<P> 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 - high (4 m) 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 . </P> <P> 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 m) of material were injected into the Columbia River, reducing the river's depth by 25 feet (8 m) for a 4 - mile (6 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 million cubic yards (50 million cubic metres) of sediment were dumped along the lower Cowlitz and Columbia Rivers . </P> <P> The May 18, 1980, event was the most deadly and economically destructive volcanic eruption in the history of the United States . Approximately fifty - seven people were killed directly from the blast and 200 houses, 47 bridges, 15 miles (24 km) of railways and 185 miles (298 km) of highway were destroyed; two people were killed indirectly in accidents that resulted from poor visibility, and two more suffered fatal heart attacks from shoveling ash . U.S. President Jimmy Carter surveyed the damage and said it looked more desolate than a moonscape . A film crew was dropped by helicopter on Mount St. Helens on May 23 to document the destruction . However, their compasses spun in circles and they quickly became lost . A second eruption occurred the next day (see below), but the crew survived and were rescued two days after that . The eruption ejected more than 1 cubic mile (4.2 km) of material . A quarter of that volume was fresh lava in the form of ash, pumice and volcanic bombs while the rest was fragmented, older rock . The removal of the north side of the mountain (13% of the cone's volume) reduced Mount St. Helens' height by about 1,280 feet (390 m) and left a crater 1 to 2 miles (2 to 3 km) wide and 2,100 feet (640 m) deep with its north end open in a huge breach . </P> <P> More than 4,000,000,000 board feet (9,400,000 m) of timber was damaged or destroyed, mainly by the lateral blast . At least 25% of the destroyed timber was salvaged after September 1980 . Downwind of the volcano, in areas of thick ash accumulation, many agricultural crops, such as wheat, apples, potatoes and alfalfa, were destroyed . As many as 1,500 elk and 5,000 deer were killed, and an estimated 12 million Chinook and Coho salmon fingerlings died when their hatcheries were destroyed . Another estimated 40,000 young salmon were lost when they swam through turbine blades of hydroelectric generators after reservoir levels were lowered along the Lewis River to accommodate possible mudflows and flood waters . </P>

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