The annual mean yearly runoff in western Washington during the 1980 water year ranged generally from 80 to 90 percent of normal, except for the Olympic Peninsula, which was about 100 percent of normal. Precipitation was deficient throughout the area during October, November, January, and May, and was excessive in the northern half of western Washington during December.

Floods were experienced at many streams on the Olympic Peninsula in December. The heaviest flows (approximately 50-70 year recurrence intervals) occurred December 17 on Calawah and Bogachiel Rivers; the peak flow of Hoh River was between 25- and 50-year recurrence interval. Chehalis River, a lowland stream in the southern portion of the region, had below normal runoff, except for October, December, and April, and annual runoff was 89 percent of normal. Skykomish River, a mountainous stream of the northern Cascade Range, had an annual runoff that was 87 percent of normal and monthly flows were below normal except for December (200 percent normal), February to April, and September.

On May 18, 1980, Mount St. Helens violently erupted. During the eruption, a massive debris avalanche, moving down the north side of Mount St. Helens, was blasted into the North Fork Toutle River valley, depositing approximately 3 billion cubic yards of material in the upper 17 miles of the valley. Mudflows quickly developed in the South Fork Toutle River and in the Lewis River tributaries of Smith Creek, Muddy River, and Pine Creek. At least 11,000 acre-ft of water, mud, and debris were deposited in Swift Reservoir between 9 a.m. and noon on May 18. A massive mudflow originated on the debris pile in the North Fork Toutle River valley and caused widespread destruction as it moved downstream through the Toutle and Cowlitz Rivers. Considerable deposition occurred in these river channels and in the channel of the Columbia River, which was closed to shipping for about 1 week. Channel capacity of the Cowlitz River was reduced from 76,000 to 7,300 cubic feet per second (at the flood stage of 23.0 feet). Dredging of the shipping channel in Columbia River commenced shortly after May 18, and dredging was started in July in the lower reach of Toutle River and Cowlitz River.

Return to:
[Washington State Hydrology Menu] ...