Conowingo Dam and the Chesapeake Bay

Last week Governor Hogan of Maryland announced that Maryland will request bids to test dredging and reuse of the dredged material from the Conowingo Dam reservoir. The test will involve removing about 25,000 cubic yards of sediment and is estimated to cost $4 million. This is the first step to remove 31 million cubic yard of sediment that have almost filled the reservoir which Maryland has estimated will cost $250 million to dredge. Dredging the reservoir is essential to protect the Chesapeake Bay.

The Conowingo Dam is a large hydroelectric dam on the Lower Susquehanna River about 10 miles upstream from where the river flows into the Chesapeake Bay at Havre De Grace, Maryland. The three dams at the downstream end of the Susquehanna River have been important in mitigating the downstream transport of nitrogen, phosphorus, and suspended sediment from the Susquehanna River watershed to the Chesapeake Bay. The Conowingo, the last dam in a series of three traps polluted sediment from the Susquehanna River in its 9,000 acre reservoir.

The U.S. Environmental Protection Agency (EPA) placed a contamination limit called the TMDL (total maximum daily load for nutrient contamination and sediment) on all the states in the Chesapeake Bay Watershed and Washington DC. The TMDL sets a total limit for the entire watershed of 185.9 million pounds of nitrogen, 12.5 million pounds of phosphorus and 6.45 billion pounds of sediment per year that represents a 25% reduction in nitrogen, 24% reduction in phosphorus and 20 % reduction in sediment from the 2011 levels. The pollution limits were then partitioned to the various states and river basins based on the Chesapeake Bay computer modeling tools and monitoring data.

When the EPA allocated the nitrogen, phosphorus and sediment reductions among the Chesapeake Bay states, the EPA believed that the Conowingo Dam would continue to trap polluted sediment for an additional quarter of a century and counted the sediment and nutrient removal of the Conowingo Dam in the design of the nutrient and sediment reductions. Subsequent studies by the U.S. Geological Survey (USGS) and the Army Corps of Engineers found that sediment was gathering at the dam at a faster rate than assumed by the EPA’s model.

The three dams on the lower Susquehanna River (Safe Harbor, Holtwood, and Conowingo) and their associated reservoirs (Lake Clarke, Lake Aldred, and Conowingo Reservoir) have been the topic of a number of studies (Langland, 2009; Langland, 1998; Langland and Hainly, 1997), all of which generally conclude that the pools behind the dams were on a long-term trajectory to becoming filled. Recently, the USGS has concluded that the dam’s sediment reservoir is almost full. There are nearly 200 million tons of sediment, nutrients, and other pollutants from the Susquehanna River trapped behind the dam.

Analysis of water-quality data collected during the large flood in the aftermath of Tropical Storm Lee (September 2011), as well as high flows that occurred in March 2011, indicate a significant decrease in the effectiveness of the reservoir system at trapping nitrogen, phosphorus, and sediment. Rather than reaching capacity in 2030 to 2035 as originally projected by the EPA, the Conowingo Dam is already 95% full and will be full and cease protecting the bay from sediment within the next three years.

The Conowingo Dam will no longer be able to trap sediment in the Susquehanna River and prevent them from entering the Chesapeake Bay. The Susquehanna River flows 464 miles from Cooperstown, New York to Havre De Grace, Maryland collecting sediment and nutrient runoff along the way. The Susquehanna drains an area of more than 27,000 square miles and is the single largest source of fresh water flowing into Chesapeake Bay. The river currently provides nearly half of the Bay’s freshwater, 41% of its nitrogen, 25% of its phosphorus and 27% of its sediment load. Without the Conowingo removing sediments containing nitrogen and phosphorus before the waters reach the Chesapeake Bay that contamination load will increase.

EPA can’t just assign increased reductions to Maryland, Pennsylvania and New York. The sources of most of this sediment is non-point source in origin. To meet the long term goals of the Chesapeake Bay TMDL needs the Conowingo Dam to trap excess sediment from the river –it is the cheapest way to meet the goals.

Also, the Conowingo Dam cannot be left full. It will not exist in some gentle equilibrium. The Conowingo and its sister reservoirs will not be constantly filled to capacity with sediments because of short-term changes from severe storms that cause scour and a subsequent reduction in exported sediments until the scoured amount is refilled. Therefore, the amount of sediment transported out of the reservoirs will not always be in equilibrium with the amount of sediment transported into the reservoirs.

After large storm events there will be plumes of sediment and nutrients released into the river and the Chesapeake bay damaging any progress made by the TMDL program. However Governor Hogan works out who will contribute to the costs, the Conowingo Dam needs to be dredged.

See Also: Hirsch, R.M., 2012, 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: U.S. Geological Survey Scientific Investigations Report 2012–5185, 17 p.