Magnitude and frequency data for historic debris flows in Grand Canyon National Park and vicinity, Arizona

Debris flows occur in 529 tributaries of the Colorado River in Grand Canyon between Lees Ferry and Diamond Creek, Arizona (river miles 0 to 225). An episodic type of flash flood, debris flows transport poorly-sorted sediment ranging in size from clay to boulders into the Colorado River. Debris flows create and maintain debris fans and the hundreds of associated riffles and rapids that control the geomorphic framework of the Colorado River downstream from Glen Canyon Dam. Between 1984 and 1994, debris flows created 4 new rapids and enlarged 17 existing rapids and riffles. Debris flows in Grand Canyon are initiated by slope failures that occur during intense rainfall. Three of these mechanisms of slope failure are documented. Failures in weathered bedrock, particularly in the Hermit Shale and Supai Group, have initiated many historic debris flows in Grand Canyon. A second mechanism, termed the fire-hose effect, occurs when runoff pours over cliffs onto unconsolidated colluvial wedges, triggering a failure. A third initiation mechanism occurs when intense precipitation causes failures in colluvium overlying bedrock. Multiple source areas and extreme topographic relief in Grand Canyon commonly result in combinations of these three initiation mechanisms. Interpretation of 1,107 historical photographs spanning 120 years, supplemented with aerial photography made between 1935 and 1994, yielded information on the frequency of debris flows in 168 of the 529 tributaries (32 percent) of the Colorado River in Grand Canyon. Of the 168 tributaries, 96 contain evidence of debris flows that have occurred since 1872, whereas 72 tributaries have not had a debris flow during the last century. The oldest debris flow we have documented in Grand Canyon occurred 5,400 years ago in an unnamed tributary at river mile 63.3-R. Our results indicate that the frequency of debris flows ranges from one every 10 to 15 years in certain eastern tributaries, to less than one per century in other drainage basins. On average, debris flows may recur approximately every 30 to 50 years in individual tributaries, although adjacent tributaries may have considerably different histories. Peak discharges were estimated in 18 drainages for debris flows that occurred between 1939 and 1994. Typically, discharges range from about 100 to 300 cubic meters per second (m3/s). The largest debris flow in Grand Canyon during the last century, which occurred in Prospect Canyon in 1939, had a peak discharge of about 1,000 m3/s. Debris-flow deposits generally contain 15 to 30 percent sand-and-finer sediment; however, the variability of sand-and-finer sediment contained by recent debris flows is large. Reconstitution of debris-flow samples indicates a range in water content of 10 to 25 percent by weight;. Before flow regulation of the Colorado River began, debris fans aggraded by debris flows were periodically reworked by large river floods that may have been as large as 11,000 m3/s. Impoundment of the river by Glen Canyon Dam in 1963, and subsequent operation of the reservoir have reduced the magnitude of these floods. Flow releases from the dam since 1963 have only partly reworked recently-aggraded debris fans. Significant reworking of new debris-flow deposits now occurs only during river discharges higher than typical power plant releases, which currently range between 142 and 510 m3/s.