Six days after the 1980 eruption, a gage house was installed (May 24, 1980) at the Highway 504 bridge over the North Fork Toutle River at Kid Valley. Gage height was recorded at either of two bridge piers at the site during the period of record. Discharge measurements were made at the bridge until spring 1981 when a cableway was constructed 950 ft downstream. Continuous daily sediment discharges begin with July 1981. Intermittent values of daily sediment discharge that were available for water year 1981 are plotted as solid circles ( fig. 33 ).
Sediment transport in the North Fork Toutle River was dominated by erosion of the debris-avalanche deposit from the 1980 eruption, with additional sediment derived from the thick lahar deposits. Sediment yield from the North Fork Toutle River was, for several years, the highest of any sizable stream in North America during the period of record, with an average sediment yield during 1982-84 of 101,000 tons/mi2. Extreme sediment concentrations (50,000 to 200,000 mg/L) persisted for several days during storm flows. At such times, steep river gradients and high sediment concentrations produced tall standing waves that were unusually smooth. For several years after the 1980 eruption, sand and fine gravel were deposited in the river channel following storm flow.
Rivers with unstable channels of sand and fine gravel are uncommon in the Cascades Range. Mountain streams with beds of coarse gravel are usually gaged on a bimonthly schedule, but the extreme sediment transport rates and unstable streambeds at Kid Valley required weekly or biweekly measurements to maintain accurate records.
As the U.S. Army Corps of Engineers expected, the sediment-retention dam N1 was soon overwhelmed by extremes of sediment discharge. The permanent SRS, constructed upstream from the confluence with the Green River, was closed during November 1987, and reduced sediment-discharge rates were attributed to deposition behind the SRS ( fig. 34 ).
Gaging stations were installed at several sites on the North Fork Toutle River upstream from the Kid Valley station. These included the Coldwater Lake exit canal and the North Fork Toutle River above Bear Creek near Kid Valley. Daily sediment discharges were not computed for these stations. Sediment samples were collected at the North Fork Toutle River above Bear Creek from 1984 to 1987. During that period, stream discharges above 1,000 ft3/s consistently produced sediment concentrations greater than 10,000 mg/L ( fig. 35 ). This station was located near the distal end of the debris-avalanche deposit and upstream from the SRS.
