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Abstract

Debris flow activity in the Whitney Creek basin of Mount Shasta is caused by incisement of soft pyroclastic beds in upper fan areas, and is the dominant late Holocene geomorphic process. A variety of geologic and botanical techniques permit the dating of many debris flows. These methods aid in the interpretation of recent denudation rates and late Quaternary geomorphic changes at Whitney Creek gorge. Geologic techniques used for dating and interpreting debris flows included carbon-isotope analyses of wood and charcoal samples, stratigraphic relations, analysis of aerial photography, and particle-size analyses of sediment deposits. Relatively recent debris flows were dated dendrochronologically using tree ages, eccentric growth-ring patterns following tree tilting by a debris flow, suppression and release sequences, and corrasion scars caused by debris flow impacts on tree trunks. Results indicate intense debris flow activity along upper Whitney Creek during recent centuries; a minimum of 10 debris flows are identified for the last 420 yr.

Sediment yields and denudation rates estimated from debris flow frequency and volume data suggest that activity has been most intense in the last five centuries. Sediment thicknesses on lower parts of the Whitney Creek fan appear sufficient to account only for deposition rates during late Holocene time. If present rates of deposition had prevailed throughout Holocene time, the average thicknesses of the lower fan deposits would be at least eight times greater than they are.

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