| 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). -- Major and Scott, 1988 |
