Particle-size distributions of hundreds of suspended-sediment samples were determined routinely during the period 1980-90 with fall-velocity and sieving methods (Guy, 1969). Size distributions of suspended sediment sampled at gaging stations are plotted in figures 78 , 79 , 80 . Median particle sizes at all stations ranged from fine silt to fine sand. Although exploratory analyses were made to detect relations between stream discharge, sediment size, and time, no obvious trends were found. Other extensive studies of particle-size relations with discharge have acknowledged the complexity of the relations, with inconclusive results (Ashmore, 1986; Walling and Moorehead, 1987).
An inverse relation between concentration and median particle size can be demonstrated for suspended-sediment samples collected at the Toutle River at Tower Road. Concentrations were divided into two groups, with 20,000 mg/L as the division line ( fig. 81 ). Median particle sizes in the group 20,000 to 128,000 mg/L were often finer than the group 2,010 to 19,800 mg/L. In the lower concentration group, the average of median diameters was 0.093 mm, compared with an average of median diameters of 0.064 mm in the higher concentration group. Increases in fine concentration during storm flow were described in the section "Peaks and Lags of Sediment Concentrations."
