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velocities ranged from 3,3 to 12.6 feet per second, Traveltimes of the flood were determined from eyewitness accounts. The leading flood wave took 3.28 hours to travel 12,5 miles (average 3,8 miles per hour). Flood peaks were 2.1 to 30 times the 500-year flood for selected locations along the flood path. Geomorphic and sedimentologic evidence suggest that it probably was the largest flood in these basins, at least since the retreat of the glaciers several thousands of years ago, Geomorphic effects of the flood resulting from the dam failures were profound. Channels were widened tens of feet and scoured from 5 to 50 feet locally. In the Roaring River valley, alternate river reaches were either scoured or filled, depending on valley slope. Generally, reaches steeper than 7 percent were scoured, and reaches less than 7 percent were filled. In the Roaring River, 56 percent of the channel was scoured, some by as much as 50 feet, and 44 percent was filled with coarse sediments, 2 to 8 feet thick, An alluvial fan of 42,3 acres, containing 364,600 cubic yards of materi- al, was deposited at the mouth of the Roaring River. The fan has a maximum thi ckness of 44 feet and an average thi ckness of 5.3 feet, The 1 argest boulder known to have moved in the flood, 14x17.5x21 feet and weighing an est i mated 452 tons, was located on the all uvi a 1 fan. Down the flow axi s, average particle size changes from 7,5-foot boulders to fine sand and silt in a distance of 1,900 feet. The alluvial fan dammed the Fall River, forming a lake of 17 acres upstream from the fan. Satisfactory results were obtained from the dam-break model, but not without significant difficulties in proper operation of the model. To cali- brate the mode 1, Manni ng n-va 1 ues between 0.1 and 0.2, or an average of 78 percent greater than field-selected values, were required; subcritical flow was verified. The occurrence of numerous debris dams caused localized backwater, resulting in predominantly subcritical flow. However, when these debris dams broke, flow probably was supercritical for a short distance until another debris dam formed. Without the extensive calibration of the model and the assumption of subcritical flow, results would have been significantly different, Peak discharges from dam-break modeling reflect water-only discharges; total discharge may have been considerably higher on the Roaring River and on the Fall River immediately downstream from Cascade Lake dam from sediment and debris, At Horseshoe Falls and for a short reach downstream from Cascade Lake dam, geomorphic and sedimentologic evidence indicates the flow temporarily became a debris torrent, or a coarse-grained, organic, and water-rich type of debri s flow, The sedi ment and debri s may have bul ked the peak water flow by 50 to 60 percenL The range of di fference of observed and modeled peak discharges varied from -3,200 cubic feet per second to 600 cubic feet per second. The range of difference of observed and modeled maximum flood depth was -1. 3 to 2.6 feet and averaged 1. 0 foot. The range of difference of observed and modeled leading edge of traveltime was -0,4 and 0.15 hour. \ I, I Comparisons were made for hypothetical breach widths of (1) 25 feet and (2) 200 feet. They were compared with model results of the actual breach width of 55 feet: 2 ~.