Seven years of sediment samples collected at the Toutle River at Tower Road show the horizontal distribution of sediment concentration at a cross section. Using the EDI method, each sample was collected at the centroid of a 20-percent discharge increment. Sediment concentrations at five centroids were analyzed for 183 cross-section samples from the period September 30, 1983, to September 1990. Samples at each centroid were divided into sand and fine concentrations. Mean sand and fine concentrations for each set of five cross-section samples were computed from sediment-size data. The centroid values differ from the mean concentrations by the percentages shown in figure 18 . Fine concentrations are distributed evenly across the channel; sand concentrations at any particular centroid often differ greatly from the mean.
Concentrations of cross-section samples ranged from below 10 to greater than 80,000 mg/L, so a possible relation between concentration and variable distribution of sands was examined, as follows. Sand concentrations were sorted into five ranges ( table 5 ). The relation was determined by computing the percent difference of concentration from the mean for each centroid using
where Cc is the sand concentration at each centroid, and Cm is the mean concentration of suspended sand in the cross section. The percent differences at centroids varied widely. The variation was expressed with the standard deviation of the percent difference within each concentration range. With increasing concentration, the mean of standard deviations at all centroids decreased from 43 percent to 29 percent. The distribution of sand in the cross section apparently became more homogeneous during high rates of sand discharge. Fine concentration was better represented by single vertical sampling, including pumped samples, because fine sediment was well distributed across the sampling section. Fine concentration may have been less influenced by local bedforms and turbulence than was sand concentration.