Alpa Sridhar

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Geomorphic controls on the slackwater deposition: Example from the Mahi River Basin, western India part of Vignettes:Vignette Collection
The river basins of western India are characterized by colossal deposits of marine, fluvial and aeolian origin. A major bulk of these sediments has been deposited during the late Pleistocene under climatic and tectonic controls. However, since the formation of the present day river valleys during the early Holocene, the sedimentation has been predominantly fluvial, in response to the variations in the climatic conditions and related south west monsoon regime. The resulting hydrological changes in a river basin are largely associated to the flooding events and can be best inferred from the existing sediment records. The infrequent high magnitude flood events are commonly reconstructed using slackwater deposits. Unconsolidated sands and silts that accumulate relatively rapidly from suspension during major floods, particularly where flow boundaries result in markedly reduced local flow velocities are described as slackwater deposits. For palaeoflood studies, a slackwater sedimentation site should be optimum for both the accumulation and preservation of the relatively fine-grained sediments carried high in flood flows at maximum stage. The ideal sites of deposition and subsequent preservation of slackwater deposits are the tributary mouths, channel-margin alcoves, caves and rock shelter deposits. The local hydraulic conditions at these sites result into a drop in the flow velocity and deposit flood sediments. In an alluvial terrain however, such conditions seldom prevail and hence the slackwater sediments are not well preserved so as to give significant palaeohydrological inferences. The alluvial Mahi River of semi arid western India (Fig.1a, b) however is peculiar with deeply incised ravines occurring on either bank at the meander bends. The dissections in these ravines provide an ideal site for back flooding of the floodwater from the main channel and accumulation of slackwater sediments (Fig. 1c). Also, the ravine openings into the main stream result in a local widening of the channel leading to a sudden drop in the flow velocity. This effect coupled with eddies that form at the meander bends probably explains the deposition of slackwater sediments at these sites. Extensive slackwater deposits have been found in the ravines that open up almost perpendicular to the main channel. The deeply incised ravines at the meander bends in the alluvial reaches have preserved sizeable sequences of slackwater deposits (Fig. 2) much inland at elevations of upto 20 m above the river level and are well protected from the annual flooding of the channel as well as any other minor flood events since their deposition. The factor of channel stability is significant as far as the palaeoflood studies are concerned. An increase in flood discharge produces only a small increase in stage for overbank flow in alluvial channels. However, the carbonate enriched Pleistocene sediments which occur as 30-40 m high cliffs behave like resistant flow boundaries unlike most of the other alluvial channels and do not allow the frequent shifting of banks over broad range of flow conditions. The best exposures of slackwater deposits in the Mahi River basin comprises sediments deposited during four distinct flood events (Fig. 3). The top of the SWD is about 12 m above the river channel bed at the highest section and extends to ~150-200 m inland ward from the present bank. In between the demarcated SWD units, silt and clay couplets occur, with a basal layer of fine to very fine sand grading into darker silty / clayey units. Occurrence of such couplets suggests temporary ponding of floodwaters and deposition during waning discharge conditions. Based on the age, the occurrence of fluvial and colluvial sediments and present geomorphic expressions, a sequence of sedimentation and erosion phases can be reconstructed. The accumulation of brownish yellow silt and dark brown clay couplets (SWD 1) was followed by the deposition of brown silt (SWD 2). A phase of erosion followed, leading to accumulation of slope debris on the top. This was followed by a phase of deposition of dark brown clayey silts (SWD 3). Another phase of erosion and accumulation of slope debris occurred subsequently and was followed by deposition of dark silts (Fig. 4) (SWD 4). The sand unit with reworked rizoliths below the SWD 1 has been dated to 14±1 ka and the SWD 2 has been dated to 4.6±1 ka. The second phase of SWD accumulation occurred during the mid Holocene ~5 ka B.P. whereas the last phase was ~1.7 ka B.P. Owing to the geomorphic set up of the Mahi River basin, the slack water deposits related to high magnitude events dating back to almost 5 ka have been preserved indicating the control of the fluvial landscape in palaeoflood reconstruction.