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Last modified
7/14/2009 5:02:30 PM
Creation date
5/20/2009 10:41:36 AM
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UCREFRP
UCREFRP Catalog Number
7409
Author
U.S. Department of the Interior.
Title
Quality of Water, Colorado River Basin.
USFW Year
1991.
USFW - Doc Type
Progress Report No. 15,
Copyright Material
NO
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<br />MAIN STEM RESERVOIR QUALITY 13 <br /> <br />(low productivity in the primary food chain) on <br />the basis of area-weighted, average chlorophyll-a <br />concentrations. Lake Mohave and Lake Havasu <br />were found to be mesotrophic (medium <br />productivity) based on that trophic state <br />criterion. The oligotrophiclmesotrophic nature <br />of the reservoirs is due to low phosphorus <br />concentrations that persist in most of the middle <br />to lower Colorado River. <br /> <br />Since most of the phosphorus is associated with <br />suspended sediments, sedimentation in the <br />headwaters of Lake Powell effectively retains <br />most of the phosphorus that historically flowed <br />downstream. Suspended sediments and <br />phosphorus inputs from the Grand Canyon <br />rapidly drop out in the upper end of Lake Mead. <br />The Virgin River and Muddy River inflows to <br />Lake Mead are minor sources of phosphorus to <br />the system. Las Vegas Wash is the principal <br />tributary input of phosphorus to the <br />river-reservoir system at and below Lake Mead. <br />Most of this input is in the form ofbio-available <br />phosphorus. <br /> <br />~\ <br />i \ <br />\ <br />, <br />\ <br /> <br />The Las Vegas Wash inflow significantly <br />elevates phosphorus concentrations in the inner <br />and middle Las Vegas Bay, and it causes some <br />increase in concentrations in Boulder Basin and <br />the Hoover Dam discharge. Phosphorus loading <br />to Lake Mohave increases as a result of inputs <br />from Las Vegas Wash. Phosphorus retention in <br />Lake Mohave is low due to rapid flushing of the <br />reservoir. Most of the phosphorus discharged <br />from Hoover Dam is thus routed through Lake <br />Mohave into Lake Havasu. Additional <br />phosphorus inputs to Lake Havasu are derived <br />from the Bill Williams River and possibly from <br />pickups in the reach between Davis Dam and <br />upper Lake Havasu. <br /> <br />'!'he Las Vegas Wash inflow contributes to the <br />h~gher productivity in the downstream <br />re~ervoirs. The decrease in phosphorus loading <br />that. has occurred from Las Vegas and Clark <br />County Sewage Treatment Plants can be <br />expected to decrease productivity in Lake <br />Mohave and possibly Lake Havasu. The slight <br />decrease that occurred in chlorophyll-a <br />concentrations in Lake Mohave during 1982 <br />probably reflects the reduction in phosphorus <br />loading. Productivity in the Boulder Basin area <br />of Lake Mead has undergone a steady decline <br /> <br />since the late 1970's when phosphorus loading <br />from Las Vegas Wash began to decrease. This <br />appears to be a major factor responsible for the <br />recent decline of the fisheries in the reservoir. <br />Similar reductions in the productivity of Lake <br />Mohave will probably also result in a decline in <br />fish production. This problem should be <br />carefully evaluated in ongoing reviews of current <br />wastewater treatment practices at the city of <br />Las Vegas and Clark County Sewage Treatment <br />Plants. A relaxation oftha phosphorus <br />standards at Las Vegas Bay during the winter <br />and a tightening of the standards, as proposed <br />by Nevada, may be warranted considering the <br />low productivity in the river system and the <br />damage occurring to the beneficial uses in Las <br />Vegas Bay. <br /> <br />Lower Colorado RIver BasIn ReservoIr <br />MonItorIng Program <br /> <br />Reclamation's Lower Colorado. Region funded a <br />2-year monitoring program on Lakes Mead, <br />Mohave, and Havasu. The program was <br />implemented in December 1985 and completed <br />in November 1987. The major portion of the <br />study was conducted under contract by the <br />Environmental Research Center, University of <br />Nevada, Las Vegas. Data were collected on a <br />monthly basis at 13 stations on Lake Mead, <br />3 stations on Lake Mohave, and 3 stations on <br />Lake Havasu (2 of those were quarterly). Water <br />column profile measurements at each site <br />included pH, water temperature, dissolved <br />oxygen, and electrical conductivity. Secchi <br />depths and limnophotometer measurements <br />were made. Water samples were taken from <br />various depths to be analyzed for nutrients, <br />zooplankton, chlorophyll, and for their major <br />ions. Current velocity measurements were also <br />made. <br /> <br />A summary of the program will be completed by <br />the Environmental Sciences Section, Research <br />Laboratory Services Division, Bureau of <br />Reclamation, Denver Office. <br /> <br />Reclamation has been working with the National <br />Park Service, Arizona Game and Fish <br />Department, and Nevada Department of Wildlife <br />in the investigation of what appears to be a <br />decline in the black bass fishery of Lakes Mead <br />
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