Ghat. to beheading. The eastward retreat of the Western Ghat Escarpment was responsible Misfit rivers for the beheading of the Maharashtra Plateau The Narmada, Tapi, Son and Sabarmati as rivers (Kale and Rajaguru, 1988). well as lower Mahi, Mahanadi and Godavari do not flow through self-formed valleys, but River diversions and barbed tributaries occupy geologically formed rift valleys or Drainage network modifications are grabens and thus are misfit. The Narmada represented by drainage anomalies, such as drainage through the SONATA is particularly river diversion and barbed drainage. There atypical. This largest west-flowing river has are many examples of sharp changes in the

36 journal of indian geomorphology: volume 8, 2020 river direction and anomalous drainage some of the rivers (Fig. 2), the two dominant networks in the Peninsula. Rivers such types of geological terranes in the Peninsula Kaveri, Pennar, Mahi, Manjara, Penganga are — the granite-gneissic (including and Son are marked by prominent sharp charnockites and khondalites) terrane and bends or elbows (Fig. 1). Many southern the basaltic (Deccan Traps) terrane (Kale and tributaries of the Tapi, such as Girna, Bori, Vaidyanadhan, 2014). Apart from channel and Panzara also follow anomalous trends. morphology, the control of lithology is also They all flow in the opposite direction for reflected in the textural characteristics of considerable distances and then abruptly the bed sediments. Fluvial sediments in the turn northward and join the main river. granite-gneissic terrane are sand-dominated, Similarly, the drainage networks of the and the fluvial sediments in the Deccan Traps Subarnarekha and Brahmani-Baitarani rivers region generally show preponderance of finer are characterised by barbed tributaries and/or fractions (silt and clay) and gravels. Sand- discordant junctions. Different mechanisms bed rivers, therefore, are more common in of river diversion and barbed drainage have the granite-gneissic terrane, and elsewhere been suggested in the geomorphic literature, the bed is generally covered by coarse gravel. such as river capture, structural control, The rock-dominated river segments are tectonic tilting, etc. (Bishop, 1995). The often characterised by irregular channel barbed drainage patterns displayed by some beds, breaks in channel profiles (rapids of the west coast rivers cutting across the and waterfalls), inner channels, straths and spine of the Western Ghat, such as the Kali, bedrock gorges. Although single channel Gangavali, Aghnashini, Mandvi, Ulhas, etc. pattern is the most common channel planform are without doubt related to river capture and in bedrock rivers, multiple channel reaches subsequent drainage diversion. The are also present. The Narmada river near on Sharavati river, the highest waterfall in Punasa provides the best example of a multi- India, is also related to river capture and river channel pattern in bedrock (Kale et al., 1996). diversion. However, in all other cases, there Whereas some of the sudden changes of is inadequate geomorphic, geological and gradients or breaks in the river profiles (knick geochronological data to confidently infer points) have formed in response to more the exact process or processes involved and resistant lithology, others are manifestations the probable temporal sequence of the events. of disequilibrium conditions induced by changes in erosional base level (due to rock Bedrock-dominated rivers uplift or fall in sea level). However, since Although many Peninsular rivers have all the knick points migrate upstream, it is alluvial reaches, there are long segments often difficult to pinpoint the exact reason(s) where bedrock is exposed along the channel for their initiation (Kale and Shejwalkar, perimeter. The channel gradient of bedrock 2008). The presence of incised bedrock rivers is a semi-independent variable and channels or gorges downstream of the knick is not directly determined by the hydraulic is the common evidence of the headward regime (Howard, 1980) but by the sub- migration of the knick points. For example, strata characteristics. There are significant the Marble Rocks canyon downstream of the differences in the channel morphology on Dhuandhar Falls (15 m) on Narmada is about account of differences in the underlying 3 km long, the Kaveri gorge downstream of rocks. Although sandstones, quartzites and Shivasamudram Falls (101 m) is almost 100 limestones are present in the catchments of km long, and the Pravara gorge downstream

