Rockslide Dams in the Northwest Himalayas (Pakistan, India) and the Adjacent Pamir Mountains (Afghanistan, Tajikistan), Central Asia

Abstract

The remains of rockslide dams are widespread in the river valleys of the northwest Himalayas (Pakistan and India) and the adjacent Pamir Mountains of Afghanistan and Tajikistan, Central Asia. The region contains in excess of two hundred known rockslide deposits of, as yet, unknown age that have interrupted surface drainage and previously dammed major rivers in the region in prehistoric time. In addition, the region contains (1) the highest rockslide dam (the 1911 Usoi rockslide, Tajikistan) in the world that dams the largest rockslide-dammed lake (Lake Sarez) on Earth, (2) the largest documented outburst flood associated with a historical rockslide dam outburst (the 1841 Indus Flood, Pakistan), and (3) the world's most recent (2010) rockslide-dammed lake emergency, the Attabad rockslide dam on the Hunza River, in the Upper Indus basin of northern Pakistan. We show that some prehistoric rockslide dams in the northwest Himalayas impounded lakes with volumes in excess of 20 Gm3, significantly larger than present-day Lake Sarez. We use SRTM-3 digital terrain data and satellite imagery to analyse four major historical rockslide damming events (1) our analysis of the 1841 Indus rockslide-damming event indicates that the volume of the impoundment and subsequent outburst was 6.5 Gm3, the largest outburst from a rockslide-dammed lake in historical time, (2) the 1858 Hunza Valley rockslide dam impounded about 805 Mm3 before catastrophic outburst in August 1858, (3) the development of the 2010 Hunza rockslide-dammed lake is described in detail for the first time. It reached a maximum volume of 430 Mm3 before stable overflow of the rockslide debris began on May 29, 2010. This remains the situation as of July 25, 2010 (200 days after impoundment), (4) the filling of Lake Sarez was conditioned by excessive outflow seepage through the debris as the lake filled to the extent that a freeboard of ca. 50 m is still naturally maintained without engineering intervention. The emplacement of rockslide dams and the behaviour of their impounded lakes are critical hazards to communities and the development of infrastructure, including hydroelectric facilities, in this region of Central Asia.