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

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Relations of Streamf low and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado By Michael E. Lewis and Daniel L. Brendle Abstract To provide for the better management of streamflow in the lower Arkansas River, two main -stem reservoirs were constructed. John Martin Reservoir, constructed near Las Animas in 1948, and Pueblo Reservoir, constructed near Pueblo in 1975, provide for flood control, irrigation, municipal water supply, and recreation. Both reservoirs have the potential to alter specific conductance in the Arkansas River because of streamflow management. A change in specific conductance could affect the intended use of the water as an agricultural or domestic water supply. Step -trend analysis of streamflow and specific- conductance data for the Arkansas River was used for determining if the operation of Pueblo Reservoir or John Martin Reservoir had affected streamflow or specific conductance in the lower Arkansas River. The nonparametric Mann - Whitney - Wilcoxon rank - sum test was used for trend analysis. Streamflow and specific- conductance data collected at five streamflow- gaging stations on the lower Arkansas River and at one station on the upper Arkansas River were analyzed for trends. The station in the upper basin was included in the analysis to differentiate between trends in the lower basin that were caused by differences in the quantity or quality of inflow from the upper basin or were caused by reservoir operations in the lower basin. Data from the station in the upper basin and from the three stations located between Pueblo Reservoir and John Martin Reservoir were analyzed for trends that may have occurred after 1974, which corre- sponds to the construction of Pueblo Reservoir. Data from the two stations located downstream from John Martin Reservoir were analyzed for trends that may have occurred after the imple- mentation of a new reservoir operating plan in 1980. At the station in the upper basin, stream- flow increased significantly and specific conduc- tance decreased significantly after 1974 during the low -flow months, January, February, and March. These trends apparently were caused by the increased importation of low- specific- conductance water after 1974 from the Colorado River Basin into the Arkansas River. At the three stations located between Pueblo Reservoir and John Martin Reservoir, streamflow and specific conductance primarily were affected by Pueblo Reservoir operations. After 1974, at the two stations located 0.4 and 24 miles downstream from Pueblo Reservoir, streamflow generally increased during most months of the growing season and decreased during November through February. The streamflow trends at these two stations largely were attributed to the storage of water in Pueblo Reservoir during winter and to the release of that stored water during the growing season in order to meet downstream irrigation needs. At the station 0.4 mi downstream from Pueblo Reservoir, specific conductance decreased during most months between September and Abstract 1