The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 37 of Randha Fall (50 m) on the Pravara river Mysore Plateau) and the middle course (a tributary of Godavari) is nearly 20 km (through Biligirirangan-Mahadeswaramalai long. In addition, there are several examples hill ranges) are anomalous (Kale et al., 2014). of gorges and retreating waterfalls at the gorge-head along the northern edge of the Incised rivers Rewa Plateau in Madhya Pradesh (Kale and Most of the large rivers and their Vaidyanadhan, 2014). tributaries are partially or deeply incised in bedrock or late Quaternary alluvium and are Low-gradient rivers therefore, devoid of significant flood plains. The wide and shallow valleys of the east- The incision and excavation in the alluvial flowing rivers are occupied by low-gradient reaches was induced by stronger monsoon rivers and contrast very strikingly with the and higher river discharges in the early steep and swift-flowing west coast rivers. Holocene (Kale et al., 2003 and references Even the longitudinal valley-floor gradients therein). However, incision in bedrock is of the large Peninsular rivers are remarkably the result of slow and steady lowering of the low. River gradients provide a good idea river channels in response to long-term fall about the degree of disequilibrium conditions in the base level. Although direct geological in the drainage systems. The Hack’s (1973) evidence is lacking, it is reasonable to infer stream-gradient (SL) index has been widely that with the onset of coolhouse conditions used to characterise a stretch of the river about 34 Ma (Fig. 3), the eustatic sea level or the entire longitudinal river profile. The has been steadily declining, and all the rivers average SL index computed from source draining into the adjoining seas have adjusted to mouth can be used to compare rivers of to this decline in the erosional base level since different sizes and length (Brookfield, 1998). Oligocene times in general, and during rapid In general, the average SL index should changes associated with Quaternary glacial decrease with an increase in the catchment cycles in particular. area. The plot given in Fig. 4 indicates that the average SL index for the Peninsular Seasonal hydrologic regime rivers is less than 120. In comparison, the Monsoon is the defining characteristic of average SL index values for the Himalayan the tropical climate over the Indian Peninsula, rivers (Indus, Ganga, Brahmaputra and all at least during the last 8–10 Ma (Kale and their major tributaries) are several times Vaidyanadhan, 2014). The Indian monsoon higher and range between 600 and 700 precipitation regime is dominated by high to (Brookfield, 1998). As the stream-gradient extreme rainfall seasonality (Nandargi et al., index is considered as a proxy for stream 2017). All the rivers of the Indian Peninsula power, it is reasonable to infer that the rivers are monsoon-fed and, therefore, the discharge of this ancient landmass are modest in terms regimes are characterised by sharp peaks and of power and competence. Fig. 4 shows that seasonality. The seasonality index (R), which for the given drainage basin area, the average is the ratio between maximum and minimum SL values for Narmada and Kaveri are much average monthly discharges (Thomas and higher (>100) than expected due to higher Kale, 2011), is significantly higher for the elevation in the source areas (Amarkantak Peninsular rivers than the Himalayan rivers and Talkaveri, respectively). Further, while (Fig. 5). The seasonality index is particularly the Narmada occupies the SONATA rift zone, elevated for Peninsular rivers with smaller in case of Kaveri the upper course (over the drainage basin areas and lower mean monthly

38 journal of indian geomorphology: volume 8, 2020 Figure 4. Plot of average stream gradient Index versus the Figure 5. Plot of seasonality index (R) against the mean drainage basin area for major Peninsular rivers. The trend monthly discharge for the major Peninsular (hollow circles) line shows that the SL index decreases with an increase in and Himalayan rivers (filled circles). Basic data from Central the catchment area. Water Commission (CWC) and other sources. discharges (Fig. 5). This in other words means Peninsula has been subjected to very slow that the rivers are morphologically active denudation since the Mesozoic times. Even only for a short duration during the water in the high rainfall zone of the Western Ghat, year. Most of the geomorphic work of erosion the erosion rates since the Neogene have been and transportation is performed during large- exceptionally low, below 8 m Ma–1 (Jean et magnitude floods that occur from time to al., 2020). The results further indicate that the time (Kale, 2003). Nilgiri plateau (~2600 m ASL), the highest plateau in the Peninsula, has remained Subdued regional denudation