Magnitude and frequency of debris flows, and areas of hazard on Mount Shasta, northern California

Abstract

Debris flows on Mount Shasta, northern California, have occurred frequently during the late Holocene in response to rapid runoff from melting snow and ice. Glacial-meltwater streams that deeply incise unstable pyroclastic and related flow deposits typically from debris flows when high discharge rates cause slope failures within steep-walled gorges. The landslide material either adsorbs streamflow quickly and becomes a slurry or it briefly dams the stream and converts to a debris flow as breaching occurs. All glacial-meltwater streams on Mount Shasta have been repeated debris-flow activity during the last 500 years. During this period, large-magnitude, potentially destructive flows on Mount Shasta have occurred at a rate of four per century, but smaller flows contained by stream channels may be 10 to 20 times more numerous. The smaller debris flows on Mount Shasta pose little hazard to human life or property, whereas larger, out-of-channel flows could cause minor damage. Only the City of McCloud and inhabited areas on the Whitney-Bolam fan appear to be threatened by possible debris-flow activity. None of the streams lacking glacial meltwater have had significant debris-flow activity during late Holocene time. Sediment yields from upper fan areas of Mount Shasta are very high, but most of the sediment, moved by debris flows from upper slopes is redeposited on lower fan areas, locally causing extensive and rapid aggradation of the fan surface. Little sediment enters a through-flowing stream network. Correspondingly high denudation rates in the sediment source areas for debris flows (the deep gorges) suggest that a high frequency of debris flows may be unique in recent centuries. (USGS)