The inception and evolution of supra-glacial lakes on debris-covered glaciers in the Nepal Himalaya
In the Himalaya, ablation of debris-mantled valley glaciers has resulted in the formation of large, potentially unstable moraine-dammed lakes. In recent decades several Glacier Lake Outburst Floods (GLOFs) have occurred, which have resulted in destruction of land and infrastructure for several tens of kilometres down valley. As a result, the growth of supra-glacial lakes is considered to be a potential hazard to human lives and livelihoods. Therefore, there is a need for better understanding of the factors involved in lake emergence and subsequent evolution, in order to prepare for and alleviate future GLOF events. On the basis of the above premise, a detailed study of one glacier in the region, the Ngozumpa, was undertaken. The specific objectives of the study were to: 1) study initial lake inception; 2) understand glacial lake development cycles; 3) monitor lake development rates; and 4) provide a baseline for future glaciological studies on the Ngozumpa Glacier. It was found that certain criteria are requisite for glacial lake formation, and that the evolution of lakes on the glacier is controlled by the englacial base level. Two types of lake exist on the Ngozumpa; 1) ‘Perched Lakes’, occupying closed basins above the englacial water table, and 2) lakes controlled by the altitude of outflow through the lateral moraine. A single example of the second type, Spillway Lake, currently exists on the Ngozumpa. Although ‘Perched Lakes’ contribute to overall downwasting of the glacier, they are subject to rapid drainage once connection is made to englacial conduits. Spillway Lake has the potential to expand into an unstable lake if present trends continue.