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I I I I I I I I I I I I I I I I I I I 2.1 2.2 SECTION 2 HYDROLOGY FLOOD HYDROLOGY 2.1.1 Gage Records Three USGS streamflow gaging stations exist along the main stem of the Elk River. These gages are located at Milner, Clark and Above Clark. The length of record for the three gages vary. 2.1.2 Flood Frequency Analysis Hydrology used in this study was provided by the Colorado Water Conservation Board (CWCB). A flood frequency analysis was performed using 20 years of streamflow data at the Milner gage on the Elk River. The resulting peak discharges for the frequencies used in this analysis are presented in Table 2,1. Table 2.1 - Flood Hydrology Discharges Description Discharge 2-year 4,030 cfs 5-year' 4,770 cfs 10-year 5,180 cfs 50-year 5,920 cfs 100-year' 6,190 cfs 500-vear' 6,750 cfs June 1997* 5,370 cfs Low Flow' 500 cfs *Frequencies used in this study The official peak discharge for the 1997 flood at the Milner gage is 5,370 cfs, as determined by the USGS. It was determined by the USGS that approximately another 400 cfs by-passed the gage and flowed over County Road 42 during the flood, bringing the total flow just upstream of the gage to 5,770 cfs (Appendix A). As seen in Table 2.1, the 1997 peak discharge would be between a 10- and 50-year flood event. WATERSHED HYDROLOGY INVESTIGATION 2.2.1 Drainage Basin The Elk River watershed is illustrated in Figure 2, I, The drainage basin consists of approximately 425 square miles, A listing of sub-areas within the watershed is presented in Table 2,2, This breakdown provides an estimate of the drainage areas associated with major tributaries to the Elk River, Flooding within the Elk River basin is primarily due to snowmelt runoff, and events tend to be oflong duration with a more gradual rise and fall of the hydrograph. This is in comparison to 5