From: David A Gabel, ENN
Published August 31, 2012 09:15 AM

As Susquehanna Nears Sediment Capacity, Chesapeake Bay Likely to Suffer

The Susquehanna is a mighty river in the northeastern United States which feeds into the Chesapeake Bay in Maryland. The river meanders through the Appalachian Mountains of Pennsylvania and into upstate New York. It is the longest river on the American east coast which drains into the Atlantic, and its watershed is the 16th largest in the country (27,500 square miles / 71,000 square km). The Susquehanna River carries sediment and runoff from the wilderness and farmlands of Pennsylvania and New York, and deposits them into the Chesapeake. However, too much nutrient runoff is a bad thing for the Bay ecosystem, so dams have been constructed to trap the sediment. A new report from the USGS found that the reservoirs behind those dams have reached their sediment capacity, with potentially disastrous consequence for the Bay.

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This is particular a problem for reservoirs directly before the Bay, near the mouth of the Susquehanna. As a result of large storms, above-normal flows of suspended sediment and nutrients are making their way downstream. Too many nutrients in the Bay, deprive the water of oxygen required for fish and a healthy aquatic ecosystems.

"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."

There are three reservoirs at the lower reaches of the Susquehanna River, just above the Bay. They are Safe Harbor Dam and Holtwood Dam in Pennsylvania, and Conowingo Dam in Maryland. The increased deposit of sediment creates a more narrow channel for water to flow through. This makes the water move faster, increasing the scouring, or sudden removal of large chunks of sediment.

Major storms, or "significant flow events" in the past few years include Tropical Storm Lee in 2011 and the heavy rains of Spring 2011. What is required now is a break in these flow events, or some serious dredging may have to be done. Otherwise, the fragile, recovering Chesapeake Bay will have to be degraded once again.

For more information: Lower Susquehanna River Watershed Assessment

Image credig: Jane Thomas (UMCES-IAN)

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