rates largely unaffected by erosion during the It is evident from the analyses of the last 200 Ma (Mandal et al., 2015b). Studies sediment load data available for a number of from other parts of the world have shown gauging sites, that the Peninsula is subjected that denudation rates are remarkably high to very slow denudation (Fig. 6). Although the in the tectonically active terrains (Himalaya Godavari river transports the highest sediment and Andes, for example) and over the stable load in the Peninsula, its annual sediment load shields and cratonic domains, the rates are is several times less than the annual sediment generally modest (Thomas and Kale, 2011). load of the conjoined Ganga-Brahmaputra The main implication of these recent studies (> 500 x 106 tons yr–1; Rahman et al., 2018). is that most of the major topographic features Rivers draining the Dharwad craton, namely of the landscape observed today have been the Kaveri and the Pennar have some of the inherited from the geological past and have lowest erosion rates (Gupta et al., 2012). In not been significantly modified during the recent years, the regional erosion rates on recent geological epochs. the longer geological time scales have been estimated on the basis of apatite fission-track, Quaternary alluvial formations apatite (U-Th) / He thermochronology, 10Be Quaternary deposits in the valleys of cosmogenic radionuclide measurements and the Peninsular rivers are noticeably limited 40Ar/39Ar dating (Gunnell et al., 2007; Sahu in areal extent and thickness. The only et al., 2013; Mandal et al., 2015a; Mandal et exceptions are the structurally-controlled al., 2015b; Jean et al., 2020). These studies river reaches of the Narmada, Tapi-Purna, indicate that, by and large, the southern Mahi, Sabarmati and Son,and the deltaic The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 39 plains on the east coast. There are indications Ponton et al., 2012; supplementary material). that even the bedrock gorges of Narmada may The transition from Iron Age to the early have been completely filled and buried by the Historic period occurred around 2600 years Pleistocene deposits (Gupta et al., 2007). In before present (Mishra, 2001). the Deccan Traps terrane, thick alluvial-fill Numerous types of human interventions deposits are exposed in the upper reaches have affected the Peninsular river systems of the Godavari and Krishna in general since the introduction of agriculture during and downstream of the bedrock gorges in the Chalcolithic period. The increase in particular. Elsewhere the riverine deposits soil erosion during the late Holocene are shallow or absent. From the existing (Meghalayan Stage) has been attributed to radiometric ages (14C, OSL, U/Th) as well increased agricultural activity in Deccan as fossil fauna, stone age tools (Acheulian (Giosan et al., 2017). In the last few decades, to Mesolithic) and the ~75,000 years old the natural functioning of almost all the Toba volcanic ash (tephra) bed occurring streams and rivers in the sub-continent has within these deposits, it appears that alluvial been seriously affected by human activities, sediments were deposited mainly during such as changes in the land use/ land weaker monsoon phases (glacial periods) and cover within the catchments, land surface incision was induced by stronger monsoon modifications, construction of dams and (inter-glacial periods) during the mid to late weirs, flow diversion, sand mining, ground Quaternary Period (Kale and Rajaguru, 1987; water extraction, etc. Studies based on Rajaguru et al., 1993; Williams et al., 2006; satellite data and historical archives reveal Chamyal and Juyal, 2008 and references a significant loss of forests and conversion therein). of forested land into cropland area during 1880–2010 in the Indian sub-continent (Tian Anthropogenic impact et al., 2014). Some of the key consequences Human presence in the India Peninsula of human impact include flow modifications, encompasses a span of at least 1.5 million years. decline in the sediment fluxes, bank erosion, The Peninsula is rich in prehistoric (Stone siltation, pollution and coastal erosion. River Age, Chalcolithic and Iron Age) and historic gauge data indicate an increase in the inter- sites (Mishra, 2001). Radiometric dates of an and intra-seasonal variability in the river early stone age or Palaeolithic (Acheulian) discharges of most of the large Peninsular site from Tamil Nadu indicates that the rivers due to river flow regulation and water hominins had occupied the southern part of diversion. Due to decline in the groundwater the Peninsula during the early Pleistocene, level and base flow, many streams and