A GEOMORPHIC ASSESSMENT OF FLOOD HAZARDS OF THE RISHI VALLEY IN THE TRANSHIMALAYA, LADAKH, NORTHERN INDIA

Flood events in the Himalaya of northern India have caused significant social and economic damage. In the Transhimalaya of northern India, the village of Karzok (32°57'53.0''N, 78°15'42.3''E, 4600 m asl) is located in the Rishi Valley downstream of a glacially fed river; the Rishi Valley lies within the current monsoon track and ends in the enclosed lake Tso Morari. Modern risk of catastrophic flooding of Karzok was assessed using geomorphic mapping, terrace surface characterization, and paleohydraulic analysis. Geomorphic mapping reveals an extensive debris fan of approximately 6,000 km² that flowed into the Rishi Valley from the southwest and likely filled the valley to its northern wall. A single flight of river terraces is preserved in the debris fan ~4 m above the active channel. Boulder measurements from the debris fan were used to estimate a maximum paleodischarge of 40,000 m³/sec and a channel full depth of ~21 m, which implies that terrace formation occurred after debris fan boulders were evacuated. Based on terrace surface characterization, paleodischarge through the Rishi Valley has not superseded the estimated paleodischarge since debris fan deposition. Additionally, basin analysis suggests the amount of energy required to generate a flood event of this magnitude is not currently present in the valley, but may have been ca. 80 ka when the valley was fully glaciated. Granitic boulders on the debris fan were sampled for cosmogenic radionuclide analysis to constrain the timing, rates, and nature of these processes. We speculate that the formation of the debris fan, its subsequent incision, and removal of debris fan boulders from the active channel are related to valley-wide deglaciation. A modern flood event of magnitude comparable to the maximum paleodischarge is unlikely since glacial ice is currently restricted to small glaciers adjacent to the high peaks (>5700 m als) that rim the valley to the south and southwest. Despite the low risk of glacially-induced flooding, the Rishi Valley is not exempt from the risks associated with monsoon and cloud burst flooding.