• Swapnamita Choudhury

      Articles written in Journal of Earth System Science

    • Morphodynamics of the Kulsi River Basin in the northern front of Shillong Plateau: Exhibiting episodic inundation and channel migration

      Watinaro Imsong Swapnamita Choudhury Sarat Phukan Bhagawat Pran Duarah

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      The present study is undertaken in the Kulsi River valley, a tributary of the Brahmaputra River that drains through the tectonically active Shillong Plateau in northeast India. Based on the fluvial geomorphic parameters and Landsat satellite images, it has been observed that the Kulsi River migrated 0.7–2 km westward in its middle course in the past 30 years. Geomorphic parameters such as longitudinal profile analysis, stream length gradient index (SL), ratio of valley floor width to valley height (Vf), steepness index (k/s) indicate that the upstream segment of the Kulsi River is tectonically more active than the downstream segment which is ascribed to the tectonic activities along the Guwahati Fault. 14C ages obtained from the submerged tree trunks of the Chandubi Lake, which is located in the central part of the Kulsi River catchment suggests inundation (high lake levels) during 160 ± 50 AD, 970 ± 50 AD, 1190 ± 80 AD and 1520 ± 30 AD, respectively. These periods broadly coincide with the late Holocene strengthened Indian Summer Monsoon (ISM), Medieval Warm Period (MWP) and the early part of the Little Ice Age (LIA). The debris which clogged the course of the river in the vicinity of the Chandubi Lake is attributed to tectonically induced increase in sediment supply during high magnitude flooding events.

    • Mid–late Holocene fluvial aggradational landforms and morphometric investigations in the southern front of the Shillong plateau, NE India

      Watinaro Imsong Swapnamita Choudhury Sarat Phukan

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      The present study is an attempt to understand the antiquity of the preserved fluvial landforms and its response to the climate–tectonics nexus through geomorphological investigations along the Jadukata, Umpung, Umngot and Umtongoi rivers in the southern front of the Shillong plateau (SP), NE India. Sedimentological characteristics, chronological analyses and morphotectonic parameters were used to describe the spatial and temporal variability in the patterns of aggradation, landform evolution and neotectonic influences in the study. Our results indicate that valley aggradation processes occurred around the transitional zone in the southern front of the SP during the mid–late Holocene era along with a hiatus in sediment deposition after 4.3 ka. Sediment generation and aggradation is modulated by precipitation anomalies associated with the Indian Summer Monsoon (ISM) variability whereas morphometric analysis suggests that activity along the Dauki–Dapsi fault has been contributing to the uplift-related deformation. Sedimentological observation supported by optically stimulated luminescence chronology obtained on palaeoflood deposits, valley-fill fluvial terraces and alluvial fans indicate their deposition during three pluvial phases: (i) 5.3–4.3 ka, (ii) 2.4–1.0 ka and (ii) 0.7–0.3 ka. Our data indicate that valley aggradation and geomorphic processes in the southern part of SP responded to short-term changes in the ISM variability with contributions from the morphotectonic activities associated with the Dauki–Dapsi fault during the late Holocene period.

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