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In the foothills of Colorado, annual flood flows are derived from snowmelt at higher elevations in the mountain regions, from rainfall at lower elevations in the plains or plateau regions, or from a combination of rain falling on snow or mixed-population hydrology. When snowmelt- and rain-generated peaks were examined separately (which is necessary in mixed- population flood regions) for 69 unregulated streams in the the foothills region of Colorado in the South Platte, Arkansas, and Colorado River basins (Elliott et aI" 1982), flood-frequency analysis indicates different trends based on elevation. The location of 27 selected study sites in the South Platte RiVer basin are shown in Figure 1, Flood-frequency relations for two sites analyzed in the Clear Creek drainage basin just west of Denver indicate that the change from snowmelt- to rainfall-dominated flooding occurs abruptly through a small range in elevation. Clear Creek near Golden (site 11) (Figure 2A) has a gage elevation of 5,735 feet (1,748 meters), is a snowmelt-dominated stream for floods less than the 10-year flood, and a rainfall-dominated stream for floods in excess of the 10-year flood. The flood of record at this site is 5,890 cubic feet per second (166.8 cubic meters per second) as a result of an intense thunderstorm over the drainage area at an elevation less than 7,500 feet (2,300 meters). In contrast, for Clear Creek near Lawson (site 10) (Figure 28) at an elevation of 8,080 feet (2,963 meters), the snowmelt-runoff floods predominate to the 500-year flood. The flood of record at this site is 2,240 cubic feet per second (63.4 cubic meters per second) resulting from snowmelt, and the largest rainfall flood of record at this site is 1,500 cubic feet per second (42.5 cubic meters per second). /..3