Widespread aggradation in the mountainous catchment of the Alaknanda–Ganga River System: timescales and implications to Hinterland–foreland relationships

Abstract

Cut-and-fill type fluvial terraces are ubiquitous in the Lesser Himalayan zone of the Alaknanda–Ganga (Ganges) rivers system, which flows perpendicular to Himalayan lithotectonic units and traverses a steep climatic gradient. Lithofacies analysis of the sedimentary sequences of cut-and-fill terraces indicated that the valley aggradation took place via (1) channel bar development and excess sediment supply, (2) debris flows composed of mixed rounded to sub-rounded lithoclasts, resulting from episodic high intensity rainfalls in the upper catchment or (3) debris flows or rockfalls generated by local landslides. The luminescence chronology indicates that valley aggradation took place in two phases: ∼49 –25 ka and 18 –11 ka. The incision of the fill started soon after 11 ka. Paleoclimatic records from marine sediments indicate that the aggradation and incision in the Alaknanda–Ganga River has oscillated in-phase with global climatic variations. Glaciation –deglaciation processes in the upper catchment produced huge amounts of sediment between 63 and 11 ka, which was fluvially transferred to the lower valley via several cycles of erosion and deposition, leading to extensive aggradation. The climatic amelioration at ∼11 ka and the completion of deglaciation processes led to increased fluvial discharge and decreased sediment supply, conditions conducive for incision of the alluvial fills. Records from the Indo-Gangetic plain and the Ganga Delta demonstrate that the phase of aggradation was regional but that incision in the foreland started at least 2 –3 ka later, after 7 ka. Bedrock incision rates, as calculated from dated alluvial covers of terraces that are separated by bedrock steps, are spatially variable and fall within the range of rates reported from across the Himalaya. These estimated rates, however, are higher than the basin average erosion rates calculated using isotopic mass balance in riverbed sediments. This study suggests that during the last 50 ka river dynamics in the Himalayas were dominated by monsoon variability and the role of tectonic activity was limited to bedrock incision in few reaches only.