Of the sediment-inundated streams around Mount St. Helens, by far the highest sediment-transport rates occurred in the Toutle River. Four gaging stations were operated at least from 1982 to 1990 in that basin, at the Green River, the North and South Fork Toutle Rivers and on the mainstem Toutle River. Near the mouth of the Toutle River, the gradient is about 16 ft/mi (0.003). The Cowlitz River, which receives the Toutle River, has a drainage area of 2,240 mi2 at Castle Rock, and the river gradient (0.0003) is about one-tenth that of the Toutle River.
Sediment deposition in the slower moving Cowlitz River from lahars and storm flows in the Toutle River reduced flow capacity to 13,000 ft3/s and disrupted water supplies. Dredging of the channel commenced immediately to accomodate potential flood flows of 50,000 ft3/s. The Cowlitz River had been the primary water supply for the riverside communities of Castle Rock, Kelso, and Longview. Several weeks passed before the communities regained use of the water supply, and the highly sedimented water required expensive treatment. As dredging of sediment from the Cowlitz River continued, channel flood capacity was evaluated by the U.S. Army Corps of Engineers with computer models of sediment transport (Brown and Thomas, 1981). Mudflows that might be generated by pyroclastic flows onto snow threatened to exacerbate the deposition problem. To address these concerns (Dunne and Leopold, 1981), the U.S. Geological Survey collected sediment data at gaging stations on the lower Toutle and the Cowlitz Rivers. For the first few years after the 1980 eruption, data collection at Toutle River at Highway 99 and Cowlitz River at Castle Rock was given priority over stations nearer Mount St. Helens, to assist in flood warning for the flood-prone communities.