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

Home Browse Search

2,000 W 1,800 F_ 2 Z 1,600 W U Cr CL 1,400 07(n Z ? W WW ,200 U) U CCW UW 1,000 z -0 U 61 800 Z� �a D 600 a Z O U 400 U LL U W W 200 07 / COMPLETION OF PUEBLO RESERVOIR • • • • • • • • • • • • • ♦t S' 01 i i i i i 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 Figure 14. Specific conductance at station 07109500 (Arkansas River near Avondale), 1969 -94. result in increased dilution and decreased specific conductance. There are two likely causes of the increased specific conductance in June and August: (1) Mixing of low - specific- conductance and high - specific- conductance water in Pueblo Reservoir resulted in increased specific conductance in the reser- voir outflow during months that historically had the lowest specific conductance, and (2) dissolved - solids loads from major tributaries increased. Apparently, both situations occurred and together caused increases in specific conductance during June and August. A lack of data prevented an analysis of streamflow and specific- conductance trends in the St. Charles River (fig. 1), which is tributary to the Arkansas River about 4.5 mi upstream from station 07109500. Adequate data did exist for general trend analysis at Fountain Creek (fig. 1), the other major tributary in the reach, although there were not enough data to do monthly trend analysis. The median specific conductance at station 07106500 (Fountain Creek 1992 1994 at Pueblo) decreased from 1,500 gS /cm in 1969 -74 to 1,380 gS /cm in 1975 -94. Although specific conductance decreased in Fountain Creek at Pueblo, it remained substantially larger than specific conduc- tance in the Arkansas River near Avondale. As previ- ously mentioned in this section, the median annual streamflow at station 07106500 increased from about 37,000 acre -ft /yr in 1969 -74 to about 67,000 acre -ft/yr in 1975 -94, with streamflow increasing during all seasons. The median specific conductance in Fountain Creek was converted to an equivalent dissolved - solids concentration based on relations described by Cain (1987). The estimated median dissolved - solids concentrations were multiplied by the median annual streamflow to obtain an estimate of the median annual dissolved -solids load contributed by Fountain Creek to the Arkansas River in 1969 -74 and 1975 -94. Based on these estimates, dissolved - solids loading increased about 58 percent from 53,000 to 84,000 tons /yr. 26 Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado