Controls on sediment storage in wide mountain valleys — a case study from Srinagar (Garhwal) valley, NW Himalaya

The valley-fill deposits in a tectonically active setting play a key role in understanding a river’s response to climatic/tectonic shifts. These deposits are a result of the interaction between the regional and the local processes, which in turn are controlled by tectonics and climate. In this study, valley-fill deposits in the widest reach of the Alaknanda River (a headwater tributary of the Ganga River)—the Srinagar valley—in the northwest Himalaya is investigated for the interaction between, 1) the axial channel (representing regional processes) and local tributary (representing local processes) and 2) axial channel and hillslope (representing local processes). We identified eight levels of terraces and five levels of debris flow surfaces in the Srinagar valley. The terraces are composed of axial river deposits and local stream deposits, whereas debris flow surfaces mainly consist of hillslope deposits. Strath terraces are also present towards the western part of the valley. The 1D resistivity survey and field data show that thick sedimentary cover caps the bedrock in the eastern part of the Srinagar valley. Geochronology data based on optically stimulated luminescence (OSL) dating suggests that some of the sediments in the Alaknanda valley are older than 80 ± 5 ka. Integration of results shows that the width of the Srinagar valley is controlled structurally by the presence of the North Almora Thrust (NAT), a regional structure present in the Alaknanda River basin. The valley width increases around the central part of the Srinagar valley; this increase in width has provided accommodation space and resulted in thick sedimentation. Major filling and evacuation of the Srinagar valley occurred between 80 ± 5 ka and 8 ± 1 ka. The study demonstrates that the long-term interaction of the axial river, tributaries, and the hillslope processes can often be masked by the younger sediments to give a deceptive morphology and evolution history, if not thoroughly investigated.