Clark Fork Ice Dam

Quick Facts
Clark Fork, Idaho
12,000 to 17,000 years ago a 4000' tall ice dam blocked the path of the Clark Fork River creating glacial Lake Missoula. At the end of the last ice age this ice dam failed, releasing more water than is held in modern day Lake Ontario and Lake Erie combined. 600 cubic miles of water rushed through this area and down Lake Pend Oreille destined for the Pacific Ocrean.

One of the most intriguing questions about the catastrophic flooding is how the ice dam failed. Various mechanisms for glacial outburst floods have been proposed: Ice erosion by overflow water, subglacial failure by flotation, deformation of ice by water pressure, and erosion of subglacial tunnels by flowing water.

One model suggests a self-dumping phenomenon. In this mechanism, floodwaters are released when the lake level reaches nine-tenths the height of the ice. At this depth the ice becomes buoyant, subglacial tunnels form and enlarge, and drainage occurs until hydrostatic pressure is decreased and the ice again seals the lake. The self-emptying model is used to explain the numerous cycles in the rhythmite deposits and to interpret each cycle as a separate flood.

Even so, only the total collapse of the ice dam can explain the largest of the catastrophic foods. Sub-glacial tunneling and enlargement due to thermal erosion progressing to collapse have also been proposed, as well as catastrophic failure due to water pressure. All are dependent on the configuration of the ice dam and structure of the ice.


Ice Age Floods National Geologic Trail

Last updated: May 27, 2022