The cataclysmic eruption of Mount St. Helens, in southwestern Washington, on May 18, 1980, produced devastating mudflows that delivered large quantities of debris to the lower reaches of the Cowlitz River and the Columbia River near Longview, Wash. To obtain information on the character of the "new" sediment and its disposition in the rivers, data were collected during a 9-day period in mid-August 1980. Measured suspended-sediment discharges indicate that material is being scoured from the reach of the Columbia River directly seaward from the Cowlitz River mouth at Columbia River mile (CRM) 68.0, and that some of the scoured material is redeposited between CRM 63.8 and CRM 54.0. Most material in transport is of silt and clay size. Pumice particles larger than 2.0 millimeters in diameter are randomly distributed throughout the bed material (bottom sediment). Mean size of bed material finer than 2.0 millimeters in the reach directly seaward from the Cowlitz River mouth is less than half the size of similar material directly upstream from the mouth and is somewhat finer than pre-eruption material. Specific gravity of bed material finer than 2.0 millimeters in the reach below CRM 73.5 is about 2.65, and the majority of particle-shape factors (determined indirectly) are between 0.5 and 0.8.
These values are similar to comparable values for bed materials at CRM 106.4. On the basis of this information, corrections for unusual sediment properties are not necessary in sediment-transport or dredging-earthwork computations for the Columbia River.
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