During the 1980 eruption, glaciers and snowfields at the head of the South Fork Toutle and the Muddy Rivers were partially melted, which generated destructive lahars in those channels (Lipman and Mullineaux, 1981). Widespread erosion of lahar deposits began during storm flow on November 6-8, 1980. Samples collected near peak discharge approached 100,000 mg/L in the South Fork Toutle River and 180,000 mg/L in the Muddy River. The range of maximum concentrations decreased to around 10,000 mg/L by 1984. Storm-flow concentrations still reached that range in 1990 at both streams. The envelopes of sediment concentration for both stations show a gradual expansion as minimum concentrations decreased to between 1 and 10 mg/L ( fig. 69 ).
Sediment concentrations increased suddenly and briefly in late summer at the Muddy River and the South Fork Toutle River at Camp 12, particularly during 1987 through 1990. Glacial meltwater and snowmelt would have been the likely sources of stream discharge at those times. There was no direct observation of sediment input to the streams, although occasional mass wasting seems likely in the steep upland valleys.
