Sept. 18, 2001 | Colorado River - Grand Canyon | Coconino County, Arizona

On at least thirteen different occasions, lava dams in Grand Canyon catastrophically failed and released huge volumes of floodwater downstream on the Colorado River. These floods created outburst flood deposits which have been recognized by geologists only within the last 10 years. The lava initially erupted across the channel of the river, blocking it for enough time that a reservoir developed behind the dam. Hydrostatic pressure or spillover caused the dams to fail and the water was instantaneously released downstream. Here near river mile 195 is a spectacular example of an outburst flood deposit that fills an old channel of the Colorado, now in its new channel to the left of the photo. (Photo and caption by Wayne Ranney)