rivers roughly between 1.7 and 1.1 Ma (Pappu et are drier now for a longer duration during the al., 2011) and the Middle Palaeolithic culture non-monsoon season. Many middle and lower occurred between 0.38 and 0.17 Ma (Kumar order seasonal streams have acquired the et al., 2018). Archaeological evidence further regime characteristics of ephemeral streams. suggests that while the early to mid-Holocene The sediment fluxes also reveal a remarkable was dominated by Mesolithic culture, the decline in the last few decades in general agriculture based Deccan Chalcolithic and the current period in particular (Gupta et cultures in central and and the al., 2012). Analyses of the annual sediment Deccan Neolithic culture in southern India load data show consistently lower sediment flourished approximately between 4500 and quantities downstream of the large dams 2800 years before present (Mishra, 2001; (Nageswara Rao et al., 2010). The results

40 journal of indian geomorphology: volume 8, 2020 Figure 6. The long-term average annual flow and the annual sediment load of the major Peninsular rivers. Basic data from Gupta et al. (2012) further show pronounced coastal erosion in that the Peninsular landscape is remarkably the past four decades, particularly along the stable and tectonically inactive (Mandal et deltas on the east coast, due to trapping of al., 2015a). In addition, old apatite (U-Th- sediments by large dams (Nageswara Rao et Sm) / He ages of the rocks exposed on the al., 2010). divides, high plateaus (Nilgiri Hills) and valleys in southern India suggest remarkably Role of tectonics and climate in the low exhumation rates during the last 200 evolution of the Peninsular landscape Ma (Mandal et al., 2015b). As expected, the The role of tectonic activity is commonly erosion rates are comparatively higher along invoked by geoscientists to explain high the Western Ghat Escarpment and along relief topography and drainage network active river channels (Mandal et al., 2015a anomalies, even in the case of stable, ancient and b). The 40Ar/39Ar geochronology of shields bordered by passive continental laterites, occurring over the Mysore Plateau margins, such as the Indian Shield (Roy, and the coastal lowland (Karwar/Goa), lends 2006; Valdiya, 2016 and references therein). a strong support to the hypothesis of tectonic Richard et al. (2016), primarily based on stability of the Peninsular landscape during the analysis of river profiles extracted from the Cenozoic (Jean et al., 2020). The ages DEM and geophysical data, concluded of the laterites indicate very low denudation that Neogene rock uplift is responsible for rates over the Mysore Plateau and the coastal the modern-day relief of the Peninsula. As lowland, and unexpected locational stability the highest elevations in the Peninsula are of the Western Ghat Escarpment since the observed in the Nilgiri-Anaimalai-Palniarea mid-Eocene (~47 Ma) times (Beauvais et al., (> 2600 m ASL), they concluded that the 2016; Jean et al., 2020). Neogene (23.03 to 2.58 Ma) uplift initiated Here it is pertinent to mention that, in spite in the south and propagated northward along of the evidence of regional tectonic stability the western margin (Richard et al., 2016). of the landscape, there are indications that However, millennial-scale erosion rates some areas within the Peninsula may have determined from cosmogenic 10Be indicate experienced domal uplift or block tectonics

The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 41 during the post-Gondwana-breakup (Roy, fan (axial length ~20 km) at the exit point 2006). Some known evidence are presented of the Dhadgaon gorge. The unusually thick below: exposed deposits (>20 m), characterised by The Pachmadhi sandstone beds, consisting rounded to sub-rounded pebbles and cobbles, of fluvial sediments deposited by an ancient were dated to about 90ka (Chamyal and Juyal, river system in the Gondwana rift basin in 2008, and references therein). The clast size the Triassic, are today occurring >1 km ASL decreases away from the fan apex. The nature over the Pachmadhi plateau (1352 m ASL of deposits clearly suggests intense bedrock at Dhupgarh). The Pachmadhi plateau is the erosion in the Dhadgaon bedrock gorge, highest plateau in the Satpuda range. It is a most likely induced by rock uplift related to very well established fact that a river channel, the movements along the Son-Narmada or without exception, occurs at the lowest Tapi Faults (Copley et al., 2014). Similarly, elevation in all types of topographic settings downstream of the Pravara gorge in the upper (mountains, plateaus, plains). Therefore, the Godavari basin, and in the lower reaches of occurrence