Terminal motions of Longbasaba Glacier and their mass contributions to proglacial lake volume during 1988–2018

Abstract. The interaction between a glacier and its glacial lake plays an increasingly important role in glacier shrinkage and proglacial lake expansion, and it increases the risk of glacial lake outburst floods (GLOFs). Longbasaba Glacier is directly contacted by a moraine-dammed lake with a high outburst risk in the central Himalayas, and has drawn a great deal of attention from scientists and local governments. Based on Landsat images and in-situ measurements, the evolution records of the shrinkage of Longbasaba Glacier and the corresponding expansion of its proglacial lake were determined for 1988–2018, and the mass contributions of glacier shrinkage to the increase in lake water volume were assessed. During the past three decades, Longbasaba Glacier has experienced a continuous and accelerating recession in glacier area and length but accompanied by the decelerating surface lowing and ice flow. Consequently, Longbasaba Lake has expanded significantly at an accelerating rate. The glacier surface lowering played a predominant role in the mass contribution of glacier shrinkage to the increase in lake water volume, while ice avalanches were the main potential trigger for failure of moraine dams and subsequent GLOF events. Due to the areal expansion, decreasing mass contributions from parent glacier shrinkage, and some mitigation measures by local governments to improve the drainage systems, the potential risk of outburst for Longbasaba Lake has continuously decreased during the last decade.