Evidence of debris flows is common along the glacially fed streams of the Mount Shasta volcano, California. These streams are Whitney Creek, Bolam Creek, Mud Creek, Ash Creek, Gravel Creek, and Inconstance Creek. Debris flows since 1580 can be documented by studying trees damaged by or growing on debris flows. Dates obtained from tree-ring analysis proved consistent with documented records of debris flows. Debris-flow dates in conjunction with geomorphic evidence permit the development of magnitude and frequency relations over a period of about 400 years. Dendrogeomorphic evidence shows that debris flows are a common occurrence on many Mount Shasta streams. Debris flows traveling at least 2 kilometers have occurred at a frequency of about 10.5 debris flows per century; certain basins are more active than others, the Gravel Creek and Mud Creek basins are among the most active, and Inconstance Creek basin is the least active. Small debris flows are more frequent and usually do not move as far downslope as large debris flows. Cyclic scouring and filling by debris flows, in and adjacent to stream channels and on debris fans, is suggested by dendrogeomorphic evidence and seems to be related to event magnitude and frequency. Distinct periods of heightened debris-flow activity and quiescent periods occur in individual basins and are apparently related to glacial and valley wall stability and meltwater supply. Debris flows of all magnitudes appear to be the major surficial geomorphic agent during noneruptive times that sculpture the channels and develop large alluvial fans.
A water-resources appraisal of the Mount Shasta area in northern California was completed to document present hydrologic characteristics so that comparisons could be made of possible future changes due to land use or volcanic activity. Data collected during this study include glacial areas and volumes, streamflow, sediment concentrations, temperature, and water chemistry of the ground water and springs. The lower flanks of Mount Shasta consist of broad, smooth aprons of pyroclastic-flow, debris-flow, and fluvial deposits. Incised channels on upper parts of the mountain tend to terminate on the flanks. Only four streams Mud, Whitney, Squaw Valley, and Ash Creeks have sufficient streamflow to reach downstream main channels or to travel more than about 6 miles from the summit. The difference in low flows of the various streams is attributed to variations in glacial melt, storage of ground water in the materials that compose the mountain and the alluvial fans, and the rain-shadow effect of the mountain on precipitation. Annual precipitation in the vicinity of McCloud, southeast of the summit, is about 53 inches, whereas on the northwest side, it is about 18 inches. Although Mount Shasta is an old volcano, lava flows and other volcanic activity have occurred within the last 10,000 years. The most recent of Mount Shasta's summit vents is Hotlum Cone, which may have erupted as recently as 200 years ago. Many springs issue from fracture joints in lava and lava tubes around the mountain; the main groundwater movement is generally away from the slopes of Mount Shasta. Where groundwater data were available, the direction of movement is in a downslope direction from areas of recharge near Black Butte and Mount Shasta to areas of discharge near Weed and Mount Shasta (city).
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