Mudflow Hazards Along the Toutle and Cowlitz Rivers from a Hypothetical Failure of Spirit Lake Blockage [open pdf - 394KB]
From the Abstract: "The debris avalanche accompanying the May 18, 1980, eruption of Mount St. Helens, in southwestern Washington, buried the former outlet channel of Spirit Lake, located 5 miles north of the volcano, to a depth ranging to 500 feet. Since that time, Spirit Lake has had no natural outlet and its water surface and contents have increased significantly. Recent studies of the debris dam stratigraphy and soil properties, and of erosion on the surface of the blockage, have led to concern that the lake may someday breach through or spill over the top. A study was made by the U.S. Geological Survey (USGS) to determine the extent of inundation that might result downstream in the Toutle and Cowlitz Rivers if a hypothetical breach should occur and generate a mudflow flood of catastrophic proportions."
Water-Resources Investigations Report 82-4125
U.S. Geological Survey: https://www.usgs.gov/