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, N. ... VI Q (low productivity in the primary food chain) on the basie of area-weighted, average chlorophyll_s concentrations. Lake Mohsve and Lake HSV8SU Were found to be mesotrophic (medium productivity) based on that trophic state criterion. The o!igotrophiclmesotrophic nalure of the reservoirs is due Ullow phosphorus concentrations that persist in most or the middle to lower Colorado River. Since most of the phosphorus is associated with 8uspended aediments, sedimentation in the headwaters of Lake Powell effectively retains most of the phosphorus that historically flowed downstream. Suspended sediments and phosphorus inputs from the Grand Canyon rapidly drop out in the upper end of Lake Mead. The Virgin River and Muddy River inflows to Lake Mead are minor sources of phosphorus to the sysU!m. Las Vegas Wash is the prinCipal tributary input of phosphorus to the river-reservoir system at and below Lake Mead. Most of this input is in the fonn ofbio-available phosphorus. The Las Vegas Wash inflow significantly elevates phosphorus concentrations in the inner and middle Las Vegas Bay, and it causes some increase in concentrations in Boulder Basin and the Hoover Dam discharge. Phosphorus loading to Lake Mohave increases as a result of inputs from Lalil Vegas Wash. Phosphorus retention in Lake Mohave ilillow due to rapid flushing of the relilervoir. Most of the phosphorus discharged from Hoover Dam is thus routed through Lake Mohave into Lake Havasu_ Additional phosphorus inputs to Lake Havasu are derived from the Bill Williams River and possibly from pickups in the reach between Davis Dam and upper Lake Havasu. The Las Vegas Wash inflow contributes to the higher productivity in the downstream reservoirs. The decrease in phosphorus loading that has occurred from Las Vegas and Clark County Sewage Treatment Plants can be expected to decrease productivity in Lake Mohave and possibly Lake Ha\'asu. The slight decrease that occurred in chlorophyll-a concentrations in Lake Mohave during 1982 probably reflects the reduction in phosphorus loading. Productivity in the Boulder Basin area of Lake Mead has undergone a steady decline MAIN STEM RESERVOIR QUALITY 13 since the late 1970's when phosphorus loading from Las Vegas Wash began to decrease. This appears to be a major factor responsible for the recent decline of the fisheries in the reservoir. Similar reductions in the productivity of Lake Mohave will probably also result in a deeline in fish production. This problem should be carefully evaluated in ongoing reviews of current wastewater treatment practices at the city of Las Vegas and Clark County Sewage Treatment Plants. A relaxation of the phosphorus standards at Las Vegas Bay during the winter and a tightening of the standards, as proposed by Nevada, may be warranted considering the low productivity in the river system and the damage occurring to the beneficial uses in Las Vegas Bay. Lower Colorado River BasIn Reservoir MonitOring Program Reclamation's Lower Colorado Region funded a 2-year monitoring program on Lakes Mead, Mohave, and Havasu. The program was implemented in December 1985 and compleU!d in November 1987. The major portion ofilie study was conducted. under contract by the Environmental Research Center, University of Nevada, Las Vegas. Data were collected on a monthly basis at 13 stations on Lake Mead, 3 stations on Lake Mohave, and 3 stations on Lake Havasu (2 of those were Quarterly), Water column profile measurements at each site included pH, water temperature, dissolved oxygen, and electrical conductivity, Secchi depths and Iimnophotometer measurements were made. Water samples were taken from various depths to be analyzed for nutrients, zooplankton, chlorophyll, and for their major ions. Current velocity measurements were also made. A summary of the program will be completed by the Environmental Sciences Section, Research Laboratory Services Division, Bureau of Reclamation, Denver Office. Reclamation has been working .....ith the National Park Senice, Arizona Game and Fish Department, and Nevada Department of Wildlife in the im'estigation of what appears to be a decline in the black bass fishery of Lakes Mead