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Mount Shasta, the southernmost stratovolcano in the Cascade Range (41.4°N) has
frequently produced lahars of various magnitudes during the last 10,000 yr. These include
large flows of eruptive origin, reaching more than 40km from the summit, and studies have
shown that at least 70 debris flows of noneruptive origin have occurred during the last
1,000 yr in various stream channels. The Mud and Whitney Creek drainages have
historically produced more debris flows than any other glacier-headed channel on the
volcano. Periods of accelerated glacial melt have produced lahars in Whitney Creek with a
volume of 4 x 106 m3 and a runout distance of about 27 km from the summit. Mud Creek
flows from 1924 – 1931 covered an area of more than 6 km2 near the community of
McCloud with an estimated 23 x 106 m3 of mud. A much older lahar in Big Canyon Creek
may have deposited a volume of 70 x 106 m3 over present day Mount Shasta City and
beyond. A lahar inundation modeling tool developed by USGS analysts is used to
objectively delineate lahar inundation zones on Mount Shasta by embedding predictive
equations in a geographic information system (GIS) that uses a digital elevation model,
hypothetical lahar volumes, and geometric relationships as input. Volumes derived from
these lahar deposits are extrapolated to the selected drainages to generate probable lahar
inundation hazard zones with a focus on mapping and hazards implications. |
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