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Fall River: Cascade Lake Dam to Estes Park Powerplant Channe 1 slope increased to 8 percent (fi g. 2) through Aspengl en Camp- ground to the Estes Park powerplant (fig. 1), resulting in dramatic channel changes. Inflow floodwater to Cascade Lake overtopped the dam. The tremen- dous forces of the water caused the entire dam to fail (figs. 9A-D). At the time of failure, flood discharge was 4,500 ft3/s, and water was flowing 4.2 ft over the top of the dam. Failure of the dam released a total of 25.1 acre-ft of stored water in addition to the volume of floodwater from Lawn Lake. Because of the near- instantaneous fa i 1 ure and re 1 at i ve ly sma 11 vo 1 ume of released water, the resulting flood hydrograph from Cascade Lake dam resembled a "spike"-shaped hydrograph, superimposed on the relatively broad inflow flood hydrograph (fig. 46). The peak discharge of the inflow hydrograph was 7,210 fP/s (Site 1), and the dam failed at a discharge of 4,500 fP/s (Site 2), indicating that the dam failed just prior to the arrival of the incoming flood peak from Lawn Lake dam. Peak discharge of the Cascade Lake dam failure was extremely difficult to determine because of its inherently unsteady nature. However, a peak discharge of 13,100 ft3/s was computed 0.9 mi downstream from the dam after considerable attenuation probably had occurred. Dam-break modeling results (discussed later) suggested a peak discharge of 16,000 ft3/s resulting from the Cascade Lake dam failure. Flood depths in this high-gradient (8-percent slope), wide flood plain of the Fall River generally were less than 5 ft, as flood conditions resembled overland flow (fig. 20). Several eyewitnesses described the flow as cascading through Aspengl en Campground. The majori ty of fl ow probably was subcri t i ca 1 due to shallow depths, the low relative submergence of bed material, energy losses from transporting boul ders, and dense vegetation obstructi ons (fig. 20). Several witnesses at Aspenglen Campground described the flow depths as relatively constant, with a secondary surge that probably corre- sponded to the "spike" of the flood hydrograph, resulting from the failure of Cascade Lake dam. The two flood deaths in Aspenglen Campground occurred near the area shown in figure 20. Two campers, having been warned by other campers of an ap- proaching flood, were last seen going into the island campsites. Fall River and Big Thompson River: Estes Park Powerplant to Lake Estes Channel slope in this part of the flood path gradually decreases from the upstream end of Fall River to the downstream end on the Big Thompson River (fig. 2), with an average slope of 2.3 percent. The channel was relatively narrow in thi s reach (fi gs. 21 and 22). Many buil di ngs were located adjacent to the channel on the flood plain (fig. 21); these structures tended to slow the passage of the flood wave. This was particularly true in Estes Park, as shown in the photographs in figures 23 and 24 (taken at about the time of peak flow). Note in figures 23 and 24 that flow was very fast only in the main street of Estes Park (as indicated by the cross waves that imply supercritical flow); the majority of flow being impeded by the buildings was subcritical. 41