The Green River basin, to the north of Mount St. Helens, and the Clearwater Creek basin, to the east-northeast, were both blanketed with airfall tephra, ash, and blast deposits. These two drainage basins, which do not head at Mount St. Helens, were isolated from the effects of large lahars. Storm flow was not sampled and measured in the Green River until June 1981; storm flows in Clearwater Creek were sampled only on November 8, 1980, and in February 1982 during water years 1981-82. The initial magnitude of sediment transport from these drainage basins is therefore uncertain.
The envelopes of sediment concentration for both stations maintained a similar width over the study period, with a parallel decline in both the maximum and minimum concentrations ( fig. 70 ). The envelopes are broader and more uniform than those of gaging stations on streams dominated by the debris-avalanche deposit. Minimum sediment concentrations often reached 1 mg/L, whereas sampled sediment concentrations did not exceed 10,000 mg/L after 1981. The tephra provided sediment for transport, but did not produce high sediment discharges after initial erosion by the storms of 1980-81.
