The monitoring objective for most gaging stations was to obtain enough information to compute representative daily sediment discharges. For computation of daily sediment discharge, records of stream discharge and sediment concentration are multiplied and integrated over time. In unstable alluvial channels, sediment discharge is only poorly predicted by stream discharge. Therefore, simultaneous measurements of sediment concentration and water discharge are made as necessary for reliable records of daily sediment discharge.
The frequency of sediment-discharge measurements was adjusted as (1) concentrations increased with high flows, (2) concentrations decreased at low flow with the formation of pavement (a stable strembed surface, depleted of finer sediments), and (3) concentrations were reduced substantially by sediment-control measures. For example, sediment-discharge measurements were made daily in the lower Toutle River in the weeks immediately following the 1980 eruption. As the day-to-day variation in sediment concentration became better anticipated, measurements during low flow were made biweekly or monthly. Automatic pumping samplers and prolonged stability of streambeds in later years also permitted a reduced measurement frequency.
Complete records of daily sediment discharge for water years 1982-90 were computed for gaging stations on the Green River and on the North Fork, South Fork, and mainstem Toutle Rivers. The combined records of two stations on the Muddy River encompass the same 1982-90 period. Computed sediment discharge records for other streams near Mount St. Helens cover shorter time periods. Records of daily sediment discharge end with water year 1984 for Pine Creek at mouth, near Cougar and the Cowlitz River at Castle Rock. Periods of sediment-discharge records at gaging stations are given in table 1 .
