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

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3,500 3,000 OZ �j O 2,500 � W aU) Ec CE EL LLI 2,000 Q W W W LL 1,500 U J CO U ZO Z 1,000 500 JAN FEB MAR APR MAY JUNE JULY AUG SEPT OCT NOV DEC 1987 EXPLANATION --F- STREAMFLOW -0 - SPECIFIC CONDUCTANCE Figure 4. Relation of streamflow to specific conductance for the Arkansas River near Avondale, 1987. Las Animas. Data collected at each site were separated into two seasons: growing season (March 16— November 14) and winter- storage season (November 15— March 15). Data collected prior to 1975 were compared to data collected from 1975 through 1994. At individual stations, the record for specific- conductance data generally was shorter than the record for streamflow. Therefore, the streamflow record for individual stations was shortened to match the specific- conductance record for that station. This matching of records facilitated the direct comparison of stream- flow and specific- conductance trends at individual stations. Arkansas River Above Pueblo After construction of Pueblo Reservoir, streamflow in the Arkansas River upstream from Pueblo increased during the growing season and decreased during the winter - storage season, largely due to the effects of the WWSP. Because specific conductance tends to be inversely proportional to streamflow, specific con- ductance at this site would be expected to increase during the winter - storage season and decrease during the growing season. The opposite occurred. The median specific conductance increased slightly from 460 to 465 g6/cm between 1966 -74 and 1975 -94 during the growing season and decreased substantially from 720 to 575 g6/ern during the winter- storage season (fig. 5). H 1,100 1,000 U Z W U W goo D 2 n P O W J UUW 800 U cc U CL W U az¢ 700 U � W Z W 0 W �O N � 600 } U Q J — _� F z? 500 O 400 The changes in specific conduc- tance after construction of Pueblo Reservoir are not a direct function of changes in streamflow, but are more directly related to the storage and subse- quent mixing of seasonally low- specific- conductance water and seasonally high - specific- conductance water in Pueblo Reservoir. Low- specific- conductance water generally flowed into the reservoir during May— August and mixed with high - specific- conductance water that flowed into the reservoir during the fall, winter, and spring. The result was a narrowing of the range in specific- conductance values after 1974. Arkansas River Near Avondale After construction of Pueblo Reservoir, streamflow near Avondale increased during the growing season and decreased during the winter - storage season. The decreased winter streamflow was caused by storage of WWSP water in Pueblo Reservoir. The increase in stream- flow during the growing season partly is attributable to the release of WWSP water from Pueblo Reservoir. Increased inflow from Fountain Creek (fig. 1) also accounts for some of the increased streamflow. Population growth in the Colorado Springs area and the associated increase in unit runoff and wastewater discharge are partly responsible for the increase in the median annual streamflow of Fountain Creek at Pueblo, which increased from about 37,000 acre -ft per year in 1969 -74 to about 67,000 acre -ft per year in 1975 -94. Specific conductance near Avondale changed appreciably after 1974 (fig. 6), but the changes were different from the changes observed at the site upstream from Pueblo (fig. 5). Between 1969 -74 and 1975 -94, the median specific conductance increased from 580 to 700 g6/cm during the growing season and increased from 900 to 1,060 gS /cm during the winter - storage season (fig 6). The combined effect of reservoir operations, as observed at the site upstream from Pueblo, and an increase in the inflow of dissolved solids from Fountain Creek apparently caused the changes in specific conductance near Avondale. Although specific conductance in Fountain Creek decreased after 1974, it remained sub- stantially higher than in the Arkansas River near Avondale. The increase in 800 cc WF U z W la- W 700 1966 -74 UZ- ® 1975 -94 �UW z�U oC/) W 600 u z� W2W aW0 Fn N zQ cc F 500 a DU W g �z 400 GROWING WINTER - SEASON STORAGE SEASON Figure 5. Specific conductance for the Arkansas River above Pueblo, 1966 -94.