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Almost annually, torrential rain, glacial outbursts, and water-saturation of steep debris-covered slopes cause debris flows at Mount Rainier. Debris flows such as these develop when floods of water generated by meteorological or hydrologic events erode and incorporate the unconsolidated glacial and volcanic debris that mantles the slopes and upper drainages of the volcano. Debris flows though similar in character to eruption-related lahars are usually much smaller than lahars and flow only a few kilometers. Even the largest debris flows rarely move farther than the boundaries of Mount Rainier National Park.
Debris flows have been common in the Nisqually River drainage in the past several centuries. Between 1926 and 1955, debris flows damaged or destroyed bridges across the Nisqually River at least four times. In October 1932, glacial sediment became water saturated during heavy rain, collapsed, and caused a debris flow that destroyed the bridge downstream. A new concrete bridge with a clear span of 25 m (80 ft) survived floods in 1947 but was destroyed by a debris flow in October 1955 when heavy rains triggered an outburst flood.
The October 2-3, 1947 Kautz Creek was the site of the most voluminous debris-flow event of historical time; it covered the road with 28 feet of mud, rocks, and tree debris. Very heavy rainfall started early on October 1, and the first debris flow occurred October 2 between 10 and 11 p.m. A succession of debris flows moved down valley during the night ending at 8 a.m. October 3. An estimated 40 million m3 (52 million yd3) of debris was deposited in a fan along the Nisqually River that extended up Kautz Creek valley northward across the highway.
Between 1985 and now, more than 30 debris flows have rushed down Tahoma Creek valley. Glacial outbursts from South Tahoma Glacier during hot dry weather caused most of the debris flows, but torrential rainstorms, which are especially common in the fall, caused several others.
In summer 2001 during a period of fair weather, debris flows in Van Trump Creek caught everyone by surprise when they came roaring under a highway bridge and past a popular campground. The cause of the debris flows was a small stream of water from Kautz Glacier that topped a drainage divide and saturated glacial sediments of upper Van Trump Park on the volcano's south side. Water-saturated debris slid away in succession to form a series of debris flows that swept more than four miles down Van Trump Creek to the confluence of the Nisqually River, raising the stream channel to an elevation above the nearby roadbed.