The Integrated Water Quality Program Annual Report Water Year 2000
Susan Hueftle
Results of Lake Powell-Tailwater monitoring:
The Hydrograph:
The August water quality monitoring trip found the reservoir at a steady elevation of 3680 ± 0.1 ft (1121 m). The hydrograph reflects the drought conditions that pervaded the Colorado river basin for water year 2000 (WY00-Oct. 1999-Sept. 2000). By September, the basin was still 83% of normal precipitation. Lake Powell inflows totaled 8,134,423 af or about 71% of dam-era average, while releases totaled 9,378,000 af or about 96% of post dam releases (USBR Hydromet database).
Typically the spring inflow begins in February and peaks in early June. This year inflows didn't increase significantly until May and the bi-modal peak was on May 12th and June 3rd (30,600 cfs and 40,600 cfs, respectively, figure 1).
The Profiles:
Figure 2 provides orientation for some of the terminology and general characteristics of the reservoir. There, and in the main channel isopleths for the reservoir (figures 3a, 3b, 3c & 3d), the effect of the depressed inflows is apparent. These profiles demonstrate physical conditions (temperature, conductivity (or salinity), pH, etc.) along the thalweg of the Colorado river in the reservoir. The dimensions of the spring flood plume, as observed by the fresher water extending across the top of the lake (figs. 3c & 3d) in the conductivity plots, is less extensive than in past years which had greater inflow. Further, the specific conductance of the base inflow near Hite Marina was over 1100 µS, as opposed to the low of 470 µS at the peak of the spring runoff last May '00. By contrast, May of 1997, reflecting one of the highest inflow years in the last 14 years, had a minimum conductance of 356 µS, and the fall of that year had a maximum inflow conductance of 960 µS. The conductance of the inflow is dictated by the volume of runoff combined with other seasonal characteristics including irrigation runoff and temperature. Long-term trends near the dam are demonstrated in figure 4.
Winter mixing drove the thermo-/ chemo-cline (zones of steepest change gradient) to the depth of the penstocks (~50-60 m from the lake's surface) in January. By the peak of summer warming in August, the upper boundary of the thermocline started at 10-12 m and extended to the depth of 30-40 m throughout the lake (figures 2, 3a-3d, 4). Because of the reduced inflow volume, this year's thermocline is shallower than in recent years' (fig.4).
Lake-wide dissolved oxygen content is decreasing after last spring's lake-wide enrichment which reached a 15 year high. This was a result of 2 processes last spring and winter. For the 2nd consecutive winter Lake Powell experienced a late winter oxygenated underflow plume (figures 3a-3c). As a result of high inflows since 1993 and continuing dilution of overall ion concentrations, the density gradient separating the epilimnion from the hypolimnion has weakened, allowing the penetration of cold, well-oxygenated water into the deepest layers of the lake. This process was significantly enhanced by the use of the jet-tubes in 1996 for the experimental flood, as well as high steady discharges that followed. Given the low levels of hypolimnetic dissolved oxygen that existed prior to 1997 and the hazards of discharging low oxygen, this may offer a tool for managing hypoxia (low O2) levels in the future.
Nutrient results:
Although results are provisional, nutrient levels throughout Powell demonstrate a response to recent climatic trends. Figure 7 displays surface nutrient values from Lees Ferry to the Colorado inflow stations, and generally represents or even exaggerates trends throughout depths of the reservoir. Across the lake and in the tailwaters, phosphorus values have increased since 1993. This could be a result of the increased inflows in recent years, or could be associated with biotic interactions, and will receive extensive study in the future. Ortho-phosphate, though often near detection limits, shows similar results. Nitrate-nitrite nitrogen alone demonstrates consistently decreasing concentrations paralleling conductance trends on the reservoir. As would be expected, nutrient levels are highest at the inflow and decrease toward the dam. Seasonal trends produce the highest concentrations associated with the inflow event, with nutrients metabolized, mixed and diluted through fall and winter.
Biological results:
Much of the plankton data is in the early stages of analysis. Figure 6 summarizes the data from an aerial perspective, showing the levels of phytoplankton biovolume and zooplankton biomass present in the reservoir and tailwaters for the last year. It shows that productivity for both peak in spring and summer. Primary
GCMRC - Annual IWQP Report September 2000 1