The reduction of annual sediment discharge in the Toutle River and the upper Lewis River basins was evident over the study period. The daily records of sediment discharge show that a large percentage of the annual sediment discharge was delivered on a relatively small number of days, which were usually days of storm flow. During storm periods, stream discharge and sediment discharge increased together. Therefore, a decrease in annual sediment discharge may be explained by examining sediment discharges during storm flows over the study period.
Using the daily values of mean stream discharge and sediment discharge, changes in the two variables can be examined for the period 1982-90. The highest 1 percent of daily discharges in the period at each gaging station are plotted with the corresponding daily sediment discharge by time (figs. 51 and 52 ). Sediment discharges at high stream discharges decreased over an order of magnitude at the North Fork Toutle River at Kid Valley and the Toutle River at Tower Road (fig. 51). At the Green River above Beaver Creek, the South Fork Toutle River at Camp 12, and the Muddy River above and below Clear Creek, time trends were not obvious. Instead, peak sediment discharges showed a marginal tendency to decrease with time, except for the South Fork Toutle River at Camp 12.
The non-parametric Kendall tau analysis of the two variables showed that, although the peak stream discharges did not have a significant, monotonic correlation with time, several stations did show significant, negative correlations between peak sediment discharge and time ( table 9 ).
