Laserfiche WebLink
<br />Selenium concentrations appear to be derived from sources above <br />Davis Dam. Selenium concentrations in all media were lower in tributaries and <br />agricultural drain samples than in nearby mainstream samples. Although <br />agricultural practices in the Lower Colorado River Valley do not appear to <br />~ increase selenium concentrations, further changes to the watershed that would <br />CO raise the concentrations of selenium and other trace constituents in the food <br />OJ chain could adversely impact fish resources in the future. <br />1\":'1 <br /> <br />B. Kain Stem Reservoir Quality <br /> <br />Each summer, the upper riverine reaches of Flaming Gorge Reservoir <br />experience intense blooms of blue-green algae that seriously degrade the water <br />quality for game fish and recreational boating. There is also evidence that <br />geochemical processes in the reservoir sediments affect both the intensity of <br />the algal blooms and the salinity in the overlying water and that the algal <br />blooms in turn affect the geochemical processes. <br /> <br />At present, it is not known how effective restoration strategies, <br />including external phosphorus and biological oxygen demand (BOD) loading <br />reductions to the Flaming Gorge Reservoir, will be in reducing the intensity <br />of the algal blooms or in increasing the dissolved oxygen concentrations in <br />the water column. It is also not known how changes in the limnology of the <br />riverine reach of the reservoir resulting from mitigative measures will affect <br />fisheries and water quality in the downstream reaches of the reservoir or in <br />the tailwater. <br /> <br />Other reservoirs on the Colorado River were found to be nutrient poor in <br />1981 and 1982 (1]. Lake Powell and Lake Mead were oligotrophic, low in <br />productivity in the primary food chain, on the basis of area-weighted, average <br />chlorophyll-a concentrations. Lake Mohave and Lake Havasu were found to be <br />mesotrophic (medium productivity) based on that trophic state criterion. The <br />oligotrophic/mesotrophic nature of the reservoirs is due to low phosphorus <br />concentrations that persist in most of the middle to lower Colorado River. <br /> <br />Since most of the phosphorus is associated with suspended sediments, <br />sedimentation in the headwaters of Lake Powell effectively retains most of the <br />phosphoru~ that historically flowed downstream. Suspended sediments and <br />phosphorus inputs from the Grand Canyon rapidly drop out in the upper end of <br />Lake Mead. The Virgin River and Muddy River inflows to Lake Mead are minor <br />sources of phosphorus to the system. Las Vegas Vash is the principal <br />tributary input of phosphorus to the river-reservoir system at and below Lake <br />Mead. Most of this input is in the form of bio-available phosphorus. <br /> <br />The Las Vegas Vash inflow significantly elevates phosphorus <br />concentrations in the inner and middle Las Vegas Bay, and it causes some <br />increase in concentrations in Boulder Basin and the Hoover Dam discharge. <br />Phosphorus loading to Lake Mohave increases as a result of inputs from LAS <br />Vegas Vash. Phosphorus retention in Lake Mohave is low due to rapid flushing <br />of the reservoir. Most of the phosphorus discharged from Hoover Dam is thus <br />routed through Lake Mohave into Lake Havasu. Additional phosphorus inputs to <br />Lake Havasu are derived from the Bill Villiams River and possibly from pickups <br />in the reach between Davis Dam and upper Lake Havasu. <br /> <br />The Las Vegas Vash inflow contributes to the higher proouctivity in the <br />downstream reservoirs. The decrease in phosphorus loading that has occurred <br /> <br />1II-4 <br />