Speaking of Debris Flows...


   Thunderstorms on August 20th an 22nd resulted in a few noteworthy changes to the river. Most notable was a debris flow in Tanner Canyon at mile 68 on the 22nd. The Tanner Canyon debris flow was about the largest we’ve seen since the Monument Creek debris flow in 1984, and caused substantial changes to the rapid there. If you haven’t run Tanner Rapid since August, then keep your eyes open on your next trip; it’s really changed. Also changed are the stage-discharge relations of the upper pool above the rapid.

   The debris flow apparently initiated in the first tributary of Tanner Creek upstream of the river. According to an eyewitness, Sue Rhodes, the debris flow occurred shortly after cessation of a severe thunderstorm that built quickly and dropped several inches of rain and sleet along the base of Comanche Point. The storm lasted less than sixty minutes! The intense rainfall mobilized unconsolidated colluvium several hundred meters above the river, and from the looks of what’s left, the place must have really been shaking; I’m talking about big boulders moving in ways that still never fail to amaze me. The initiation setting along the base of Comanche Point is classic for Grand Canyon debris flows. Check it out next time you get a chance.

   The initial debris flow pulse of the Tanner debris flow, termed the “bouldery snout,” was quite large, but probably lasted only a matter of seconds; long enough, though, to dam the main channel of the creek temporarily. The boulder dam then re-mobilized and carried at least two dozen large boulders down to the river resulting in a new debris fan, and a rapid’s transformation. The debris flow was followed by a prolonged flash-flood of muddy water that was about one-quarter as large, but lasted from two to three hours. This watery flood eroded about 800 cubic meters of the new fan’s sediment, which was transported into the river. The new debris fan contains about 10,000 cubic meters of boulders, sand and mud, and has constricted the river channel by at least 30 meters. It will be interesting to see how much of this material is washed away during the proposed experimental flood scheduled for 1995. I estimate that the new fan should be completely inundated by the 45,000 cfs flow.

   In addition, there was also a debris flow at Cardenas Creek on the same day, though it had almost no effect on the river channel. The Cardenas debris flow didn’t transport nearly as many large boulders as the Tanner Canyon debris flow, and was composed mostly of finer sediment derived from the Dox Formation. On August 20th, debris flows also occurred in at least two small tributaries downstream of Havasu Creek. At mile 158, a debris flow buried the First Chance camp under three to four meters of debris, rendering it virtually unusable by river parties. Upstream of Havasu, the Last Chance camp was also buried under numerous boulders. Lastly, there was a debris flow on the same day at mile 160 on the right side of the river. This debris flow deposited many large boulders on the existing debris fan and in the river, creating yet another, small, new rapid. The debris flows of the 20th were also triggered by thunderstorms which lasted several hours and affected the river corridor from Havasu Creek down to Fern Glen; notice all the new talus that was produced by this storm next time you’re floating that stretch. The USGS gage at Havasu Creek was also destroyed during the storm by rock fragments that exploded from a very large boulder after it landed in the creek bed; I’m not sure, but I think that it may have come all the way down from the Esplanade Formation. This rockfall must have gotten the attention of whoever might have been hiking the creek that day! Were any of you there? I would love to hear about it.

   The debris flows of August followed a continuing trend in terms of their impacts to river resources. This trend consists mainly of progressive elimination of sand bars through burial and (or) erosion when debris flow sediments are deposited at the river. Relatively small debris flows also continue to deposit large boulders in the river channel, increasing the severity of existing rapids, and creating new ones. Without the large annual flows that occurred before the dam, we should expect more of the same as tributaries continue doing their jobs in forming Grand Canyon. The proposed experimental flood of 1995 will give us a chance to evaluate how some of the tight spots, like 24-Mile, Tanner, Fossil, Specter and Bedrock Rapids, might be opened-up by occasional, prescribed flows exceeding present interim releases.

   One last word. To date I know of no serious injuries or fatalities attributed to debris flows in Grand Canyon. In my opinion, this is mainly due to the infrequency of debris flows there, and the keen judgement of professional guides who know the ways of flash-floods from experience. Never underestimate the potential of the side canyons when camping on or near debris fans. Remember that a debris flow is as likely to occur in the middle of the night as in the day, and may be triggered by as little as one hour of intense rain. In many cases, the very steep tributaries mean very little travel time once a debris flow is initiated. That means precious few seconds for you and your clients to get out of the way of a fast-moving boulder train. The potential energy of 5,000 feet of rock contained within drainages often less than one square kilometer in size, means little advance warning for river parties to evacuate a threatened camp. Remember, it only takes one debris flow to ruin a great trip.

Ted Melis