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

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May and peaks in the middle or latter part of June. The recession of snowmelt runoff usually is supple- mented by off - channel reservoir releases in the upper basin in July and August. Natural base flow and reservoir releases maintain streamflow during the low -flow period from October through March. At station 07099400 (Arkansas River above Pueblo), located 0.4 mi downstream from Pueblo Dam, stream- flow is regulated by the operation of Pueblo Reservoir. The hydrograph for station 07099400 is similar to the hydrograph for station 07096000, except that stream- flow is considerably smaller at station 07099400 during November through March. At station 07109500 (Arkansas River near Avondale), located about 24 mi downstream from Pueblo Reservoir, streamflow is affected by Pueblo Reservoir operations and by substantial tributary inflow from Fountain Creek and the St. Charles River. The shape of the hydrograph for station 07109500 is similar to the hydrograph for station 07099400, except that tributary inflow from occasional rainfall runoff produces distinctly larger peaks, and the streamflow is substantially larger at station 07109500 during winter, owing to tributary inflow from Fountain Creek. At station 07124000 (Arkansas River at Las Animas), which is located 110 mi downstream from Pueblo Reservoir, the magni- tude of streamflow is substantially smaller than at the upstream stations. The decrease in streamflow is attrib- utable to irrigation diversions. As previously discussed in the "Description of Study Area" section, much of the streamflow in the river downstream from La Junta (fig. 1) consists of irrigation- return flow during parts of most years. Downstream from John Martin Reservoir at stations 07130500 (Arkansas River below John Martin Reservoir) and 07133000 (Arkansas River at Lamar), the annual hydrographs are regulated by the storage and release operations in John Martin Reservoir. The reservoir gates typically are closed during November through March, and all inflow is stored; winter stream- flow at stations 07130500 and 07133000 is main- tained by ground -water discharge. Requests for reser- voir releases of stored water by downstream irrigators usually begin in the first 2 weeks of April and continue episodically through October. Rainfall runoff that is generated upstream from the reservoir is attenuated by storage in the reservoir. Streamflow between John Martin Reservoir and Lamar is substantially decreased by irrigation diversions. Specific conductance in the Arkansas River markedly increases downstream from Pueblo (fig. 4). The downstream increase in specific conductance is a function of the evaporative concentration of dissolved solids. In 1990 -93, during a period of inten- sive data collection, the median specific conductance increased downstream from 276 µS /cm at Canon City to 3,855 µS /cm at Lamar (fig. 4). Miles (1977) reported that the dissolution of soluble sedimentary materials between Canon City and Pueblo Reservoir increases specific conductance in that reach. From Pueblo to Lamar, specific conductance primarily increases because of the consumptive use of irrigation water and the concomitant increase in the concentra- tion of dissolved solids. The rate of increase in specific conductance is larger downstream from Catlin Dam (fig. 4) because irrigation -return flow composes a larger percentage of streamflow in this reach than it does upstream from Catlin Dam (Cain, 1987). RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS IN THE ARKANSAS RIVER At Canon City Station 07096000 (Arkansas River at Canon City) is located in the upper Arkansas River Basin about 28 mi upstream from Pueblo Reservoir (fig. 1). Streamflow and specific- conductance trends were analyzed at this site to determine if the quantity and quality of water flowing from the upper basin into the lower basin were different during the period before the completion of Pueblo Reservoir (pre -1975) compared to the period after the completion of Pueblo Reservoir (post- 1974). Streamflow and specific- conductance data were available for 1964 -94. Streamflow and specific - conductance data from 1964 through 1974 were compared to data from 1975 through 1994. The median annual streamflow at station 07096000 increased from about 520,800 acre -ft/yr in 1964 -74 to about 538,600 acre - ft/yr in 1975 -94; however, this difference in streamflow was not statisti- cally significant (p= 0.92). Although there was no significant change in the median annual streamflow, the daily mean streamflow changed substantially in several months of the two periods (fig. 5). Daily mean streamflow increased significantly (p< -0.05) between 1964 -74 and 1975 -94 in January, February, March, April, June, October, November, and December (table 2). Most of the increases occurred during low RELATIONS OF STREAMFLOW AND SPECIFIC - CONDUCTANCE TRENDS TO RESERVOIR OPERATIONS 11 IN THE ARKANSAS RIVER