Formation, evolution and geomorphological context of Himalayan moraine dams: Mount Everest region, Nepal

Moraine dams are unstable accumulations of unconsolidated sediment and relict glacier ice, original formed during the Little Ice Age or earlier. Dams are at increasing risk of failure as glaciers recede with climatic warming, since downwasting of the glacier surface results in the growth of lakes. Thousands of people perished and infrastructure destroyed from glacial lake outburst floods in the 20th century, particularly in the tropical Andes and the Himalaya, whilst the potential for further disasters is increasing as the number of lakes grows. Here we evaluate ground observations on the structure and composition of moraines, combined with an assessment of glacier dynamics from remote sensing techniques from several glaciers in the Mt Everest region of Nepal. These glaciers range from one with incipient lake-formation (Khumbu Glacier), to one with a fully developed moraine-dammed lake (Imja Glacier) to one with a flood-breached moraine (Chukhung Glacier). The moraines grew from accretion of multiple sedimentary facies of basal glacial and supraglacial origin. Some moraines have a decaying ice core. Folding and thrusting are believed to have been prime factors in moraine growth during the Little Ice Age phase of active flow. Using ASTER imagery, glacier surface gradients and velocities have been quantified. The subsequent formation of moraine-impounded lakes is favoured by a debris-covered tongue, the coalescence of supraglacial lakes and negligible ice velocities. With the supply of ice to these flat glacier tongues much reduced, the number of potentially hazardous lakes will grow at an accelerating rate.