A reconnaissance of 36 tributaries of the Colorado River indicates that debris flows are a major process by which sediment is transported to the Colorado River in Grand Canyon National Park. Debris flows are slurries of sediment and water that have a water content < 40% by volume. Debris flows occur frequently in arid and semiarid regions. Slope failures commonly trigger debris flows, which can originate from any rock formation in the Grand Canyon. The largest and most frequent flows originate from the Permian Hermit Shale, the underlying Esplanade Sandstone of the Supai Group, and other formations of the Permian and Pennsylvanian Supai Group. Debris flows have reached the Colorado River on an average of once every 20 to 30 yr in the Lava-Chuar Creek drainage since about 1916. Two debris flows have reached the Colorado River in the last 25 yr in Monument Creek. The Crystal Creek drainage has had an average of one debris flow reaching the Colorado River every 50 yr, although the debris flow of 1966 has been the only flow that reached the Colorado River since 1900. Debris flows may actually reach the Colorado River more frequently in these drainages because evidence for all debris flows may not have been preserved in the channel-margin stratigraphy. Discharges were estimated for the peak flow of three debris flows that reached the Colorado River. The debris flow of 1966 in the Lava-Chuar Creek drainage had an estimated discharge of 4,000 cu ft/sec. The debris flow of 1984 in the Monument Creek drainage had a discharge estimated between 3,600 and 4,200 cu ft/sec. The debris flow of 1966 in the Crystal Creek drainage had a discharge estimated between 9,200 and 14,000 cu ft/sec. Debris flows in the Grand Canyon generally are composed of 10 to 40% sand by weight and may represent a significant source of beach-building sand along the Colorado River. The particle size distributions are very poorly sorted and the largest transported boulders were in the Crystal Creek drainage. Reworking of debris fans by the Colorado River creates debris bars that constrain the size of eddy systems and forms secondary rapids and riffles below tributary mouths. (See also W89-09239) (Lantz-PTT)
