Mountain Island Lake is an impoundment of the Catawba River in North Carolina and supplies drinking water to more than 600,000 people in Charlotte, Gastonia, Mount Holly, and several other communities. The U.S. Geological Survey, in cooperation with the Charlotte-Mecklenburg Utilities, conducted an investigation of the reservoir to characterize hydrologic and water-quality conditions and to develop and apply a simulation model to predict the response of the reservoir to changes in constituent loadings or the flow regime.
During 1996-97, flows into Mountain Island Lake were dominated by releases from Cowans Ford Dam on Lake Norman, with more than 85 percent of the total inflow to the reservoir coming from Lake Norman. Riverbend Steam Station discharges accounted for about 12 percent of the inflows to the reservoir, and inflows from tributary streams contributed less than 1.5 percent of the total inflows. Releases through Mountain Island Dam accounted for about 81 percent of outflows from the reservoir, while Riverbend Steam Station withdrawals, which were equal to discharge from the facility, constituted about 13 percent of the reservoir withdrawals. About 5.5 percent of the withdrawals from the reservoir were for water supply.
Strong thermal stratification was seldom observed in Mountain Island Lake during April 1996-September 1997. As a result, dissolved-oxygen concentrations were only infrequently less than 4 milligrams per liter, and seldom less than 5 milligrams per liter throughout the entire reservoir, including the coves. The Riverbend Steam Station thermal discharge had a pronounced effect on surface-water temperatures near the outfall.
McDowell Creek, which drains to McDowell Creek cove, receives treated wastewater from a large municipal facility and has exhibited signs of poor water-quality conditions in the past. During April 1996-September 1997, concentrations of nitrate, ammonia, total phosphorus, and chlorophyll a were higher in McDowell Creek cove than elsewhere throughout the reservoir. Nevertheless, the highest chlorophyll a concentration measured during the study was 13 micrograms per liter-- well below the North Carolina ambient water-quality standard of 40 micrograms per liter. In the mainstem of the reservoir, near-bottom ammonia concentrations occasionally were greater than near-surface concentrations. However, the relatively large top-to-bottom differences in ammonia and phosphorus that have been observed in other Catawba River reservoirs were not present in Mountain Island Lake.
External loadings of suspended solids, nitrogen, phosphorus, and biochemical oxygen demand were determined for May 1996-April 1997. Flows through Cowans Ford Dam contributed more than 80 percent of the biochemical oxygen demand and nitrogen load to the reservoir, with McDowell Creek contributing about 15 percent of the biochemical oxygen demand load. In contrast, McDowell Creek contributed about half of the phosphorus load to the reservoir, while inflows through Cowans Ford Dam contributed about one-fourth of the phosphorus load, and the McDowell Creek wastewater-treatment plant contributed about 15 percent of the total phosphorus load. The remainder of the phosphorus loadings came from Gar Creek and the discharge from the Riverbend ash settling pond.
Mountain Island Lake is a relatively small (11.3-square-kilometer surface area) impoundment. An area of 181 square kilometers drains directly to the reservoir, but much of this area is undergoing development. In addition, the reservoir receives treated effluent from a municipal wastewater-treatment facility.
The two-dimensional, laterally averaged model CE-QUAL-W2 was applied to Mountain Island Lake. The model was configured to simulate water level, water temperature, and 12 water-quality constituents. The model included the mainstem, four coves, three point-source discharges, and three withdrawals.
Simulated water levels generally were within 10 centimeters of
Additional Publication Details
USGS Numbered Series
Mountain Island Lake, North Carolina; analysis of ambient conditions and simulation of hydrodynamics, constituent transport, and water-quality characteristics, 1996-97