Late Quaternary floods and their control on aggradation and incision in the Indus River, Ladakh Himalaya

Rivers in the Himalaya have extensively been used as a tool to understand the mechanism of valley aggradation and incision resulting due to precipitation variability and ongoing deformation. The suture zone tectonics control the landscape building along the upper Indus River, however, intensified monsoon encroaches on the region and modify the surface processes. The valley is filled with outwash fans and aggradation pulses at ~ 52 ka, ~ 28 ka and ~16 ka when the monsoon was strengthened. At the younger time, during ~13 to ~9 ka, the Indus valley has suffered an incision phase marked by exposure of bedrock near the Indus-Zanskar confluence.

The present study constrained the paleodischarge of the Indus River during periods of established river aggradation, incision and flood-time to understand climatic settings during the enhanced sediment-load modulated by the increased discharge. At the valley filling time (47–23 ka), clast geometric data of the largest imbricated clasts from the riverbed as well as the aggraded sequences were utilized to calculate discharges. Sand-silt couplets marked as slack water deposits (SWDs) of age 14–10 ka at Indus-Zanskar confluence were used to constrain the paleodischarges during net river incision. The catchment-scale paleodischarge derived from valley fill sequences and SWDs ranges from 834±47 to 4457±253 and 19030 to 47954 cumecs. Incision-time discharges were three to ten-fold higher than from the aggradation time observed that the aggradation in the Himalayan rivers occurred in transient time (33–21 ka and 17–14 ka) when the sediment load in the rivers increased just after the glaciation. Hence, the aggradation in the Indus River has occurred when the sediment to water ratio was higher and the river carrying capacity has reduced, subsequently, the incision was initiated when sediment to water ratio reduced and the river sediment carrying capacity increases during post-glacial climatically wet phase (early Holocene).