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METEOROLOGY. HYDROLOGY. BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 65 2300 MDT produced rises on several streams with resultant crests almost as high as the earlier ones. The major damage occurred in Glen Haven from West Creek which enters the town from the southwest. Downstream from Glen Haven, damage was limited mainly to the highway, which generally parallels the river to Drake. The peak discharge of 888 cubic feet per second, which occurred on North Fork Big Thompson River about 0.1 mile upstream from Fox Creek (Site 13). in- dicates the small contribution to downstream floodf1ows from thet part of the basin west of Glen Haven. Similar results were obtained for Fox Creek where a peak discharge of 1,300 cubic feet per second occurred at Site 14, 0.2 mile above the mouth. Peak discharges on West Creek (Site 151 and Devils Gulch (Site 16) were 2,320 cubic feet per second and 2,810 cubic feet per second, respectively. Again, only a small part of the West Creek basin was hit by the heavy rain- falL Conversely, the entire basin of Devils Gulch received extremely heavy rainfall resulting in the high unit runoff of 3,090 cubic feet per second per square mile from the 0.91-square-mile basin. Downstream from Glen -Haven, the unnamed tributary which enters the river from the south at Glen Haven picnic ground (Site 171 had a peak discharge of 9,670 cubic feet per second from a drainage area of 1.38 square miles. This peak-runoff rate of 7,010 cubic feet per second per square mile is the second _highest unit. runoff rate for the flood, being exceeded only by Dark Gulch (Site 8) which heads due south of this stream. Peak discharges for Black Creek (Site 18) and Miller Fork (Site 19) were 1,990 cubic feet per second and 2,060 cubic feet per second, respectively. The peak stages occurred on these two streams about 2300 MDT; both streams were reported as being dry during the 2100-MDT flood period. A peak discharge of 3,240 cubic feet per second occurred on the lInn~med tributary (Site 20) which enters North Fork from the south about 3.2 miles west of Drake. The unit discharge of 2,570 cubic feet per second per square mile is less than that of other streams which head in the same general area, but much of this basin lies near the eastern edge of the storm area. Flood runoff in streams east of this point rapidly decreased with no evidence of flood runoff on North Fork tributaries at Drake. At Drake, the North Fork Big Thompson River (Site 21) had a peak discharge of 8,710 cubic feet per second. This peak discharge greatly exceeds the previous max- imum discharge of 1,290 cubic feet per second which occurred on June 16, 1965. The hydrograph from the gaging station (operated by the Colorado Division of Water Resourcesl on the North Fork at Drake (fig. 50) shows that one peak occurred at 2110 MDT, possibly caused by backwater from the Big Thompson River. The stream then receded until 2135 MDT and rose again to a peak stage of 9.21 feet at 2140 MDT. The second peak which occurred in the vicinity of Glen Haven about 2300 MDT was not recorded at this site because of the plugged gage intakes. BIG THOMPSON RIVEll BASIN, DRAKE TO MOUTH OF CANYON The peak discharge of the Big Thompson River about 0.4 mile east of Drake (Site 22) was 30,100 cubic feet per second. The flood crest at this location occur- red a few minutes after 2100 MDT or approximately 30 minutes prior to the flood crest on the North Fork at Drake. At 2100 MDT, the approximate time of the Big Thompson River crest stage at Drake,the discharge of the North Fork was about 4,500 cubic feet per second. By adding this discharge to the peak discharge of the Big Thompson River at Site 12, a discharge of 32,700 cubic feet per second is obtained, which is only about 9 percent greater than the measured peak discharge of 30,100 cubic feet per second. Fortunately. the flood 9000 llllOO~ !f 7OOa~ 8 w '" eooo~ a: ~ m 5000~ !.l .. ::> 4000 l- v ii!: u1 " 3000 I- ~ is 20001- 1000 I- a 2400 I I - - - - - ( - 600 1200 1600 24DD JULY 31. 1976 FIGURE 50.-Discharge hydrograph for the North Fork Big Thompson River at Drake (Site 21) until 2300 MDT on July 31. 1976. IE