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GEOMORPHIC EFFECTS OF THE FLOOD 43 FIGURE 39.-The 17-ocre lake formed in Horseshoe Park by the dsmming of the Fall River, taken in August 1983 (aerial). Downstream is to, the left. the edge of the alluvial fan (fig. 39). In July 1983, the lake had a surface area of 17.0 acres (0.0265 mi'), and appeared to be a permanent hydrologic feature for the foreseeable future. FALL RIVER: HORSESHOE FALLS TO CASCADE LAKE DAM The Fall River in Horseshoe Park between the junc- tion with the Roaring River and the downstream limit of Horseshoe Park at Site 1 just upstream from Cascade Lake dam is a sinuous meandering stream that falls 66 ft in 1.8 mi, or a slope of 0.7 percent (fig. 2). A view of Horseshoe Falls and Horseshoe Park is shown in figure 40. Surficial geology in Horseshoe Park consists of cohesive silts and clays deposited on the floor of a late Pleistocene lake. Lawn Lake floodwaters passed over and across the meanders, but the velocity of flow and duration of overbank flows were not sufficient to erode or disrupt the lakebed materials into which the mean- ders had formed (fig. 41). In Horseshoe Park maximum flood depth was about 10 ft, and maximum width was 1,300 ft measured on the aerial photograph. Water was out of the banks on the Fall River in Horseshoe Park for less than 4 hours,' except for local ponded water. The photograph in figure 41 was taken about a week after the flood; it clearly shows that no immediate visible modification oc- curred to the meandering channel pattern of the Fall River. In the spring of 1983, about 1 year after the Lawn Lake flood, numerous sandy point bars had formed in the Fall River channel in Horseshoe Park. Large amounts of sandy bedload, produced from a high spring runoff eroding bare and exposed channel banks along the Roaring River, and from the alluvial fan area, had moved into Horseshoe Park. Bedload and channel measurements by John Pitlick (1985) indicated that parts of the Fall River channel have been completely filled with sandy sediment. After the flood passed, much of the flood plain of Horseshoe Park was covered with a thin veneer of fine- grained silt and sand. No mineral sediments coarser than very coarse sand were transported through Horse- shoe Park, although some vegetal debris was trapped in the willows lining the channel. Sediment thickness varied from about 1 ft in locally protected places near the channel at the upstream end of Horseshoe Park, to thin drapes at the edges of the flow.