Changes in sediment and nutrient storage in three reservoirs in the lower Susquehanna River Basin and implications for the Chesapeake Bay

Abstract

The Susquehanna River contributes nearly 50 percent of the freshwater discharge to the Chesapeake Bay in a year of normal or average streamflow. The river also transports the greatest amount of nutrients (estimates of nearly 66 percent of the nitrogen and 40 percent of the phosphorus load) from all nontidal areas in the Chesapeake Bay Basin. Excessive nutrients in the Bay result in algal blooms that decrease the amount of light reaching submerged aquatic vegetation, and upon decomposition, deplete the oxygen in the water. In a normal-flow year, the Susquehanna River also contributes about 25 percent of the sediment load from non-tidal areas to the Bay. Suspended sediments also reduce light needed by submerged aquatic vegetation and can smother living-resource habitat and obstruct fish gills. A reservoir system consisting of Lake Clarke, Lake Aldred, and Conowingo Reservoir is formed by three consecutive hydroelectric dams on the Lower Susquehanna River. Safe Harbor Dam, which forms Lake Clarke, was built in 1931. Holtwood Dam, the smallest of the three, was built in 1910 to form Lake Aldred. The largest and most downstream dam, Conowingo Dam, was built in 1928 and forms Conowingo Reservoir. Since construction, the reservoirs have been filling with sediment and sediment-associated nutrients. The upper two reservoirs have reached their capacity to store sediments and generally no longer trap nutrients and sediments. Conowingo Reservoir has not reached storage capacity, however, and is currently trapping about 70 percent of the suspended-sediment load, 2 percent of the total-nitrogen load, and 40 percent of the total-phosphorus load that would otherwise be discharged to the Chesapeake Bay (Ott and others, 1991). In 1990, 1993, and 1996, the U.S. Geological Survey collected information on the depth to sediment in the reservoirs to determine the remaining sediment-storage capacity in the reservoir system and to estimate when the reservoirs will reach sediment-storage capacity. In addition, sediment cores were collected and analyzed in 1993 and 1996 to determine the nutrient mass remaining in the Conowingo Reservoir. The 1996 data collection followed a major flood in the Susquehanna River Basin.