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48 GEOMORPHICALL Y EFFECTIVE FLOODS Fig. 2_ Airphoto of the failure of Reservoir No_ 3. Centralia, Wash. (from Costa, 1994). I / 3. STREAM POWER, GEOMORPHIC WORK, AND CHANNEL CHANGES --- Stream power per unit boundary area (w) expressed in watts per square meter 0N/m'), is computed from w '" 'YQS/w where 'Y is specific weight of the fluid (9800 N/m' for clear water), Q is discharge, S is energy slope, and w is water-surface width. Peak stream pO'fer per unit bound- ary area at the sites of indirect-discharge estimales for the Centralia flood, 3,300 W/m', and for the Porter Hill flood, 2,900 W/m', are among the largest values ever documented for historic flows [Baker and Costa, 1987]. The historic floods in other basins that generated similar or smaller values of peak stream power per unit bound- ary area all were considered geomorphically effective according to our inspections, and the authors' reports. In contrast, in spite of the magnitudes of the peak stream power of the two dam-failure floods in Oregon and Washington, the erosional effects on downstream alluvial channels were unimpressive. Characteristics of the original channel at Centralia are not known. The floodplain is about 20 m wide and bounded by a bedrock ridge on one side, and roadfill on the other. The slope is about 0.09, and the surface is grass-covered and regular, with three or four widely-spaced large trees. Floodplain sediment consists of gravel and cobbles in a silt and clay-rich matrix. During the flood, the original channel in the small valley was enlarged, and a l.5-m headOllt formed. The 20-m-wide floodplain was entirely inundated by about one meter of water flowing at aDout 4.2 mis, but neither the floodplain nor floodplain vegetation were destroyed or greatly modified (Figure 3). Most of the visible change in the floodplain is attributable to deposition of coarse sediment from the preceding debris flow, not the water flood. At Porter Hill, the floodplain slopes at about 0.10 and consists of open forest and moss-covered stumps. The surface is covered with leaf litter. ferns, and a few fallen trees. Floodplain sediment consists of poorly-drained and unstratified gravel, silt, and clay. Following the flood at Porter Hill, moss was still intact on the upstream side of trees below high-water marks, and ferns and leaf litter were virtually undisturbed. It was nearly impossible to tell that a large flood, 10 m wide, and 1 m deep, had recently -