Evidence for surface lowering at supraglacial lakes on debris-covered glaciers in the Himalayas

Elevation changes at supraglacial lakes during the period 1992-2008 were compared with surface elevation changes of glaciers in lake-free regions, on the Khumbu Glacier and the Ngozumpa Glacier, Nepal Himalaya. Ngozumpa Glacier showed remarkable surface lowering of supraglacial lakes during this period, as compared with elevation changes of the glacier in lake-free zones. Supraglacial lakes with high turbidity were associated with areas of large surface lowering. Glacier surfaces surrounding these lakes tend to be covered with relatively thin debris layers. Supraglacial lakes with low turbidity were located at glacier termini, where glacier surfaces were slightly uplifted; in these areas, glacier surfaces were covered with a thick debris mantle. Supraglacial lakes with high turbidity were connected to englacial conduits, and/or were surrounded by ice cliffs, and thus received an inflow of meltwater; the water in these lakes is warmed prior to outflow through englacial conduits. On the other hand, supraglacial lakes with low turbidity do not contribute to glacier ablation, as they have no inflow or outflow. Therefore, supraglacial lakes with low/high turbidity located on the lower parts of a glacier are reliable indicators of small/large amounts of surface lowering. Areas that show minor elevation changes indicate the location of banks that are damming lake water; areas with large surface lowering may indicate the location of large and expanding glacial lakes.Characteristics of supraglacial lakes;