Sediment loads in the North Fork Toutle River and the mainstem Toutle River were dominated by sediment transport from the debris-avalanche deposit. Sediment concentrations at the North Fork Toutle River at Kid Valley and the Toutle River at Tower Road are compared in figure 68 . At both stations, maximum sediment concentrations exceeded 100,000 mg/L during 1981-84. During the storm flows of 1980-81, debris-avalanche deposits slumped into the flow intermittently, several cubic meters at a time. Downstream from the debris-avalanche deposit, channel banks composed of lahar deposits and pre-existing soils were eroded and delivered to the flow. Mass wasting of channel banks probably sustained the extreme sediment concentrations.
The beginning of a gradual decrease in maximum sediment concentrations is apparent in 1985 and 1986 (fig. 68). Sediment-control works, constructed by the U.S. Army Corps of Engineers in 1980 and 1981 along the North Fork, the South Fork, and the mainstem Toutle Rivers, had temporary effects on sediment transport. However, substantial decreases of sediment concentration were measured downstream from the permanent SRS after its closure in November 1987.
In summer 1981 at the North Fork Toutle River at Kid Valley, sampled concentrations approached 10 mg/L during low flow; such low concentrations were not measured again until the ponding of the North Fork Toutle River by the SRS in 1987. The envelopes of sediment concentration for the North Fork and the main
stem Toutle Rivers enclose the minimum concentrations along a sustained plateau (fig. 68). The plateau appears where sediment concentrations rarely dropped below 1,000 mg/L from water years 1982 to 1987. During the same 5-year period, maximum sediment concentrations declined from more than 100,000 mg/L to less than 50,000 mg/L.
The sustained plateau of sediment concentrations at low flow indicates that sediment was perpetually available for transport. The similarity between the two envelopes of sediment concentration shows that transport rates in the mainstem Toutle River were dominated by sediment sources in the North Fork Toutle River.
