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Sediment deposition in Lake Clarke, Lake Aldred, and Conowingo Reservoir, Pennsylvania and Maryland, 1910-93

January 1, 1994

The Susquehanna River carries a significant amount of the sediment and the nutrient load transported to the Chesapeake Bay. Three large hydroelectric dams are located near the mouth of the Susquehanna River. The three dams and associated reservoirs are Safe Harbor (Lake Clarke) and Holtwood (Lake Aldred) in southern Pennsylvania and Conowingo (Conowingo Reservoir) in northern Maryland. Two of these reservoirs, Lakes Clarke and Aldred, have reached a state of equilibrium with sediment transport in the river. The third, Conowingo Reservoir, continues to accumulate sediment as well as particulate organic nitrogen and particulate phosphorus. Bottom-elevation surveys of Conowingo Reservoir made in 1959, 1990, and 1993 indicate that the reservoir will reach equilibrium with sediment transport of the river during the next 10 to 20 years. Data collected from 1985-89 indicate that the Susquehanna River transports about 1,780 million pounds of sediment, 147 million pounds of nitrogen, and 5.1 million pounds of phosphorus to the Chesapeake Bay during a year of normal streamflow. Once equilibrium is reached in the Conowingo Reservoir, these loads may increase to levels currently transported by the river to the reservoirs, about 6,600 million pounds of sediment, 153 million pounds of nitrogen, and 9.1 million pounds of phosphorus per year. These higher loads may effect progress made on reducing nutrient loads and should be considered when planning future programs.