| On May 18, 1980, Mount St. Helens violently erupted, sending billions of cubic yards of mud, ash, rock fragments, and debris down the North and South Forks of the Toutle River, the Cowlitz and Columbia Rivers, and other streams. A total of 35.6 million cubic yards of mudflow material was deposited in the lower Cowlitz (downstream from the Union Pacific Railroad bridge) and Columbia Rivers. The arrival of this material at the Columbia River, during a period of the daily tidal cycle, when the flow was at a minimum or quite probably reversed and flowing upstream resulted in its deposition in an area 7 miles upstream and 2 miles downstream from the junction of the Cowlitz and Columbia Rivers. The thickness of the deposit was at a maximum near Coffin Rock, where a hole 140 feet deep was filled with 90 feet of sediment. Incomplete filling, compaction, and (or) erosion caused a l0-foot depression to form in the surface of the newly deposited material at this location. On May 23, 1980, the presence of two scour channels on each side of a tonguelike deposit (delta) of very fine sand and silt indicates that the coarse-grained sediment and water surge on the morning of May 19 initially scoured the lower 0.5 mile of the Cowlitz River bed. A minimum of 0.4 million cubic yard of material was scoured from the Cowlitz River bed and subsequently redeposited in the Columbia River before the 1.3-million-cubic-yard delta was formed. -- Haeni, 1983 |
