Post-Eruption Changes in Channel Geometry of Streams in the Toutle River Drainage Basin, 1980-82, Mount St. Helens, Washington

The May 18, 1980, eruption of Mount St. Helens, Washington, generated a debris avalanche, lateral blast, lahars, and tephra deposits that altered mainstem and tributary channels within the Toutle River drainage basin. Channel cross sections were monumented and surveyed on North Fork Toutle River and its tributaries, on South Fork Toutle River, on Green River, and on Toutle River in 1980 and 1981. These streams drain the north and west flanks of the volcano. The network of channel cross sections was surveyed more frequently following periods of higher flow. The repetitive cross-section surveys provide measurements of bank erosion or accretion and of channel erosion or aggradation. These data can be used to determine erosion rates, and to identify sources and storage sites of sediment in sediment budget computations. This report presents channel cross-section profiles constructed from the survey data collected during water years 1980 through 1982.