When the ash column from Mount St. Helens subsided on the evening of May 18, 1980, the valleys below had been reshaped with enormous lahars and the broken remnants of the mountain. To assess the immediate hazards and to anticipate the future of the altered landscape, the U.S. Geological Survey prepared a complex program of sediment-transport monitoring. Ideally, sediment discharge from the devastated lands would be monitored by synoptic, continuous data collection at most gaging stations for discharge and sediment concentration. Logistical constraints of personnel and equipment often restricted data collection to three or four stations when sediment discharges were high, and only intermittent measurements could be obtained. Of three basic objectives set for sediment-transport monitoring, computation of daily suspended-sediment discharge was the most successful. Research-quality data collected during several unique sediment-laden flows fulfilled another objective. Sediment-transport data were collected at all gaging stations for total sediment discharge; much of the organized and published data is suitable for sediment-transport research.
Analysis of sediment-transport data from streams near Mount St. Helens has produced regional information on the magnitude of sediment discharge and the return toward pre-eruption water quality. After the lahars of May 18-19, 1980, about 170 million tons of sediment had been transported in suspension from the Toutle to the Cowlitz River by September 30, 1990. This amounts to less than 10 percent of the sediment in the debris-avalanche deposit. Sediment concentrations at similar discharges decreased over the study period, as did the annual sediment discharges at gaging stations. During storm flow in 1990, streams with sediment loads dominated by debris avalanche or lahar deposits reached sediment concentrations that still exceeded pre-eruption levels.
Data collection for sediment discharge records included repetitive measurements of stream discharge and sediment concentration. Accuracy of sediment discharge records was increased by obtaining frequent measurements during periods of extreme sediment transport. Data collected during storm flows gave good estimates for peak sediment concentration and for changes in stage-discharge relations. Because data were often collected synoptically at several gaging stations, sediment budgets and travel times of flood waves could be calculated with reasonable accuracy.
Sediment transport in the North Fork Toutle River and the Toutle River was dominated by erosion of the debris avalanche and associated lahar deposits. Annual sediment discharge from the Toutle River basin in water year 1990 was 6 percent of that in water year 1982. Maximum sediment concentrations were near 100,000 mg/L through water year 1984. Minimum concentrations decreased below 100 mg/L after the closure in 1987 of the North Fork Toutle River SRS. Typical storm-flow concentrations decreased by an order of magnitude over the study period.
Two lahar-dominated streams, the South Fork Toutle River and the Muddy River below Clear Creek, decreased in annual sediment discharges between 1982 and 1990. Annual sediment discharge of the Muddy River below Clear Creek in 1990 was one-fifth of that in water year 1982. During storm flows in water year 1989, both streams produced sediment concentrations that were near those measured in water years 1982-84. At the South Fork Toutle River, sporadic changes in annual sediment discharge, and the high sediment discharge of 1990, suggest that sediment-transport rates were sustained by continued erosion of lahar deposits and bank material.
Two blast- and ashfall-affected drainage basins, the Green River and Clearwater Creek, may have had high sediment discharges in water year 1981, but daily values were not computed. Subsequent water years were marked by a gradual decrease in annual sediment discharges. Both maximum and minimum sediment concentrations in the Green River and Clearwater Creek decreased by less than a factor of 10 between 1982 and 1990.
Instrumentation, field methods, and data-processing techniques were developed by the Cascades Volcano Observatory to handle the extremes of sediment transport and the copious sediment data. These improved methods and instruments provided more adequate tools for acquisition of sediment data on large rivers than were previously available. Ample field experience was gained with automatic sampling and with sturdy installations of redundant gage orifices. Motorized staylines were used for restraint of cable-suspended equipment in storm flows. Experimental instruments for sediment data collection were deployed at gaging stations near Mount St. Helens; some instruments were retained for routine use.
Urgent demands for sediment data were met with improved methods of data processing. Laboratory sediment data were computerized for automatic retrieval in computation of sediment discharge and for sediment-transport research. Automated transfers of sediment data, from laboratory to staff to the database WATSTORE, minimized the time spent on data processing.
Sediment data collected at gaging stations near Mount St. Helens between 1980 and 1990 have provided quantitative answers to questions about sediment transport by storm flows and about long-term changes in sediment transport of streams affected by volcanic debris flows. Further examination of the available sediment data by river engineers and earth scientists of all disciplines is welcomed.