Small landslide types and controls in glacial deposits: Lower Skagit river drainage,northern cascade range,Washington |
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| Authors: | Paul L Heller |
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| Institution: | (1) U.S. Geological Survey, Menlo Park, California;(2) Present address: Department of Geosciences, University of Arizona, Tuscon, Arizona |
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| Abstract: | Observations of 167 small, shallow landslides spanning a 22-year period on extensively logged slopes of Quaternary terraces
in the lower Skagit and Baker Valleys, Washington, shows that there is a relationship between the common slope failures in
this area and the slope angle, stratigraphy, and logging practices. Landslide frequency increases upvalley, as do mean annual
precipitation and the frequency of perched water tables.
Debris slides are most common, occur on steep slopes (>50%) composed of sand and gravel, and are most abundant in areas previously
logged by the clear—cut method. Debris flows occur on shallower slopes (>30%) where the stratigraphy leads to perched water
tables. Debris flows larger than 600 m2 in area appear to be unrelated to logging practices. Slump flows, described here for the first time, occur on similar slope
angles and stratigraphic situations as debris flows. They differ mainly by the presence of semiconsolidated material, usually
till, at the slide head. Where till is breached—commonly along road cuts—water infiltration is increased, saturating underlying
fine-grained deposits, which then fail by debris flowage. Secondary slumping of till happens when the slope steepens during
debris flow failure.
Small landslides surrounding Lake Shannon may contribute up to 80% of the total particulate matter yield to the fluvial system
at present, increasing lake sedimentation by a rate of 5 mm/yr. |
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