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a characteristic "wall of water;" in this case, that "wall" was estimated to be 25 to 30 ft high by a number of eyewitnesses. The leading edge of the wave front, although very steep, probably was not a vertical wall of water, This steep wave front probably was accentuated by the large amount of debris, The 1 eadi ng edge of the fl ood wave was movi ng at an average of 9,1 mi /h in the Roaring River (fig. 15), The peak probably was very close to the leading edge of the flood wave (see "dam-Break Mode 1 i ng"), The re 1 at i ve ly slow speed of the leading edge was due to retardance by the vegetation and resulting debris dams. The leading edge probably was very similar to the 1889 flood that result- ed from the failure of South Fork dam near Johnstown, Pa., which claimed 2,209 lives. In the 1889 flood, eyewitnesses described the flood as carrying extremely large amounts of debris and traveling much slower than expected, "From time to time as the flood entered narrow places in the valley, the massed debris acted as a dam and the giant flow seemed to slow and stop; then the front would boil and seethe and huge trees, ejected by overwhelming pressures, would shoot into the air as the flood once more surged ahead." (Clark, 1982, p, 141), Ample evidence of large debris dams remained in the Roaring River (fig, 17). The 1982 flood also carried large amounts of debris, including very large boulders (fig, 168). For these conditions, the theory and application of conventional energy and flow-resistance concepts (such as Manning's n-values) probably are not appl icable. The occurrence of numerous debris dams caused local ized backwater, resulting in predominately subcritical flow. However, when these debris dams break, flow probably was supercritical for a short distance until another debris dam formed. Turbulence was extremely high, as observed by Stephen Gillette at Horseshoe Falls, where he saw boulders and trees being thrown into the air, Across the large alluvial fan formed at the base of Horseshoe Falls, water spread, The depth, width, velocity, or cross-sectional area of flow are unknown. Early in the flood, the flow path was down the major axi s of the fan, Thi s was the area of the fan where the 1 argest boul ders and thi ckest sediments were depos i ted. Soon aftel' the fl ood wave arri ved, the mai n fl ow path became plugged wi th sedi ment and debri s; on the fall i ng 1 imb of the hydrograph, the main flow gradually shifted to the right side of the fan, This migration of the main flow effects from the left to right parts of the alluvial fan is indicated by the distinctly finer-grained deposits on the right side of the fan, flow paths visible on large-scale aerial photographs, and the geographic position of the boulder berms formed during the first few minutes of the flood, Fall River: Horseshoe Falls to Cascade Lake Dam The confluence of the Roaring River with the Fall River is at the up- stream end of Horseshoe Park (fi g, 1), The Fall Ri ver streamflow pri or to the fl ood was estimated to be about 150 ft3 / s, The channe 1 slope fl attened 39