During its approximately 50,000 year history, Mount St. Helens episodically has contributed large volumes of granular volcaniclastic sediment into the Lewis River valley and its tributaries. Although much of this sediment resulted in thick valley fill near the volcano, lahars (volcanic debris flows) did travel long distances down the Lewis River valley. Deposits of large lahars coat valley walls at least 50 km (kilometers) downstream from the volcano to heights of more than 30 m (meters) above the present valley floor. Similar slope-mantling deposits (dated at 3,920 +/- 365 radiocarbon years) have been identified at a distance of 35 km from the volcano. These deposits clearly indicate that valley-filling lahars from relatively recent eruptive periods traveled long distances down the Lewis River valley. Paleohydrologic analyses suggest that these flows had discharges of more than 100,000 m3/s (cubic meters per second).
Flow transformation has been an important sedimentologic process in the Lewis River valley. Downstream from Mount St. Helens, lahars generally transformed from dense, high-strength debris flows to less-sediment-rich runout flows (hyperconcentrated streamflow evolved from a distal lahar). Beyond 50 km, the transformations of lahars from Mount St. Helens accelerated in response to valley widening. Consequently, sedimentation at the mouth of the Lewis River valley was dominated by fluvial processes. The process of flow transformation in the downstream decay of lahars in the Lewis River valley also was influenced by the noncohesive nature of the lahars (less than 2 percent clay). Many lahars in the valley probably did not originate as fully developed mass flowage on the slopes of the mountain, but rather as water surges or underdeveloped lahars that bulked by erosion of stream alluvium from the tributary valleys draining the mountain.
Because the crater of Mount St. Helens now faces north, lahars are unlikely to affect the southerly draining Lewis River valley during the present eruptive period, unless large-scale explosive activity resumes or the location of the vent changes. If lahars generated as a direct result of an eruption threaten Swift Reservoir, they probably will not exceed a volume of 170 million m3 (cubic meters).
Although lahars constitute a potential hazard to the Lewis River valley, the constraining design event during a future eruptive period is a debris avalanche. Studies conducted since 1980 indicate that debris avalanches have occurred several times in the volcano's brief history. A debris avalanche of even one-half the size of that of 1980 would probably have devastating consequences if it were to occur in the Lewis River drainage basin.
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