of river deposited sediments at the Par river in north Konkan, thick alluvial the highest elevation indicates uplift and the deposits are present. These unusually thick role of tectonics. The present elevation of the Quaternary fluvial deposits indicate an fluvial beds has been attributed to significant episode of intense valley erosion and incision post-Deccan Traps uplift in the area (Sheth, in bedrock, most likely induced by tectonic 2007). uplift. Only a fall in the base level of erosion Lignite beds of Mio-Pliocene age, known rather than climate change can account for as Ratnagiri Beds, occur up to 8–10 km inland such intense erosion of the bedrock. The and up to 150 m above the present sea level presence of incised bedrock meanders and in south Konkan and Goa. These clay beds, prominent knickpoints upstream as well as in often overlain by thick laterites, contain plant the adjoining basins (Mandvi, Dhamanganga, fossils and indicate deposits of nearshore to etc.) lends a strong support to the hypothesis estuarine environments (Badve et al., 2002). of block or domal uplift in this part of the Only two mechanisms can explain the present Deccan Traps region (Kale and Shejwalkar, elevation of these marine/estuarine deposits 2008). — higher sea level or tectonic uplift along Typical alluvial fans at the mountain the coast. Although tectonic uplift is likely, front, although common along the Himalayan other types of supporting evidence is yet to foothill zone, are rare features in the Indian be identified and presented. shield, perhaps because of the tectonic U/Th and 14C dates of marine terraces, stability of the landscape. However, a ~10 km beach rocks, marine shells and corals provide wide and ~7 km long, symmetric alluvial fan indications of higher sea levels or possible has been developed by a lower order tributary tectonic uplift along the southern tip of India of the Sagileru river in the Pennar basin (Kale, (Kanyakumari to Rameswaram) and along the 2009). In a rocky terrain and an erosional Saurashtra coast during the late Quaternary landscape, such large depositional features (Banerjee, 2000; Sharma et al., 2017). are anomalous. Therefore, this and other Thick river deposits occur immediately smaller fans developed along the east-facing downstream of some long bedrock gorges, front of the Nallamalai hills (coinciding with for example, downstream of the Dhadgaon the Cuddapah Boundary Thrust) as well as gorge on Narmada. These coarse gravel the deep incision of the Krishna river within deposits of Narmada (Plate 1B) form a large the Nallamalai hills further north (Fig. 1),

42 journal of indian geomorphology: volume 8, 2020 possibly indicate the control of tectonics in Peninsula valleys were largely acquired this part of the Cuddapah basin. Needless during this very warm and moist phase. to state, the role of lithology (erodibility of A second phase of intense weathering the rocks) and structure cannot be ruled out and lateritisation/bauxitisation over the completely. Indian Peninsula has been identified by Alluvial fans of modest size are also Jean et al. (2020) during the Miocene (Fig. present along the southern slopes of the 3). This phase closely follows the globally Satpuda ranges in the Tapi-Purna basin. identified warmer period from ~17 to 14 Ma, Vertical offset in the fan surfaces indicates known as the Miocene Climatic Optimum movement along the Tapi Fault during the (Westerhold et al., 2020). It is likely that Holocene (Copley et al., 2014). This evidence the laterites overlying the Ratnagiri lignite plus the historical records of seismicity beds in Konkan and Goa, formed during this (Allah Bund-1819, Coimbatore-1900, warmer phase or during the Mid-Pliocene Satpuda-1938, Anjar-1956, Koyna-1967, Warm Period (~3.29–2.97 Ma). Multiple Broach-1970, Killari-1993, Jabalpur-1997, lines of evidence suggest that the present Bhuj-2001, etc.) along or close to the major monsoon climate was firmly established over lineaments/fractures suggest deformation and the Indian sub-continent during the Miocene, reactivation of pre-existing structural features about 8–10 Ma ago (Kale, 2014). The onset in the Indian Shield (Roy, 2006). of monsoon climate might have further Another factor that has played an enhanced the rates of weathering and fluvial important role in the evolution of Peninsular erosion, especially in the Western Ghat zone landscape during the Cenozoic is the (Jean et al., 2020). By the end of Neogene, climate. Reconstruction of the Cenozoic the Peninsular landscape had, more or less, global climate by scientists indicates warm acquired its present surface configuration. to hothouse conditions during Palaeocene During