Complete sediment-discharge records are available for the mainstem Toutle River for the period May 18, 1980, to September 30, 1990. Records were computed for the Toutle River at Highway 99 (May 18, 1980, to September 30, 1982; fig. 38 ) and the Toutle River at Tower Road (June 8, 1981, to September 30, 1990) ( fig. 39 ). Samples collected on May 18 and 19, 1980, were used to estimate the sediment discharge of the Toutle River lahars at about 153 million tons over those 2 days. Suspended-sediment discharge from the Toutle River for water years 1981-86 totaled about 152 million tons, which nearly equals the estimate for the lahars of May 18-19, 1980. Excluding lahars, sediment discharge from the Toutle River for the period 1980-90 was 167 million tons.
The Highway 99 bridge over the Toutle River near the mouth was used for monitoring sediment input to the Cowlitz River, but the site (Toutle River at Highway 99 bridge) had practical deficiencies. The bridge site was abandoned temporarily in September 1980 because of the uneven flow approach to the bridge and unstable stage-discharge relations. Operations were moved to a new gage house at the Toutle River near Silver Lake, which was established as a gaging site in 1929. The steep and narrow cross section had mean velocities greater than 7 ft/s even at medium flows. Stage-discharge relations also were unstable at the Toutle River near Silver Lake, and operations were resumed at the Toutle River at Highway 99 about December 2, 1980. High flows on December 26, 1980, at the Toutle River near Silver Lake undermined the river bank on which the new gage house was situated, and the house collapsed into the storm flow. Gaging operations continued at the Toutle River at Highway 99 through September 1982.
The gaging station at Tower Road was established March 5, 1981. The straight reach and constant width provided more stable stage-discharge relations than found at the other Toutle River gaging sites. Sediment-discharge measurements for the mainstem Toutle River were made at this station during the 9-year period 1982-90.
The sum of annual sediment discharges from the North Fork and the South Fork Toutle Rivers was lower than annual sediment discharge at Tower Road by 12 percent in 1982 and by 22 percent in 1983. Erosion of sediment deposits in stabilization basins near the Tower Road gaging station may have caused discrepancy beyond measurement error. To examine the discrepancy, daily sediment discharges at the North Fork Toutle River at Kid Valley for the period 1982-90 were subtracted from daily sediment discharges at the Toutle River at Tower Road for the same period. The differences are plotted by time in figure 40 . Positive differences were greater and more frequent during 1982-84. Negative differences (daily sediment discharge greater at Kid Valley than at Tower Road) were less frequent and were not apparent after 1987. The positive differences most likely resulted from sediment discharge from the South Fork Toutle River. By 1984, annual sediment discharge from the South Fork Toutle River was only 2 percent of that from the North Fork Toutle River.
