Flux of Nitrogen, Phosphorous, and Suspended Sediment from the Susquehanna River Basin to the Chesapeake Bay During Tropical Storm Lee, September 2011, as in Indicator of the Effects of Reservoir Sedimentation on Water Quality

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Concentrations of nitrogen, phosphorus, and suspended sediment are measured at the U.S. Geological Survey streamgage at Conowingo Dam at the downstream end of the Susquehanna River Basin in Maryland, where the river flows into the Chesapeake Bay. During the period September 7–15, 2011, in the aftermath of Tropical Storm Lee, concentrations of these three constituents were among the highest ever measured at this site. These measurements indicate that sediment-storage processes behind the three dams on the lower Susquehanna River are evolving. In particular, they indicate that scouring of sediment (and the nitrogen and phosphorus attached to that sediment) may be increasing with time. Trends in flow-normalized fluxes at the Susquehanna River at Conowingo, Maryland, streamgage during 1996–2011 indicate a 3.2-percent decrease in total nitrogen, but a 55-percent increase in total phosphorus and a 97-percent increase in suspended sediment. These large increases in the flux of phosphorus and sediment from the Susquehanna River to the Chesapeake Bay have occurred despite reductions in the fluxes of these constituents from the Susquehanna River watershed upstream from the reservoirs. 


The water and the nitrogen, phosphorus, and suspended sediment—the three constituents for which there are watershed-specific goals under the Chesapeake Bay Total Maximum Daily Load (TMDL) requirements (U.S. Environmental Protection Agency, 2012)—that are derived from the Susquehanna River Basin and flow past Conowingo Dam are critically important to the ecological condition of the Chesapeake Bay. Reductions in nutrients are needed to limit algae blooms that die and sink to the bottom of the Chesapeake Bay and consume oxygen, resulting in hypoxic zones where fish and shellfish cannot survive. In addition, suspended sediments and these algae blooms limit the penetration of light that is important to grasses and other aquatic life on the bottom of the Chesapeake Bay. Estimates indicate that, on average, the Susquehanna River contributed nearly 47 percent of the freshwater and 41 percent of the nitrogen, 25 percent of the phosphorus, and 27 percent of the sediment load to the Chesapeake Bay during 1991–2000 (Gary Shenk, U.S. Environmental Protection Agency, written commun., 2012). In September 2011, Tropical Storm Lee produced an estimated 4 to 7 inches of rainfall throughout much of the Susquehanna River Basin, with precipitation in some areas exceeding 12 inches. This storm produced very large, although not unprecedented, flooding in various parts of the basin. It was widely noted in the media that the outflows from the Susquehanna River past Conowingo Dam produced a large plume of sediment that was visible in satellite photographs for several days after the flood peak. The visible plume extended at least as far as the mouth of the Potomac River, approximately 100 miles downstream from Conowingo Dam. A satellite image from September 13, 2011 (4 days after the peak streamflow at Conowingo Dam), is shown in figure 1. It was widely recognized that a volume of water and sediment of this magnitude would have transported large amounts of nitrogen and phosphorus to the Chesapeake Bay (Blankenship, 2011).

National Aeronautics and Space Administration Moderate Resolution Imaging Spectroradiometer (MODIS) photograph from the Terra sa

The U.S. Geological Survey (USGS) streamgage on the Susquehanna River at Conowingo, Maryland (hereafter referred to in this report as “Susquehanna River at Conowingo”), has a 44-year streamflow record (1968–2011). The maximum daily discharge during the flood of September 2011, associated with Tropical Storm Lee, was 709,000 cubic feet per second (ft3/s) on September 9, 2011; this value was the second largest annual maximum daily discharge recorded for water years 1968–2011. The largest occurred during the flood of June 1972, a result of Hurricane Agnes, with a maximum daily-mean discharge of 1,120,000 ft3/s. The third and fourth largest events in the period were 662,000 ft3/s in 1975 and 622,000 in 1996, respectively. Therefore, the Tropical Storm Lee flood event, although large, was of a magnitude that can be expected to occur about once every 20 years as determined from the data recorded at this streamgage. 

An analysis of the transport of sediment and nutrients during a flood event can benefit from an examination of discharge over a period of several days, rather than the peak discharge alone. The 2011 flood, when measured as an average of the highest 7 consecutive days of flow, was the third largest in the period of record (387,000 ft3/s, compared to 712,000 ft3/s in 1972 and 391,000 ft3/s in 1993). Seven-day average maximum flows for 8 of the 44 years of record exceeded 300,000 ft3/s. By this measure, the Tropical Storm Lee flood falls well within a range that can be expected to occur once every 10 to 15 years. 







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