On Wednesday the Chesapeake Executive Council, made up of the Governors of Maryland, Pennsylvania, and Virginia, the Administrator of the U.S. EPA, the Mayor of Washington, D.C. and the Chair of the Chesapeake Bay Commission, met for their annual meeting to discuss strategies to improve the Chesapeake Bay and watershed.  After the meeting, Governor Rendell of Pennsylvania committed to supporting research into the increasing threat of sediment build-up behind the hydroelectric dams on the lower Susquehanna River. Most people probably haven’t heard about the issues with the dams, so I would like to share some information.

Since the building of the four dams, these dams have trapped much of the sediment and phosphorus coming from Pennsylvania. Of the lower Susquehanna River dams, York Haven, Safe Harbor, Holtwood, and Conowingo, all but Conowingo, the furthest south, have reached capacity and are no longer trapping sediment. Sediment behind Conowingo Dam and the loss of its sediment retaining capacity now present imminent and substantial threats to the Chesapeake Bay.

The first threat to the Bay is a repeat of the “catastrophic pulse”. During 4 days in 1972, flood water from Tropical Storm Agnes carried 4 years worth of sediment and pollutants down the Susquehanna River. When the flood water reached the lower Susquehanna dams it scoured another 8 years of sediment that had been trapped in the reservoirs (most from Conowingo). This “catastrophic pulse” of 12 years worth, or 30 million tons of sediment combined with the surge of freshwater to inflict the biggest single damaging event ever recorded in the Chesapeake Bay. Over the past 35 years this sediment has built up to a level exceeding 1972 levels, creating a threat of damages even greater than that experienced in 1972. Scientists agree that the question is not if this will occur again, but only a matter of when.

The second threat builds as the Conowingo Pool reaches sediment storage capacity causing a massive increase of sediment and phosphorus to the Chesapeake Bay. The Conowingo Dam currently collects and retains about 65% of the sediment coming down the Susquehanna. This essentially makes the Conowingo Dam the biggest single mitigation tool, or “Best Management Practice”, in the Chesapeake Bay. Once sediment trapping capacity is reached, the average load of sediment from the Susquehanna to the Chesapeake Bay will increase by up to 250% (from 1.2 million tons to 3.2 million). This sediment will contain as much as 3.5 million pounds of phosphorus, an increase of 30 to 40%. These increases will have major effects on the Chesapeake Bay from the size of “dead zones”, to feeding and breeding capabilities of aquatic species, to channel dredging frequency and costs. 

One major storm, or simply letting the sediment build up, will both lead to a catastrophe. In New Orleans we saw what can happen when warnings like these are ignored. These threats must be dealt with, and the Chesapeake Executive Council has shown great forethought in making this a priority. I hope this explanation helps our citizens and legislators understand the need for immediate action.  

From the Mighty Susquehanna,
Michael R. Helfrich
Lower Susquehanna RIVERKEEPER®

 

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