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Mount Rainier volcano dominates the landscape of a large part of western
Washington. It stands nearly 3 miles higher than the lowlands to the west and
1.5 miles higher than the surrounding mountains. The base of the volcano
spreads over an area of about 100 square miles, and lava flows that radiate from
the base of the cone extend to distances of as much as 9 miles. The flanks of
Mount Rainier are drained by five major rivers and their tributaries. Clockwise
from the northwest the major rivers are the Carbon, White, Cowlitz, Nisqually,
and Puyallup. Each river flows westerly through the Cascade Range and, with the
exception of the Cowlitz, empties into Puget Sound near Tacoma, Washington. The
Cowltiz joins the Columbia River in the southwestern part of the State to flow
to the Pacific Ocean.
Each major river in Mount Rainier National Park occupies a deep canyon whose floor is 1,000-3,000 feet below the adjacent divides. Valley-floor gradients are 100-400 feet per mile near the park boundaries and increase markedly upstream. The valley floors of Tahoma Creek, the North and South Puyallup Rivers, and the Mowich River have gradients of 700-800 feet per mile in their upper reaches and are among the steepest in the park. The volcano's summit towers 9,000-11,000 feet above valley floors only 3-6 miles away. The flanks of the volcano itself have slopes mostly between 25 degrees and 30 degrees, although those of Willis Wall on the north side are between 40 degrees and 45 degrees. Partly because of its position astride a high, dissected part of the Cascade Range, Mount Rainier does not have broad peripheral aprons of laharic and fluvial deposits like those fringing the base of Mount Shasta in northern California and some large stratovolcanoes elsewhere. Instead, large lahars originating on the volcano in Quaternary time repeatedly flowed far down the canyons of the Cascade Range, and some came to rest beyond the mountain front in parts of the adjoining Puget Sound lowland (Crandell, 1963b). Within the mountains, lahars typically are found on valley floors, although the deposits of some of the largest lahars also veneer valley walls to heights of hundreds of feet. Some even mantle divides that are a thousand feet or more above valley floors near the volcano. Lahars that formed within the last thousand years commonly form low terraces or veneer other unconsolidated deposits that form the terraces. Older lahars are often found interbedded with fluvial terrace deposits, duff horizons and stumps that represent old forest floors, and thin layers of pyroclastic material. |
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