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Show concentration in hypolimnetic deep waters of Lake Powell. Overall, the TDS concentration in Lake Powell is at an all- time low due to heavy runoff since water year 1983 ( Jerry Miller, Bureau of Reclamation, personal communication, 1986). II. E. 4. Reservoir nutrient dynamics and productivity. Nutrient dynamics within Lake Powell are characterized by the high removal of phosphorus, which results from the deposition in sediment of phosphorus bound to silt and clays close to the tributary inflow points. Because phosphorous bound in sediment is biologically unavailable, it cannot contribute to lake productivity. Paulson and Baker ( 1984) estimate that Lake Powell traps 98 percent of the total phosphorus and 46 percent of the total nitrogen entering from the tributaries. Thus, with the exception of certain areas near the inflow of the tributaries, nutrient concentrations are low, with mean soluble reactive phosphorus concentrations ranging from 0.002 - 0.003 mg/ L and mean total phosphorus concentrations ranging from 0.008 - 0.010 mg/ L in 1981 and 1982 ( Paulson and Baker 1984). Because of its sediment dynamics and great depth, Lake Powell appears on Arizona's Clean Lakes Priority List as having the lowest trophic index in the state ( least eutrophic). Although extensive data is not available, the low nutrient concentrations found in Lake Powell indicate that primary productivity may be limited except in areas close to the source of nutrient inflow, such as the inflow points of the Colorado, San Juan, and Escalante Rivers. Thus, the river arms may be highly productive for fish and phytoplankton even though the lake itself is nutrient- poor. II. E. 5. Reservoir sedimentation and heavy metal deposition. Sedimentation is the deposition of silt and soil on the reservoir bottom when silt- loaded river waters enter the lake. The energy of flowing water, required to keep the silt in suspension, is much reduced in the slowly circulating lake, and sediments quickly settle to the lake bed below the mouths of the rivers. To a lesser degree, windblown soil from the shoreline contributes to sediment accumulation. Sedimentation gradually fills in lakes, although hundreds of years may be required to complete the process. Heavy metals are sometimes associated with deposited sediments. Graf ( 1985) states that the weathering of natural source rocks in the lake's watershed is responsible for most of the estimated 2200 kg of mercury deposited annually in Lake Powell. Also, elevated selenium concentrations in fish have been reported for Colorado River stations in Arizona ( Bill Kepner, U. S. Fish and Wildlife Service, personal communication, 1985), with some concentrations high enough to cause reproductive problems in the affected fish. The Bureau of Reclamation periodically measures reservoir- bottom profiles at Lake Powell to allow modeling of the sedimentation process. This information can be used to project reservoir life and to determine the usable life of potential development areas. A complete series of 15 |