Richard H. Jahns Distinguished Lecture: The Formation and Failure of Natural Dams

Lakes dammed by landslides, moraines, and glaciers have drained suddenly to produce floods orders of magnitude larger than nival or rainfall floods. Reservoirs that form behind landslide dams pose a threat to upstream infrastructure. Most landslide dams fail soon after they form by overtopping and incision; the failure may produce destructive downstream floods. Lakes dammed by end and lateral moraines are susceptible to failure because they are steep-sided and consist of loose, poorly sorted sediment that may be ice-rich. Rapid incision of a moraine dam may be caused by a large overflow triggered by an avalanche or rockfall. As climate warms, lakes impounded by glaciers may drain suddenly following a long period of stability due to progressive wastage of the glacier dam and formation of subglacial or ice-marginal channels. Most outburst floods display an exponential increase in discharge, followed by an abrupt decrease to background levels. Peak discharges are controlled by lake volume, dam morphology, and materials, failure mechanism, and downstream topography and sediment availability. Most moraine-dammed lakes formed in the past century as glaciers retreated from bulky moraines constructed during the Little Ice Age. The lakes soon began to fail as climate warmed. With continued glacier retreat, the supply of moraine-dammed lakes will be exhausted, and the threat they pose will diminish. Glacier-dammed lakes typically have gone through a period of cyclic or sporadic outburst activity, lasting up to several decades, since climate began to warm in the late nineteenth century. The outburst floods from any one lake ended when the glacier dam weakened to the point that it could no longer trap water behind it. However, with continued glacier retreat, the locus of outburst activity may shift up-glacier to sites where new lakes develop in areas that are becoming deglaciated.