Geomorphology of debris flows and alluvial fans in Grand Canyon National Park and their influence on the Colorado River below Glen Canyon Dam, Arizona

Abstract

Debris flows in at least 529 Grand Canyon tributaries transport poorly-sorted clayto boulder-sized sediment into the Colorado River, and are initiated by failures in weathered bedrock, the "fire-hose effect," and classic soil-slips often following periods of intense rainfall coincident with multi-day storms. Recent debris flows had peak-discharges from about 100-300 m3/s. Twentieth-century debris flows occurred from once every 10-15 years in eastern tributaries, to once in over a century in western drainage areas. Systemwide, debris flows likely recur about every 30-50 years, and the largest recent flows were initiated during Pacific-Ocean storms in autumn and winter. Three idealized hydrographs are inferred for recent debris flows based on deposits and flow evidence: Type I, has a single debris-flow peak followed by a decayed recessional streamflow; Type II, has multiple, decreasing debris-flow peaks with intervening flow transformations between debris flow and non-debris flow phases; and Type III, may have either a simple or complex debris-flow phase (begin as either Type I or II), followed by a larger streamflow peak that reworks or buries debris-flow deposits under streamflow gravel deposits. From 1987 through 1995, at least 25 debris flows constricted the Colorado River, creating 2 rapids and enlarging at least 9 riffles or rapids. In March-April, 1996, reworking effects of a 7-day controlled flood release (peak = 1,300 m³/s) on 18 aggraded debris fans in Grand Canyon were studied. Large changes occurred at the most-recent deposits (1994-1995), but several other older deposits (1987-1993) changed little. On the most-recent fan deposits, distal margins became armored with cobbles and boulders, while river constriction, flow velocity, and streampower were decreased. Partial armoring of fan margins by relatively-low mainstem flows since the debris flows occurred, was an important factor limiting fan reworking because particles became interlocked and imbricated, allowing them to resist transport during the flood. Similar future floods will accomplish variable degrees of fan reworking, depending on the extent that matrix-supported sediments are winnowed by preceding mainstem flows.