Reducing pollution in the Susquehanna River watershed could ease the environmental effects of an essentially full reservoir behind Conowingo Dam, according to a final report from the Lower Susquehanna River Watershed Assessment Team released in March.

For decades, the reservoir behind Conowingo Dam—as well as those behind the Holtwood and Safe Harbor dams—has trapped particles of sediment flowing down the Susquehanna River, along with the nutrients that are often attached.

But a draft report from the LSRWA team released in November 2014 indicated this reservoir is full—and the final report upholds these findings: no substantial changes were made to the findings or recommendations of the report between the draft and final phases.

According to the report, the reservoir is trapping smaller amounts of sediment and nutrients and, during large storms, sending more of these pollutants into the Susquehanna River more often.

The report indicates that reducing pollution loads, particularly nutrients, upstream of the dam would provide a more effective solution than various strategies for managing sediment at the dam itself, such as dredging or bypassing.

Among the findings in the report are--

-- Dams Not Longer Trapping Sediment: Each of the three reservoirs’ sediment trapping capacity is greatly reduced and that each reservoir has reached an end state of sediment storage capacity. The evaluations carried out through this assessment demonstrate that Conowingo Dam and Reservoir, as well as upstream Safe Harbor and Holtwood Dams and their reservoirs, are no longer trapping sediment and the associated nutrients over the long term. Instead, the reservoirs are in a state of dynamic equilibrium.

-- Storms Scour Sediment From Behind Conowingo: During storm events 20 to 30 percent of the sediment that flows into Chesapeake Bay from the Susquehanna River is from scour of bed material stored behind Conowingo Reservoir, and the rest is from the upstream watershed, which includes scour from behind Holtwood and Safe Harbor Dams;

-- Nutrients Carried By Sediment More Harmful Than Sediment: Nutrients associated with the scoured sediment were determined to be more harmful to Bay aquatic life than the sediment itself. The particulate nutrients settle to the bottom and are recycled back up into the water column in dissolved form and stimulate algal production.

-- Minimal Benefits From Dredging: Increasing or recovering storage volume of reservoirs via dredging or other methods is possible, but the Chesapeake Bay ecosystem benefits are minimal and short-lived, and the costs are high. When sediment is strategically removed from the reservoirs behind the dams, there was a predicted minor influence on scour load (reduction) and sediment deposition (increase); there was also a predicted minor reduction in adverse impacts to Chesapeake Bay ecosystem health for a future similar storm event. Scour events would still occur, but lower amounts of sediment and associated nutrients were estimated to be mobilized during these events.

-- Nutrient Management/Mitigation More Effective: Management opportunities in the Chesapeake Bay watershed to reduce nutrient delivery are likely to be more effective than sediment reduction opportunities at reducing impacts to the Chesapeake Bay water quality and aquatic life from scour events, but these management opportunities were not investigated in detail during this assessment. The relative importance of nutrient load impacts from the lower Susquehanna River reservoirs is a finding that indicates that nutrient management and mitigation options could be more effective and provide more management flexibility, than solely relying on sediment management options only.

In 2010, the Chesapeake Bay Total Maximum Daily Load (TMDL) was established to reduce nutrient and sediment loads across the watershed. Bay jurisdictions—Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia and the District of Columbia—and federal agencies are currently in the process of submitting draft two-year water quality goals, or milestones, to achieve the nitrogen, phosphorus and sediment reduction goals of the TMDL.

At the first meeting of the PA Chesapeake Bay Watershed Implementation Planning Steering Committee meeting on April 3, DEP said the next round of watershed implementation plans will contain revised nutrient and sediment reduction goals to account for the failure of the Susquehanna River dams to traps sediment.

The final report is available on the LSRWA website.

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(Reprinted in part from the Chesapeake Bay Program Blog.)

[Posted: April 26, 2017]

5/1/2017