Sediment Buildup Behind the Conowingo Dam

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On Monday, an article appeared in The Washington Post on the Conowingo Dam in Maryland, and Governor Larry Hogan’s efforts to have the sediment behind the dam dredged. While sediment buildup behind the Dam poses an environmental threat to the Chesapeake Bay, and I think the Dam should be dredged, I don’t think this is the biggest threat to the health of the Bay. I would also like to see Hogan focus on reducing nutrient loads to the Chesapeake, from agricultural and urban/suburban runoff in Maryland. That being said, it’s good to see the current Governor tackling water quality issues for the Chesapeake Bay, and committed to reducing sediment loads to the Bay from the Conowingo Dam.

Located at the Southern end of the Susquehanna River, the Conowingo Dam was constructed in 1929. Since that time, (1929-2012), about 470 million tons of sediment have been transported from the Susquehanna into the Conowingo Reservoir, where 280 million tons of sediment have been trapped by the dam, and 190 million tons have gone on to reach the Chesapeake Bay (USGS, 2014).

Some reports differ on whether the Conowingo Dam has reached storage capacity for sediment. The USGS says yes; a 2010 EPA study said the Dam had not yet reached full capacity, and likely would not for another 15-20 years. However, the EPA stated in this same 2010 report, that, “once the Conowingo Reservoir reaches the sediment trapping capacity, the sediment and nutrient loads delivered to the Chesapeake Bay via the Susquehanna River will equal the load delivered into the reservoir system. Once storage capacity is reached, the nitrogen load will increase by 2%; the phosphorus load will increase by 40%; and the suspended sediment load will increase by at least 150%” (EPA, 2010, T-3).

The Conowingo Reservoir holds nutrients and sediment from the Susquehanna River, a tributary of the Chesapeake Bay known for its high loads of nitrogen, phosphorus, and sediment. The Susquehanna carries runoff to the Bay from agricultural fields, and urban and suburban regions in the upper half of the Chesapeake Bay watershed, from parts of New York, Pennsylvania, and Maryland. This river delivers half of the Bay’s freshwater, and contributes 40% of the nitrogen, 20% of phosphorus, and a significant amount of sediment to the Chesapeake (CBF, 2006). Should the Conowingo Reservoir reach full capacity, even more nitrogen, phosphorus and sediment will reach the mainstem of the Chesapeake Bay from the Susquehanna.

The most significant concern in regards to the Conowingo Dam, is the buildup of sediment. Major storm events cause sediment backed up behind the Dam to overflow directly into the Chesapeake Bay. When the Conowingo Reservoir is at full capacity, this risk of overflow increases. Sediment is an issue for the Bay, when excess loads delivered to the Bay block out sunlight for underwater grasses, which they need to survive. Sediment can also cover oyster beds, suffocating mature and juvenile oysters. Sedimentation can lead to poor water quality, impacting other fish and shellfish species in the estuary.

To reduce the risk of sediment and nutrient overflow from the Conowingo Dam, Governor Hogan is calling for dredging of sediment behind the dam, and wants the dam’s operating company, Exelon Corp., to pay for this estimated $250 million project. Exelon is fighting back, citing studies from the Maryland Department of the Environment and the Army Corps of Engineers that state nutrients are far more harmful to the Bay than sediment. Exelon is also arguing that the sediment from the Dam is only harmful to small sections of the Bay, and contributes a very small percentage of sediment to the estuary, (the Post article says less than 5%).

I agree that nutrients from agricultural practices, and urban and suburban runoff pose a larger threat to the Bay as a whole, and Hogan should focus more energy on reducing nutrient loads from these sectors. However, if the Conowingo Reservoir is indeed at full capacity, the Bay, even a small portion of it, cannot risk a 150% increase of sediment load, which is likely to happen with a large storm event. (For example sediment scour, or removal of sediment from behind the Dam, reached millions of tons for major storm events in the past. Tropical Storm Agnes in 1972 resulted in 13.5 million tons of sediment scour. A more recent major storm event, Tropical Storm Lee in 2011, resulted in 3.5 million tons of sediment scour). An increase in sediment load this large is sure to cause water quality issues in the upper reaches of the Bay, and harm fish and shellfish species in this region.

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