SUMMARY
In the last 500 years, at least 69 debris flows of noneruptive origin have occurred on Mount Shasta. Between 1900 and 1985, 37 debris flows occurred on the various stream channels draining the volcano, giving an average interval of 2.3 years between debris flows on Mount Shasta. During any one year, however, several streams may experience a debris flow such as in 1977 when flows occurred on Whitney, Mud, and Ash Creeks. Debris flows occurred in successive years on Mud Creek (1924, 1925, and 1926). For a given stream, the mean interval between flows on a stream is 13.3 years. Between 1900 and 1985, the greatest number of debris flows occurred on Mud and Whitney Creeks. Most debris flows occurred in July, but they also were observed in August and September.
Debris flows may be formed when banks of eruptive debris or lava flows are undermined by streamflow and collapse into the channel, causing temporary blockage. An additional potential cause of debris flow is the formation of snow and ice dams as the result of melting glaciers and snowpack, creating temporary dams that are subsequently breached. Debris flow also occurs when heavy amounts of localized precipitation fall on the slopes of Mount Shasta during the summer season (June, July, and August).
The pattern of debris-flow deposits is dependent on the capacity and sinuosity of the channel, presence of overflow areas, and patterns of deposition of previous flows. In 1985, a debris flow on Whitney Creek traveled 27 km from the summit. The depth of the deposits, ranging from 0.4 to 2.5 m and averaging 1 m, tends to decrease in a downstream direction. Cross-section shapes of debris-flow deposits are convex. Surveys of seven sites on streams on Mount Shasta indicate that the maximum height of the deposits relative to the low point (usually near the margins of the debris flow) is about 5 m. The sediment-water ratios of reconstituted debris-flow slurries at 19 sites averaged 0.68, which is equivalent to a minimum water concentration of 32 percent by volume.