In 1928, a ceremony along the banks of the lower Susquehanna River near Conowingo, Maryland, opened what was then the United States’ second largest hydro-electric project (the first being Niagara Falls). At that time, the Conowingo Dam was celebrated as an engineering feat. These days, it is more well-known for its nearly filled reservoir of nutrient and sediment pollution which is threatening the Chesapeake Bay.
Every day, 25 billion gallons of fresh water from the Susquehanna River flow through the Conowingo Dam on its way to the Chesapeake Bay. The Susquehanna River alone provides 50 percent of the Bay’s fresh water supply. The reservoir behind the Dam has trapped an average of three-and-a-half million pounds of phosphorus and four billion pounds of sediment every year since the dam opened, which is approximately a third of the phosphorus and half of the sediment that flows along the Susquehanna River and into the Chesapeake Bay annually.
The reservoir catches the nutrients and sediment from agricultural, suburban, stormwater and urban runoff that flows downstream, but the problem is that these materials are rapidly loading up the reservoir and within in a few years it should be completely full.
When the Chesapeake Bay Total Maximum Daily Load (Bay TMDL) was put into place in 2010, it was based on the assumption that the reservoir would continue trapping nutrients and sediment as it had always done and the climate would remain steady (meaning hurricanes and other extreme weather events weren’t taken into account). But new models, monitoring data and research show just how much conditions have changed.
In 2015, only five years later, a report issued by the United States Geological Survey estimated that the dam had reached 92 percent of its capacity. Another report by the Lower Susquehanna River Watershed Assessment team confirmed that since the reservoir is essentially full, it is trapping smaller amounts of incoming sediment and sending more and more of it, along with the nutrients that attach to it, over the dam and downstream into the Bay. While the sediment is detrimental enough by reducing water clarity and dissolved oxygen in the Bay, it also carries chemical contaminants such as pesticides and polychlorinated biphenyls (PCBs) in addition to nutrients. All of these harm water quality and the living resources in the Bay, even though the exact amount of toxic pollutants flowing from the reservoir are not well defined at this time.
Hurricanes and other extreme weather events have a strong impact on the dam. Imagine how the situation looked during Hurricane Agnes in 1972 and Tropical Storm Lee in 2011. The gates of the dam were opened to relieve pressure from accumulating water, churning up large amounts of built-up nutrient and sediment pollution and sending them downstream. This decimated underwater grass beds, negatively impacting marine life and fueling the growth of harmful algae blooms, leading to an increased dead zone.
It is clear that the nutrient reductions called for in the Susquehanna River watershed by the Bay TMDL is no longer enough to meet the goals to improve water quality in the deep waters of the upper Bay. The Conowingo Dam is currently owned by Exelon Corporation and physically lies in Maryland, but the Susquehanna River flows downstream from New York and Pennsylvania.
Recently, the Chesapeake Bay Program decided that the best way to address the increased pollution from the Conowingo Dam would be to create a separate Watershed Implementation Plan (WIP) for the dam. WIPs detail the steps that each watershed jurisdiction (Delaware, the District of Columbia, Maryland, New York, Pennsylvania, Virginia and West Virginia) will take to meet their pollution-reduction targets.
A separate steering committee will be tasked with writing this new WIP for the Conowingo Dam and developing a financing strategy. The committee will have representatives from each jurisdiction and the assistance of a third party. The Environmental Protection Agency will provide oversight in the development and implementation of the Conowingo WIP, evaluate and track the progress being made to reduce the additional pollution from the dam and provide technical and contractual support.
This approach will give all the jurisdictions in the Chesapeake Bay watershed the chance to participate in this massive undertaking and contribute in putting conservation practices in place to help offset the nutrient and sediment pollution loads from the Conowingo Dam.
Learn more about the Conowingo Dam.