Debris flow from 2012 failure of moraine-dammed lake, Three Fingered Jack volcano, Mount Jefferson Wilderness, Oregon

In the late spring or early summer of 2012, a flood emanated from a small moraine-dammed lake on the northeast flank of Three Fingered Jack in the Mount Jefferson Wilderness. Channel erosion or slope collapse breached the natural dam of the lake, leading to a sudden lowering of lake level by 2.8 m and discharge of 12,700 cubic meters (m³) of water. The resulting debris flow formed a bouldery deposit extending about 0.35 km downslope.

The Three Fingered Jack debris flow is one of several that have issued from moraine-dammed lakes in the Oregon Cascade Range. A thorough summary of those lakes and the hazards associated with them was published in 2001, based largely on fieldwork by Jim O’Connor and Jasper Hardison in the early 1990s. Described here are details of the 2012 event, an update to the O’Connor story begun earlier.