The sediment source of greatest concern was deposited along the North Fork Toutle River, where the upper 17 mi of the river valley was buried by the debris avalanche from Mount St. Helens. The deposit contained more than 2 mi3 of unconsolidated sediment (Meyer and Janda, 1986). Not only did the enormous deposit provide sand and gravel for continual transport, but several lakes were formed on tributary channels that were blocked by the debris-avalanche deposit. Closed depressions on the irregular surface of the debris-avalanche deposit filled with water to form ponds. The channel blockages and ponds could not be regarded as stable, and the area was closely monitored while sediment and flood-control projects were constructed to reduce the hazards to areas downstream. Temporary dams to retain sediment were constructed on the North Fork Toutle River (N1) and the South Fork Toutle River (S1) by the U.S. Army Corps of Engineers in 1980. A permanent sediment-retention structure (SRS) was constructed on the North Fork Toutle River and was first closed in November 1987.
Volcanic debris flows from the upper slopes of Mount St. Helens entered the channels of the South Fork Toutle River, Pine Creek, the Muddy River, and several tributary channels. The debris flows, or "lahars" (the Indonesian term for a rapid, transient flow of sediment and water from a volcano), were generated during the 1980 eruption from melting snow and glacial ice. Peak flow of the lahar in the South Fork Toutle River occurred within minutes of the eruption on May 18, 1980. Deposit thicknesses ranged from 3 ft in middle reaches to 7 to 13 ft in the broad alluvial reach near the mouth (Janda and others, 1981). Deposits in Pine Creek and the Muddy River were poorly sorted mixtures of clay-sized to boulder-sized particles that attained thicknesses up to 8 ft (Pierson, 1985). As the debris-avalanche deposit in the North Fork Toutle River dewatered during May 18, 1980, an immense lahar formed, which flowed down the Toutle River in the afternoon and left extensive deposits along the banks of the Toutle and the lower Cowlitz Rivers (see also "Sediment Discharges of Lahars").
The drainage basin of the Green River, a tributary of the North Fork Toutle River, was partially deforested and was blanketed with tephra (volcanic ash, pumice, and blocks) and blast material by the 1980 eruption. Clearwater Creek, a tributary to the Muddy River, was also blanketed with tephra, and forests in its upper reaches were devastated by the eruption blast. Stream runoff from the tephra-affected basins showed elevated sediment concentrations for several years following the 1980 eruption.