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22 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO geomorphic work, and to cause great modifications to the channels and valleys below the dams (as described in "Geomorphic Effects of the Flood"). TRAVELTIME Immediately after the flood, personnel of the U.s. Bureau of Reclamation interviewed residents along the flood path and compiled written statements to evaluate the dissemination of and response to the flood warnings. This information, in conjunction with streamflow-gaging station, Lake Estes inflow, and stream mileage data, pro- vided data to compute the traveltime of the flood wave. Apparently no one observed the failure of Lawn Lake dam, and the time of failure is uncertain. Campers at Lawn Lake reported hearing a roar between 0200 MDT and 0400 MDT, which probably corresponded to in- creased flows through a partial failure. The amount of water probably was quite small (perhaps less than 100 ft'/s), because campers along the Roaring River were not affected. Other information supported a small partial failure this early. L. V. Davis. the owner of Cascade Cottages at Cascade Lake dam, reported flow was slight- ly higher than usual at 0700 MDT, before the main flood wave arrived at the dam at about 0715 MDT. Based on available information, it appears that Lawn Lake dam failure occurred about 0530 MDT, just before sunrise. Because campers along the Roaring River understand- ably were more concerned with fleeing the wall of water, their estimates of time may be only approximate. Observers along the remainder of the flood path were easily able to distinguish the leading edge of muddy floodwaters from the normally clear streamflow. Accord- ing to Stephen Gillette, a truck driver forA-l 'frash Serv- ices, who was the first to report the flood at 0623 MDT, floodwaters reached Horseshoe Falls at about 0615 MDT and u.s. Highway 34 in Horseshoe Park at 0634 MDT (fig. 1). According to Mr. Davis, floodwaters reached Cascade Lake dam at 0715 MDT, causing its failure at 0742 MDT. Estes Park police reported that floodwaters reached Estes Park a little after 0830 MDT. Floodwaters reached 06733000 Big Thompson River at Estes Park streamflow-gaging station (Site 6) at 0835 MDT, and per- sonnel of the U.S. Bureau of Reclamation reported that .water levels started rising in Lake Estes at 0847 MDT. It was more difficult to determine when the peak was occurring, as no distinctive hydraulic feature existed. Both Rann Schultz and Dan Davis of the National Park Service, as well as Stephen Gillette, provided information to estimate that the peak followed the leading edge by less than 40 min, or about 0700 MDT, at U.s. Highway 34 in Horseshoe Park (fig. 1). As the flood traveled only 0.6 mi through the flatter Horseshoe Park, this estimate appeared quite long, considering that the flood was a "wall of water" in the Roaring River. Based on informa- tion provided by Rann Schultz, the peak followed the leading edge by about 25 min, or about 0748 MDT, at the Aspenglen Campground access road,located 0.4 mi downstream from Cascade Lake dam. Dave Thomas, broadcasting the flood's progress from a KSIR radio mobile-transmitter station 1.2 mi upstream from Estes Park, indicated the peak followed the leading edge by 18 min, or at 0830 MDT. Based on a stage hydrograph reconstruction at 06733000 Big Thompson River at Estes Park streamflow-gaging station, just upstream from Lake Estes, the peak followed the leading edge by 30 min or at 0905 MDT. Inflow to Lake Estes peaked between 0910 MDT and 0915 MDT (use 0912 MDT for the peak time). or 25 min after the lake began rising. Unfortunately, because of limited and approximate peak-time data, it is not possible to determine whether the peak was moving faster than the leading edge of the flood wave, as would be expected. A summary of data related to time of flooding prepared by Graham and Brown (1983) is shown in table 6 and graphicaily summarized in figure 15. The lower line in figure 15 corresponds' to the arrival time of the flood; the upper line in figure 15 corresponds to the peak time of the flood. Traveltimes for the arrival time of the flood were summarized for three channel segments, based on fairly uniform reach traveltimes (fig. 15). The speed of the leading edge of the flood (and probably the peak) averaged 9.1 milh (miles per hour) in the Roaring River, 2.1 milh in the Fall River through Horseshoe Park, and 4.0 milh from Cascade Lake dam on the Fall River to Lake Estes. Overall the speed of the leading edge of the flood averaged 3.8 milh. Consider- ing the high-gradient channels, these traveltimes were slow. Apparently, this slowness was because the chan- nels were extremely rough, and tremendous amounts of debris in the water, particularly in the Roaring River, produced the slow speed of the flood wave. DESCRIPTION OF FLOOD-WAVE CHARACTERISTICS Specific details of the flood wave are of interest in understanding flow hydraulics. Understanding of flood- wave characteristics is important as they are the major cause of property damages, geomorphic changes, and are a factor in implementing flood warnings. Flood-wave characteristics varied along the flood path; therefore, they are discussed by various segments where characteristics are fairly uniform for that segment. Channel slope is the dominant factor in establishing these segments (fig. 2.)