the Quaternary, the river systems and Eocene (~66 to 34 Ma), and hothouse underwent repeated adjustments in response conditions during early Eocene (56 to 47 to glacial-interglacial cycles and associated Ma) (Westerhold et al., 2020). Lignite sea-level changes (Kale, 2014). On the basis and other types of deposits of this period of radiometric dates and sedimentological occurring in some parts of India also indicate data of the last glacial and the present inter- hot and humid climatic conditions over the glacial periods, it appears that, in general, Peninsula. This was also the time of intense fluvial aggradation in the valleys was weathering and lateritisation/bauxitisation associated with cooler and drier periods, and over the Indian Peninsula (Jean et al., 2020). degradation was the dominant fluvial activity Most of the laterites occurring over different during the warmer inter-glacial periods (Kale lithologies formed during this hot and humid et al., 2003). phase (Kale, 2014). It is important to note here that this was roughly the time when the Ancestry and antiquity of the Peninsular Indian Plate was centred over the equator rivers during the northward drift. As a result, the Generally, the palaeo flow-paths of rivers climatic conditions were highly favourable are inferred from, either deposits relatable to for deep weathering over the divides and the existing rivers or from palaeogeographic concentrated fluvial erosion along the active maps. The fluvially sculpted landscape of the channels (Kale, 2014). It is most likely that Indian Peninsula shows poor preservation of the present dimensions and forms of the fluvial archives. The Cretaceous to Neogene

The Rivers of Indian Peninsula: Their Salient Geomorphic Characteristics 43 fluvial deposits are conspicuously absent on the intra-cratonic grabens (rift valleys), filled land. The Quaternary fluvial deposits, although with sediments of the Gondwana period. present, are mostly confined to a narrow As the palaeocurrent trends from various belt in certain stretches of the present river Gondwana basins suggest drainage alignment valleys, and record only the late Quaternary towards the NW or NE direction (Valdiya fluvial history. In the absence of continental 2016), it is logical to infer that the present fluvial records of the earlier period, inferring roughly eastward courses of these rivers were and reconstructing the ancestry of the main established (or re-aligned) sometimes after drainage courses unambiguously remains a the Gondwana period (post-Jurassic). difficult proposition. However, considering The palynological assemblages and fossils some of the well-known facts about the occurring within the sediments underlying geological history and palaeogeography of the Deccan Traps (the Bagh and Lameta India it is possible to tentatively reconstruct beds) in the lower and upper Narmada Valley, the geomorphic history of the main-stem the planktonic foraminifera in the Deccan rivers. Some of the well-established facts are intertrappean beds in the Chhindwara district, listed below: and pollen data of infratrappeans from the The general slope of the Deccan Plateau is Yavatmal area in Maharashtra suggest that towards the east over the Precambrian terrain an arm of the sea from west coast extended as well as over the Deccan Traps terrain, through the Narmada Valley, and another arm particularly to the south of the SONATA of shallow sea from the east coast extended zone. The rivers draining the Precambrian through the Godavari-Pranhita valley during terrain, such as the Kaveri and Pennar as the late Cretaceous (Keller et al., 2009; well as the Tungabhadra, do not provide any Prasad et al., 2018). These palaeogeomorphic indications of drainage reversal. This in other reconstructions imply that the existing words means that the easterly drainage has courses of the Narmada-Tapi and lower been in existence throughout the Cenozoic, Pranhita-Godavari were established after the and even before the Deccan Volcanism (late retreat of sea at the end of Cretaceous Period Mesozoic). or beginning of Palaeocene. Similarly, the The Deccan Traps presently cover an thick Ariyalur marine formations in Tamil area of over 500,000 km2. Estimates indicate Nadu suggest major marine transgression in that immediately after the end of Deccan the lower Kaveri and adjoining basins during Volcanism (~65 Ma ago) it covered about 2–3 Cretaceous (Nagendra and Reddy, 2017), times more area. The drainage network over which coincided with the time of hothouse the Traps has without doubt evolved after the conditions and higher eustatic sea levels. end of the Deccan volcanism. Therefore, the Considering all the above