Document Type

Technical Report

Publication Date

6-1981

Publication Title

Lake Mead Limnological Research Center: Technical Report Series

Publisher

Environmental Research Center, University of Nevada, Las Vegas

Volume

6

Abstract

Lake Mead has undergone a serious decline since Glen Canyon Dam was constructed 450 km upstream in 1963.

State fisheries management agencies are concerned that the decline was caused by water level fluctuations and more severe drawdowns during the bass spawning season, when the operation of Hoover Dam was altered during the post-Lake Powell period.

The construction of Glen Canyon Dam and formation of Lake Powell in 1963 drastically altered the natural discharge and temperature cycles and decreased suspended sediment and nutrient loading in the Colorado River inflow to Lake Mead. Recent studies indicate that these changes in nutrient loading have caused a decrease in the fertility and productivity of Lake Mead, and this, in turn, could have contributed to the decline in the largemouth bass fishery.

The Water and Power Resources Service initiated a detailed investigation of the chemical and biological properties of Lake Mead sediments in order to: (i) resolve questions regarding historical changes in fertility and productivity of the reservoir, (ii) better assess the cause(s) for the decline in the largemouth bass fishery and (iii) predict future impacts associated with the proposed power modifications to Hoover Dam and operation of pump-storage hydroelectric units.

Sediment cores were collected with a Vibra-corer by a commercial, oceanographic drilling firm in non-delta areas of the inner Las Vegas Bay, middle Las Vegas Bay, Boulder Basin, Virgin Basin, Bonelli Bay and the Overton Arm. Sediments were dated by 137Cs assays and analyzed for organic content, organic carbon, total nitrogen, total phosphorus, organic phosphorus, NaOH -extractable phosphorus, calcium carbonate, bulk density and water of hydration.

Individual-basin (Lower and Upper Basins) and reservoir-wide sedimentation rates were estimated for autochthonous and allochthonous organic carbon and calcium carbonate, nitrogen and phosphorus and dry weight during three periods (<1954, 1955-1962, <1963) of Lake Mead history. Autochthonous organic carbon sedimentation in the post-Lake Powell period was used with recent measurements of phytoplankton productivity to develop a regression model for predicting historic rates of productivity.

Reservoir-wide sedimentation rates and productivity in Lake Mead were relatively low during the period prior to 1954. Increased nutrient loading in years of high runoff during the 1955-1962 period caused a sharp increase in reservoir-wide sedimentation and productivity. The Upper Basin was especially productive during this period due to large inputs of suspended sediments and phosphorus. Phosphorus loading in the Colorado River decreased by over 90% in the post-Lake Powell period and caused a severe reduction in productivity in the Upper Basin. Increased phosphorus loading from the discharges of secondary-treated sewage effluents into Las Vegas Bay, combined with relatively high nitrogen loading from the Colorado River, elevated productivity in the Lower Basin. However, this was not sufficient to offset reductions that occurred in the Upper Basin, and reservoir-wide productivity decreased by 77% during the post-Lake Powell period and was equivalent to productivity in the period prior to 1954.

This decline in productivity was accompanied by a decrease in abundance of zooplankton, which comprise the principal food source for largemouth bass fry. Survival of bass fry appears to have decreased in the face of low zooplankton abundance, and this may be the cause for the historic decline of the largemouth bass fishery.

The fertility and productivity of Lake Mead could be improved to benefit the bass fishery if: (i) Hoover Dam were operated from a surface, rather than deep, discharge, (ii) pump-storage hydroelectric units were operated to recirculate nutrients in the reservoir, (iii) nutrient loading from Las Vegas Wash were maintained at current levels or allowed to increase with some type of diffuser system to minimize the point source problem in the inner Las Vegas Bay.

Keywords

Freshwater fishes; Hydrobiology; Hydrochemistry; Lake Mead (Ariz. and Nev.); Sedimentation analysis

Disciplines

Biochemistry | Environmental Indicators and Impact Assessment | Environmental Monitoring | Fresh Water Studies | Natural Resources and Conservation

Language

English