According to a new USGS report, the Susquehanna River delivered more phosphorus and sediment to the Bay during 2011 than from than any other year since monitoring began in 1978.
Flooding from Tropical Storm Lee made up a large fraction of the Susquehanna River’s inputs to the Bay for both 2011 and over the last decade. During the flooding the Susquehanna River delivered about 2 percent of total water to the Bay for the last decade; however, it delivered 5 percent of the nitrogen, 22 percent of the phosphorus, and 39 percent of the suspended sediment.
According to the report, from 1996-2011 total phosphorus moving into the Bay has increased by 55 percent, and suspended sediment has increased by 97 percent. Over this time period, total nitrogen decreased by about 3 percent overall, but showed increases during large events.
The lower reaches of the Susquehanna River, just upstream from Chesapeake Bay, include three reservoirs: Safe Harbor Dam and Holtwood Dam in Pennsylvania and Conowingo Dam in Maryland. Over the past several decades these reservoirs have been gradually filling with sediment.
“The upstream reservoirs have served previously to help reduce nutrient pollutant loads to the Chesapeake Bay by trapping sediment and the pollutants attached to them behind dams,” explained USGS Director Marcia McNutt. “Now that these reservoirs are filling to capacity with sediment, they have become much less effective at preventing nutrient-rich sediments from reaching the Bay. Further progress in meeting the goals for improving water quality in the Chesapeake will be more difficult to achieve as a result.”
“It has been understood for many years that as the reservoirs on the Lower Susquehanna River fill with sediment, there will be a substantial decrease in their ability to limit the influx of sediment and nutrients, especially phosphorus, to the Chesapeake Bay,” said Bob Hirsch, research hydrologist and author of the report. “Analysis of USGS water quality data from the Susquehanna River, particularly the data from Tropical Storm Lee in September 2011, provides evidence that the increases in nutrient and sediment delivery are not just a theoretical issue for future consideration, but are already underway.”
Another recent USGS study reported about a 25 percent reduction in nutrients and sediment concentrations just upstream of the reservoirs, reflecting the benefit of actions to improve water quality in the upper portion of the Susquehanna River watershed.
On average, the Susquehanna River contributes nearly 41 percent of the nitrogen, 25 percent of the phosphorus, and 27 percent of the sediment load to the Bay.
Sediment research was conducted as part of The USGS National Research Program in Water Resources and the USGS Chesapeake Bay Ecosystems Program. The report, titled Flux of nitrogen, phosphorus, and suspended sediment from the Susquehanna River Basin to the Chesapeake Bay during Tropical Storm Lee, September 2011, as an indicator of the effects of reservoir sedimentation on water quality, is available online at usgs.gov
source: USGS
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