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<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 />rj the tail water . <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 />oligotrophicjmesotrophic 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 />phosphorus 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 wash 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 bie-available phosphorus. <br /> <br />The Las Vegas Wash 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 Wash. 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 Williams River and possibly from pickups <br />in the reach between Davis Darn and upper Lake Havasu. <br /> <br />The Las Vegas Wash inflow contributes to the higher productivity in the <br />downstream reservoirs. The decrease in phosphorus loading that has occurred <br />from Las Vegas and Clark County Sewage Treatment plants can be expected to <br />decrease productivity in Lake Mohave and possibly Lake Havasu. The slight <br />decrease that occurred in chlorophyll-a concentrations in Lake Mohave during <br />1982 probably reflects the reduction in phosphorus loading. productivity in <br />the Boulder Basin area of Lake Mead has undergone a steady decline since the <br />late 1970's when phosphorus loading from Las Vegas Wash began to decrease. <br />This appears to be a major factor responsible for the decline of the fisheries <br />recently experienced in the reservoir. Similar reductions in the productivity <br />of Lake Mohave will probably also result in a decline in fish production. <br />This problem should be carefully evaluated in ongoing reviews of current <br />wastewater treatment practices at the city of Las vegas and Clark county <br />Sewage Treatment Plants. A relaxation of the phosphorus standards at Las <br />Vegas Bay during the winter and a tightening of the standards, as proposed by <br />Nevada, may be warranted considering the low productivity in the river system <br />and the damage occurring to the beneficial uses in Las Vegas Bay. <br /> <br />111-7 <br /> <br />- <br />