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Minimum Flow <br />Hydraulic parameters were surveyed for five transects within the study site on 26 September <br />2000. Streambed profiles and water surface elevations of the surveyed transects are shown in <br />Figures 12 and 13. Two riffle transects (2 and 4) were selected for winter and summer minimum <br />flow analysis. Riffle transect 2 lies within an area where degradation of the right stream bank <br />exists. The proposed habitat improvements would repair the stream bank in this area. The <br />wetted width of this riffle transect is substantially greater than that of the other selected riffle <br />transect (4). When reconstruction is complete, the wetted widths of transects 2 and 4 should be <br />similar and minimum flows should be similar as well. The minimum flow requirements for <br />transect 4 should be the most representative for the reach after the habitat improvement is <br />completed. The hydraulic parameters output by RHABSIM are located in Tables 3 and 4. A <br />winter minimum flow of 4 cfs and summer minimum flow of 16 cfs meet the criteria used by <br />Nehring 1979. <br />The Colorado Water Conservation Board has suggested that modeling accuracy drops when the <br />modeled flow is less than 40% of the measured flow at the time of surveying. The winter <br />minimum flow of 4 cfs is below the 40% cutoff point of 5 cfs for the measured discharge of 13 <br />cfs. However, analyzing the relationship between stage and discharge shows that this <br />relationship is consistent throughout the range of flows modeled (Figure 14). Stage and <br />discharge appear to function normally below the 40% point. <br />Minimum Flow Study, South Fork of the South Platte River — Final Report Page 16 <br />February 19, 2001 <br />