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16 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO FIGURE lO.-The middle reach (river mile 2.1) of the Roaring River showing the extensive areas of scour and fill along the channel. Indirect measurements of peak discharges could not be made upstream from Site 1 (fig. I), but peak discharges estimated from the dam-break model are discussed in the section "Dam-Break Modeling." Chan- nel and hydraulic conditions were such that a peak- discharge measurement could not be made immediately downstream from Cascade Lake dam, but was made 0.9 mi downstream (Site 3). Because of rapid attenua- tion, the measurement at Site 3 did not reflect the dam- break peak discharge at the dam (see "Dam-Break Modeling" for an estimate of peak discharge). However, the discharge over the top of the dam (fig. 9A) at the moment of failure of Cascade Lake dam was computed to be 4,500 fts/s. This measurement was made from high-water marks upstream from the dam that were set at the time of failure. Note how the water level drop- ped 1 to 2 ft on the right (far) bank, in the series of photographs of Cascade Lake dam failing (figs. 9A -D), because of failure of the dam. A peak-discharge profile was constructed from Lawn Lake dam to Olympus dam. Indirect peak-discharge measurements were considered more accurate than dam.break model results; hence, the profile reflects in- direct measurements where both were available. The peak-discharge profile (fig. 11) reflects adopted peak discharges and indicates the rapid attenuation of peak flows in the downstream direction. Greatest attenua- tion probably occurred as the flood traveled through Horseshoe Park. The peak discharge more than doubled as a result of the near.instantaneous failure of Cascade Lake dam. VOLUME Since the construction of Lawn Lake dam in 1903, there had been some question as to its storage capaci- ty, particularly the live capacity of the lake. At the time of failure, several capacities were referenced. Minimal information existed on the capacity of Cascade Lake. In District Court for Larimer County, the reservoir was decreed to have a capacity of 759.6 acre-ft, but a 1931 capacity survey indicated 817.2 acre-ft of storage at high water (Office of the State Engineer, 1983), Because of the need for storage-capacity data, land surveys were combined with aerial-photographic mapping techniques to determine pertinent preflood and maximum-flood water-surface levels, develop topographic maps, and determine storage-capacity data of both lakes. These topographic maps are shown on plates 1 and 2 in the