A program of in situ and laboratory studies was conducted in the Southwestern Region of Virginia, U.S.A. to determine the role of the deposited sediments of the Niagara Reservoir in affecting the quality of the flow of the Roanoke River above the headwaters of Smith Mountain Lake. Agronomic analysis techniques were adapted and applied to provide quantitative analysis capability for the sediments. The surficial sediments were observed to constitute a considerable reservoir of phosphorus, which, during the anoxic bottom conditions occurring during the summer months, caused an enhancement of concentrations in the water column. Conversely, during the winter months, the surficial sediments acted as a phosphorus sink, removing it from the water column. Laboratory microcosm studies made it possible to quantify the release and removal of phosphorus under aerobic and anaerobic conditions, and to observe the factors upon which the exchanges depended. The phosphorus interactions were found to be governed principally by sediment iron and organic matter content.
The deposited sediments of Niagara Impoundment showed a capacity to redistribute the impoundment influent nutrient load, causing an enhancement of the quantities released during the summer months, thus magnifying the impact on the waters of Smith Mountain Lake. The maintenance of aerobic benthic conditions could prevent the sediment releases from occurring. In the long term, sediment nutrients deposited in the past, without proper control, could provide a continuing source of phosphorus to the waters of Smith Mountain Lake, thereby reducing the effectiveness of upstream control of point and diffuse sources.