2002 Debris Flows and Floods in Grand Canyon

Given the on-going drought in the Southwest, the summer of 2002 wasn’t expected to have many floods or debris flows in Grand Canyon. The general failure of the summer monsoon this year confirmed that it shouldn’t be an active season. That all changed during and following September 7th, when several important debris flows occurred between the mouth of the Little Colorado River and Phantom Ranch. More widespread streamflow flooding caused significant changes over a wider area, and the Little Colorado River had one of its largest summer floods in a long time, peaking at about 11,500 cubic feet per second (cfs) on September 11 and pushing the Colorado River up to 21,500 cfs. Following the record debris flow season of 2001 (Fig. 1), the combined effects of the 2001–2002 debris flows changed debris fans along the river corridor more than at any period we know of since perhaps 1966.
The storms of September 7th were monsoonal only in the sense that they occurred during the summer. The Arizona monsoon generally consists of a mixture of moisture generated from the Gulf of Mexico and the eastern North Pacific Ocean. This moisture travels into Arizona guided by several large-scale circulation features, including high-pressure systems that set up over the Midwest and the Caribbean Sea (known as the Bermuda High). This summer, those guiding systems sent most of the summer’s moisture into New Mexico and west Texas, leaving us dry for much of August. What happened on September 7th was very unusual: a frontal system from the Pacific Ocean drew in low-level moisture from two tropical cyclones, one west of Baja California and one just southeast of its tip. This potent slug of moisture caused widespread rainfall in Arizona between September 6th and 10th.
The storm resulted in what appears to have been a debris flow at Sheer Wall Rapid and eleven debris flows between Lava Canyon and just upstream of Grapevine Rapid. Most of these debris flows came from river right, and many of the tributaries, or at least their contributing areas, were relatively small. Between 1984 and 2001, when debris flows have been counted in Grand Canyon, an average of 4.6 occurred per year. The 2001 summer season had the highest annual total with fifteen. The 27 debris flows in 2001–2002 have left a mark on Grand Canyon that will remain at least until another significant flood is released from Glen Canyon Dam. These flows caused changes to at least seven riffles or rapids, although none of these changes are significant to navigation of the river; eliminated one heavily used camp (lower Rattlesnake Camp); and created several low-water sand bars that may be useful if flow releases remain low.
The largest debris flow in 2002 occurred at Unkar Rapid. Unkar Creek spewed a large amount of cobbles and boulders onto its debris fan, raising its surface by perhaps as much as six feet. Most of the new deposition is on the upstream side of the debris fan and away from the beginning of the rapid, which appears to be unchanged. Something similar occurred at both Sheer Wall Rapid (from Tanner Wash on river left), except in that case flow around the left side of the debris fan has been choked off and a small sandbar has formed directly under the waterfall in the canyon’s mouth. Finally, a debris flow caused significant aggradation on the small debris fan on the left side of Sockdolager Rapid.

This debris flow, the second from Hance Creek in three years, left levees that are quite visible to those bold enough to take their eyes off the whitewater looming below.
A small but highly significant debris flow occurred at lower Rattlesnake Camp. The river grapevine reports that a private trip with a number of long-time Grand Canyon guides was camped at the lower beach that gets first shade in summer. They took a hike in the rain and returned to find their camp demolished, with the kitchen and their kayaks blown out into the river. Fortunately, no one was hurt at the time and their motorboat was undamaged, although if they had been in camp injuries and or death would have been likely. This debris flow barely reached the Colorado River but deposited considerable sediment throughout the camp, including some boulders up to nine feet across.
Smaller debris flows occurred at Lava Canyon, constricting the right side of Lava Canyon Rapid but not changing the run; at Basalt Canyon, increasing the drop through a previous riffle; at the Forbidden Zone across from Cardenas Creek, where an upstream canyon deposited a small new debris fan and the downstream canyon (mile 71.2R) reconnected a small island with the right bank; at a previously fastwater reach at mile 73.9, creating a small rapid; at the riffle at the mouth of Escalante Creek, shifting the channel from the right side to the center; and at the second riffle downstream of Sockdolager Rapid, creating a sizeable new rapid that is quite startling. Innumerable mass movements that can only be described as “wet avalanches” occurred between Hance and Grapevine Rapids, including a really interesting saturated slope failure on the left side just below Hance. Check that one out on your next trip.
Several significant floods occurred in tributaries this year as well. Two—at Carbon Canyon (mile 64.7 R) and Blacktail (mile 121 R)—caused changes to heavily visited sites. The Blacktail flood was one of the few that occurred in July, when a brief period of thunderstorm activity affected Grand Canyon. The September 7th storm affected a broad area, including Lees Ferry and most of the left-hand tributaries from 60-Mile Rapid down to Palisades.
On your next Grand Canyon trip, expect to see lots of new, low, muddy sand (if releases remain low) and enlarged or changed riffles or rapids from Lava Canyon Rapid to Grapevine. You might want to keep in mind that the lower Rattlesnake Camp has been eliminated from the camping choices between Unkar and Hance.
Bob Webb and the USGS debris-flow monitoring crew