facts, it is rivers draining the Deccan Traps terrain are reasonable to infer that the foundations of the young and thus Cenozoic in age. existing drainage systems of the Peninsula The Mahi and Sabarmati rivers are were laid sometimes during the post-Jurassic associated with the Cambay graben, which period over the non-Deccan Traps areas, and formed during the rifting and separation of the drainage network over the Deccan Traps India and Seychelles. This implies that the was established after the end of Deccan modern water courses of these rivers are not Volcanism (Palaeocene-Eocene). This in turn older than 65 Ma. implies that many of the river systems of the The present courses of the Pranhita- Peninsula consist of segments of different Godavari, Damodar, and Mahanadi occupy ages, and the complete integration has

44 journal of indian geomorphology: volume 8, 2020 taken place, perhaps during the early or mid tools in the last few years, there are large Cenozoic (Kale, 2014). Kaveri is one of the gaps in our understanding of the geomorphic major exceptions, because it neither occupies processes and geomorphic history of the the Gondwana basins, nor it was flooded Peninsular rivers. Several issues have and buried by Deccan Traps lava flows, nor emerged from the present review that remain affected by continental collision, marine less understood or completely unanswered. transgression (except in the delta region) or One of the main reasons for this is the decline glaciation. Further, none of the earlier studies in the number of purely field-based studies in has suggested that the Kaveri drainage was recent years. Geomorphology is an empirical disrupted during the Gondwana breakup in science as well as a field science and hence the Mesozoic. It is therefore likely that the intensive and rigorous fieldwork and field- Kaveri drainage has retained characteristics based measurements / monitoring are some of of the ancestral drainage to a considerable the fundamental requirements for successful extent. In the absence of sedimentological research. In the last few years, the focus of records, it is not possible to independently the geomorphic studies in India has gradually verify this hypothesis. However, the sharp shifted away from field-based studies towards bends and its course across the Biligirirangan remote sensing and GIS based studies. The Hill ranges (a horst) in the middle domain DEMs, satellite images and GIS are only suggest that the river has undergone some reconnaissance tools. Without adequate field changes and drainage re-arrangement during knowledge and field data, the interpretations the Cenozoic. Further, in other parts of the largely based on the processing of satellite Indian Peninsula, the presence of sharp bends images and DEM-derived morphometric data in the river courses, barbed tributaries, and are not likely to contribute significantly to head-truncated valleys similarly implies that the existing knowledge on the geomorphic most of the rivers have undergone drainage processes, feedback mechanisms and long- re-organisation to a variable extend due term legacies of human impact on landscapes. to river capture, diversion and beheading. Improved understanding of the geomorphic Although there is ample evidence now to processes and feedback mechanisms under suggest that the Peninsula has remained different landscape settings can lead to tectonically stable during the Cenozoic, better understanding of the range and there is geological and geomorphic evidence extent of human impacts and can help in of block tectonics in some parts. These and anticipating and mitigating future impacts other geomorphic evidence thus suggest of anthropogenically-driven earth system that the drainage re-arrangement or re- processes in general and the fluvial processes organisation in the major drainage basins was in particular. induced by stream piracy, structural control, block tectonics and changes in the erosional Acknowledgement base level at different time periods. The net Shantamoy Guha of the Indian Institute effect is the present surface configuration and of Technology, Gandhinagar is thanked for drainage network of the peninsula. providing copies of some of the research publications and for reading the final version. Concluding remarks It is apparent from the above discussion References that in spite of the availability of better Badve, R.M., Sakurkar, C.V. and Vartak, geochronological data and powerful analytical A.V. (2002) Palynology and depositional

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Date received : 25 December 2020 Date accepted : 31 December 2020

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