1980 Eruption of Mount St. Helens - Mudslides Flow Downstream

Mudslides Flow Downstream

The hot, exploding material also broke apart and melted nearly all of the mountain's glaciers along with most of the overlying snow. As in many previous St. Helens' eruptions, this created huge lahars (volcanic mudflows) and muddy floods that affected three of the four stream drainage systems on the mountain, and which started to move as early as 8:50 a.m. Lahars travelled as fast as 90 miles per hour (140 km/h) while still high on the volcano but progressively slowed to about 3 miles per hour (4.8 km/h) on the flatter and wider parts of rivers. Mudflows from the southern and eastern flanks had the consistency of wet concrete as they raced down Muddy River, Pine Creek and Smith Creek to their confluence at the Lewis River. Bridges were taken out at the mouth of Pine Creek and the head of Swift Reservoir, which rose 2.6 feet (0.79 m) by noon to accommodate the nearly 18,000,000 cubic yards (14,000,000 m3) of additional water, mud and debris.

Glacier and snow melt mixed with tephra on the volcano's northeast slope to create much larger lahars. These mudflows traveled down the north and south forks of the Toutle River and joined at the confluence of the Toutle forks and the Cowlitz River near Castle Rock, Washington, at 1:00 p.m. Ninety minutes after the eruption, the first mudflow had moved 27 miles (43 km) upstream where observers at Weyerhaeuser's Camp Baker saw a 12-foot (3.7 m) high wall of muddy water and debris pass. Near the confluence of the Toutle's north and south forks at Silver Lake, a record flood stage of 23.5 feet (7.2 m) was recorded.

A large but slower-moving mudflow with a mortar-like consistency was mobilized in early afternoon at the head of the Toutle River north fork. By 2:30 p.m. the massive mudflow had destroyed Camp Baker, and in the following hours seven bridges were carried away. Part of the flow backed up for 2.5 miles (4.0 km) soon after entering the Cowlitz River, but most continued downstream. After traveling 17 miles (27 km) further, an estimated 3,900,000 cubic yards (3,000,000 m3) of material were injected into the Columbia River, reducing the river's depth by 25 feet (7.6 m) for a 4-mile (6.4 km) stretch. The resulting 13-foot (4.0 m) river depth temporarily closed the busy channel to ocean-going freighters, costing Portland, Oregon an estimated five million US dollars. Ultimately more than 65,000,000 cubic yards (50,000,000 m3) of sediment were dumped along the lower Cowlitz and Columbia Rivers.