Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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Cain (1987) determined, using regression analysis, that a specific conductance of 718 gS /cm at station 07099400 was equivalent to a dissolved - solids concentration of 500 mg /L. The conversion also was assumed to be applicable at the two water - supply diversion points. Therefore, the dissolved - solids concentration at station 07099400 and at the two water - supply diversion points was estimated to exceed 500 mg /L when specific conductance exceeded 718 gS /cm. During 1966 -74, the estimated exceedance of a specific conductance of 718 RS/cm was 28 percent at station 07099400, 45 percent at the Pueblo Board of Water Works diversion, and 55 percent at the St. Charles Mesa Water District diver- sion point. During 1975 -94, after the construction of Pueblo Reservoir, the exceedance of a specific conduc- tance of 718 µ6 /cm decreased to about 1 percent at station 07099400, 4.3 percent at the Pueblo Board of Water Works diversion, and 27 percent at the St. Charles Mesa Water District diversion point (fig. 12). These results indicate that the chemical quality of the Arkansas River, in terms of specific conductance and dissolved - solids concentration, has improved in the 8.5 -mi reach between Pueblo Reservoir and the St. Charles Mesa Water District diversion since 1975, when Pueblo Reservoir was completed. The improved quality of water is attributable to two factors: (1) Decreased specific conductance in the upper Arkansas River, probably because of the increased importation of Colorado River Basin water; and (2) dilution of reservoir inflow having elevated specific conductance during low flow by low- specific- conductance water in Pueblo Reservoir. Near Avondale The largest main -stem streamflow in the Arkansas River occurs at station 07109500 (Arkansas River near Avondale) (fig. 1) because of substantial tributary inflow from the St. Charles River and Fountain Creek and because the station is upstream from most of the large irrigation canals that divert most of the flow from the lower Arkansas River. Streamflow at station 07109500 is strongly affected by Pueblo Reservoir operations because of the proximity of the station to Pueblo Reservoir and because of the absence of substantial streamflow diversions between the reservoir and the station. In terms of water quality, most of the concern and focus at this station are related to specific conductance and the suitability of the river as an irrigation supply. Streamflow and specific - conductance data were available at station 07109500 for 1969 -94. The median annual streamflow in 1969 -74 (623,000 acre- ft /yr) was not significantly different (p =0.65) from the median annual streamflow in 1975 -94 (625,200 acre - ft/yr). Similarly, the median annual streamflow from the upper basin in 1969 -74 (535,700 acre- ft /yr) and 1975 -94 (538,600 acre- ft/yr), as indicated by the record at station 07096000, was not significantly different (p= 0.69). The temporal nature of streamflow, however, changed appreciably during the two periods (fig. 13). After the completion of Pueblo Reservoir (1975), streamflow generally increased during March through October and decreased during November through February (fig. 13). The decreases in daily mean streamflow during November through February were all statisti- cally significant; streamflow increased significantly during March, April, June, August, and October (table 6). The decreased winter streamflow was caused by the storage of water in Pueblo Reservoir as part of the WWSP. As previously noted in the "Pueblo Reservoir" section, the median annual volume of water stored in Pueblo Reservoir during 1975 -94 as part of the WWSP was 42,200 acre -ft. The significant increase in streamflow during March, April, June, August, and October is attributable to the combined effects of the release of stored WWSP water from Pueblo Reservoir and increased inflow from Fountain Creek. The median annual streamflow at the tributary station 07106500 (Fountain Creek at Pueblo) increased from about 37,000 acre - ft/yr in 1969 -74 to about 67,000 acre -ft /yr in 1975 -94. The median daily streamflow of Fountain Creek at Pueblo during March through October increased 50 percent from about 48 ft3 /s in 1969 -74 to about 72 f0/s in 1975 -94. This increase in streamflow likely is partly attributable to increased unit runoff and increased municipal wastewater discharge from the Colorado Springs area. Increased unit runoff probably has resulted from the substantial growth and the associated paving of permeable surfaces in the greater Colorado Springs area. Much of the flow in Fountain Creek is derived from sewered (treated wastewater effluent) and unsewered (lawn irrigation) wastewater from Colorado Springs and several smaller municipalities in El Paso County (Edelmann and Cain, 1985). RELATIONS OF STREAMFLOW AND SPECIFIC- CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 23 IN THE ARKANSAS RIVER