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<br /> <br />FLOODING PROCESSES AND ENVIRONMENTS ON ALLUVIAL FANS <br /> <br />37 <br /> <br /> <br />FIGURE 2-5 The upper part of this debris-flow fan in Owens Valley, California, shows a classic rough. bouldery <br />surface. Courtesy of T. Dunne. <br /> <br />fan as the declining gradient reduces the conveyance capacity of the channel, forcing flow <br />overbank. The result is a smooth surface with only an occasional boulder on the lower parts of a <br />debris flow fan. On debris flow fans, streams are often confined to nondiverging, boulder-lined <br />channels left by the debris flows, and therefore they neither shift across the fan nor overtop the <br />banks in most cases, except on the lower parts of the fan where shallow channels were originally <br />formed by the dilute, low-viscosity flows described above. Of course, if the debris lining the <br />channels is gravelly rather than bouldery, the capacity for channel shifting and eventual <br />realignment by water floods is greater. <br />Many channels on debris flow fans are single-thread depressions blocked at their upper <br />ends by bouldery accumulations, so they are never invaded by stream floods or debris flows. Like <br />alluvial fans, debris flow fans are subject to varying amounts of deposition and parts or even much <br />of the fan may be inactive under the present climate. For example, the debris flow fans emanating <br />from the east side of the Sierra Nevada in the northwestern part of Owens Valley have more or <br />less ceased to accumulate since the end of the last glaciation in the mountains, and the oldest parts <br />of the fans date from previous glaciations. Parts offans debouching from the unglaciated southern <br />Sierra and from the White Mountains on the western side of Owens Valley continue to receive <br />