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

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Streamflow in the lower Arkansas River is regulated by the storage and release operations of two main -stem reservoirs, Pueblo Reservoir and John Martin Reservoir. Because of streamflow manipulation, both reservoirs can cause increases or decreases in the dilution potential in the river and, thus, can affect specific conductance. Although the specific- conductance conditions and the relations between specific conductance and streamflow in the Arkansas River have been well documented (Miles, 1977; Cain, 1985, 1987), the effects of main -stem reservoir operations on specific conductance have not been systematically studied and reported. Beginning in 1988, the U.S. Geological Survey initiated a basinwide study of water quality in the Arkansas River and of the effects of certain water - supply operations on water quality, including the relations of streamflow and specific - conductance trends to reservoir operations in the lower Arkansas River. The study was conducted in cooperation with the Colorado Springs Utilities; Pueblo Board of Water Works; Southeastern Colorado Water Conservancy District; Pueblo County, Department of Planning and Development; city of Aurora, Department of Utilities; St. Charles Mesa Water District; Upper Arkansas Area Council of Governments; Upper Arkansas Water Conservancy District; city of Pueblo, Department of Utilities; Pueblo West Metropolitan District; Fremont Sanitation District; and cities of Rocky Ford, Las Animas, and Lamar. Purpose and Scope This report presents: (1) Step -trend analysis results for streamflow and specific conductance at three main -stem Arkansas River streamflow - gaging stations located between Pueblo Reservoir and John Martin Reservoir and at two main -stem stations located between John Martin Reservoir and the Colorado- Kansas State line, and (2) a determi- nation of whether significant streamflow and specific - conductance trends are related to the operations of the reservoirs. Daily mean streamflow data and discrete specific- conductance data were used in the trend analyses. The five main -stem stations were selected for trend analysis because of the availability of long -term streamflow and specific- conductance data. Additionally, streamflow and specific- conductance trends were evaluated at station 07096000 (Arkansas River at Canon City), which is in the upper basin about 28 mi upstream from Pueblo Reservoir (fig. 1). This station was included in the analysis to help differen- tiate whether trends in the lower basin 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. Although specific- conductance trends were the main focus of this study, streamflow trends also were analyzed because streamflow and specific conductance generally are correlated; there- fore, changes or trends in specific conductance often can be explained in terms of the associated change or trend in streamflow. All streamflow and specific- conductance data used for trend analysis in this study are available from the U.S. Geological Survey National Water Data Storage and Retrieval System (WATSTORE). Addi- tionally, all streamflow and specific- conductance data used for trend analysis in this study were published in annual data reports (U.S. Geological Survey, 1959, 1960, 1961, 1962 -65, 1963a, 1963b, 1964, 1965a, 1965b,1966-75a,1966-75b,1976-95). At individual stations, the record length for specific- conductance data generally was shorter than the record for streamflow. Therefore, the streamflow - record length for individual stations was shortened to match the specific- conductance record for that station. This matching of record lengths facilitated the direct comparison of streamflow and specific - conductance trends at individual stations. Because record lengths differed between stations (table 1), trend results were not quantitatively compared between stations. At most stations, the effects of tribu- tary streamflow and specific conductance were not considered in the trend analysis because of a lack of tributary streamflow and specific - conductance data. The exception was station 07106500 (Fountain Creek at Pueblo), which is tributary to the Arkansas River near Pueblo (fig. 1) and for which there exists long- term streamflow and specific- conductance data. Streamflow and specific- conductance data from the three stations located between Pueblo Reservoir and John Martin Reservoir were evaluated for trends that might have occurred after the construction of Pueblo Reservoir in 1975. Data from the two stations located downstream from John Martin Reservoir were evaluated for trends that might have occurred 4 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado