River sedimentation caused by the May 18, 1980, eruption of Mount St. Helens, Washington, has been monitored in a continuing program by the U.S. Geological Survey. In this report, sediment discharge and changes in sediment transport are summarized from data collected at stream-gaging stations near Mount St. Helens during the years 1980 through 1990. The objectives of the monitoring program included collection of data for calculation of total sediment discharge, computation of daily suspended-sediment discharge, and detailed observations of unique sediment-laden flows. Over the 11-year period, most sediment data were collected at gaging stations on seven eruption affected streams: the Green River, the North and South Fork Toutle Rivers, the Toutle River, the Cowlitz River, Clearwater Creek, and the Muddy River.
About 170 million tons of sediment (excluding volcanic debris flows) were transported in suspension from the Toutle River basin during water years 1980–90. Another 13 million tons were transported past the gaging stations on Muddy River in the upper Lewis River basin during water years 1982–90. Long-term reductions in sediment concentration occurred within most ranges of stream discharge at streams dominated by transport from the debris-avalanche deposit and at streams in drainage basins with extensive airfall deposits. Reductions in sediment concentration were less apparent at upper ranges of discharge in two streams dominated by lahar deposits, the South Fork Toutle River and the Muddy River.
Bed material, suspended sediment, and bedload were sampled periodically and analyzed for size distributions. Bed material and bedload coarsened with time at some stations. Median particle sizes of suspended sediment did not show a simple relation with time. During water years 1980–84, bed material in the lower Toutle River was medium to coarse sand. During the same period, bed material in the North Fork Toutle River was coarse sand and fine gravel. By 1990, bedload samples collected in the North Fork Toutle River (downstream from the sediment-retention structure) were typically coarse gravel.
