Glacial impoundments and outburst floods

Abstract

Countless small and many larger lakes exist in Karakoram glacier basins. The region has a long history of outburst floods from them. Impoundments may be on the glacier, beside, in front of and even beneath the ice. The actual dams may be formed of ice, moraines or a combination of the two. Smaller glacial lake outburst floods (GLOFs) seem to occur somewhere in every year, the commonest glacier hazard. More rarely, large lakes occur and threaten much greater damage. Most have involved ice dams of a single type, where a substantial tributary glacier advances across and impounds a main river valley. Over the last two centuries, more than 100 glaciers of over 10 km in length have interfered with upper Indus and Yarkand streams. Large reservoirs have only been definitely identified with 23 glaciers, but other evidence shows many more at some time in the past. Large Karakoram ice dams develop quickly and rarely last more than a few months. The most dangerous cases, in particular at Chong Khumdan and Kyagar Glaciers, have involved two or more major outburst floods in episodes lasting several years. These GLOF hazards differ from smaller ones in the Karakoram and those receiving attention recently in the rest of the Himalaya. Specifically they require glacier advances. In the past decade or so, some of the glaciers associated with large ice dams have advanced and caused, or threatened to cause, GLOFs.