SPDSS_Task46-2_StreamGainLossEstimates

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Table 8 Gain/Loss Summary, 1950 - 2005 Reach Mass Balance cfs Estimated Baseflow cfs Other Gain-Loss cfs South Platte 1 -Waterton to Denver 93.1 75.9 17.1 South Platte 2 -Denver to Henderson 51.5 45.3 6.1 South Platte 3 -Henderson to Fort Lu ,ton 47.3 37.7 9.5 South Platte ~ -Fort Lu Eton to Kerse 263.2 219.1 44.1 South Platte 5 -Kerse to Weldona -16.2 42.7 -58.9 South Platte 6 -Weldona to Balzac 84.4 80.6 3.8 South Platte 7 - Bt~lzac to Juleshur r 252.6 255.9 -3.3 Cache la Poudre - Ft Collins to Greele 161.6 144.2 17.4 Cherr Creek - Franktown to Denver 16.4 16.5 -0.1 During the 1991 to 1994 period all monthly averages remained positive for all reaches (Figures 11-20). With the exception of the South Platte 2 -Denver to Henderson reach (Figure 13), the reaches had the highest baseflow values (representing flow from groundwater to streams) during the summer months and the lowest baseflow values during the winter. This trend likely corresponds to the general pattern of lower summer flows and application of surface water for irrigation and suggests that much of the return flow occurs within a few months following irrigation. The Soutll Platte 2 -Denver to Henderson reacl111as a wide alluvial aquifer relative to other reaches, which may result in lagging of irrigation return flows for this reach and t11e opposite pattern of baseflow. Figures 21 to 30 present the average monthly baseflow and mass balance values for the 1991 to 1994 period on a per river mile basis. Figure 21 shows the baseflow 1lydrographs posted on the map of the study area so one can evaluate spatial as well as temporal aspects of the average monthly baseflow. The baseflow values in the South Platte River reach generally range from 2 to 8 cfs/mile with the exception of higher values ii1 the South Platte 4 -Fort Lupton to Kersey reach, whose range is 4 to 10 cfs/nine (Figure 25). The average baseflow for fl1e period is presented for eac11 reach >11 Table 9. Also shown >11 this table are the average baseflow values per river mile. Table 9 Estimated Baseflow, 1991 -1994 Reach Estimated Baseflow cfs Estimated Baseflow Per River Mile cfs South Platte 1-Waterton to Denver 94.1 5.4 South Platte 2 -Denver to Henderson 87.7 5.5 South Platte 3 -Henderson to Fort Lu Eton 62.8 3.7 South Platte ~ -Fort Lu iton to Kerse 245.3 6.3 South Platte 5 -Kerse to Weldona 173.3 4.0 South Platte 6 -Weldona to Balzac 132.3 5.3 South Platte 7 -Balzac to Juleshur~ 293.1 3.0 Cache la Poudre - Ft Collins to Greele 172.0 3.1 Cherr Creek - Franktown to Denver 18.0 0.5 SPDSS Phase 4 Task 46 Technical Memorandum -Final 15 0~4i 10i0~