Abstract

During the Pleistocene, glaciers from the Sawatch Range flowed down three contiguous tributary valleys to the Arkansas River near Granite, Colorado. One glacier may have pushed the river out of its channel, and the other two crossed the river and forcibly impacted granite walls on the far side of the valley. These glaciers formed at least one ice dam reaching up to 670 feet (ft) high that blocked the Arkansas River and created a large lake. This lake was about 600 ft deep and extended 12 miles (mi) upstream to the Malta Substation below Leadville.

When the ice dam(s) broke, the entire lake drained catastrophically. The outburst tore out the distal ends of the moraines and carried the detritus down the valley in a torrent of dirty water that deposited a sheet of flood boulders 60 ft thick in a very brief time. Many of the flood boulders are tens of feet in diameter; one boulder is 45 ft by 24 ft by 15+ ft. Flood boulders can be found 150 ft up the side of the valley wall.

At least four catastrophic floods swept the Upper Arkansas Valley. These floods together are named the Three Glaciers Floods. Field evidence documents well the last two such floods, but only patches of older flood boulders attest to two earlier floods. The two more recent floods are tentatively assigned Bull Lake and Pinedale ages, which would correspond to the two most recent glacial advances.