Dams are constantly forced to choose between two things.
Hydroelectric dams create electricity by using flowing water to turn turbines.
Flowing water is essential for the habitats of many fish, mussels, and the animals that rely on them.
Scientists conducted a two-decade experiment at the Roanoke Rapids Dam in North Carolina to see if there really is this kind of trade-off.
The results produced an outcome that was not so simple.
A stretch of river received less water
Many rivers are affected when large hydroelectric dams are built because part of the river’s natural flow has to be rerouted through some sort of man-made tunnel or pipe to provide the power needed to generate electricity.
That is what happened to the Roanoke River.
Most of the water from the river had been diverted through a constructed channel connected to the powerhouse, which generated electricity.
One of the original sections of the river got much less water.
This portion of the river became known as the bypassed reach.
Why three different flow patterns were tested
Conditions in the habitats downstream from the dam changed due to reduced flow.
Many species of fish, including American Shad, American Eels, and Freshwater Mussels, depend on specific river flow conditions during certain phases of their life cycle.
A new FERC license issued in 2004 gave an opportunity to find out if changing flow rates in the bypassed reach would make these habitat conditions better.
The new FERC license called for a multi-step process.
Managers decided to try three different flow regimes in the bypassed reach.
The question was how the different flow levels would affect habitat conditions.
Managers worked together with NOAA Fisheries, State Resource Agencies, and Federal Resource Agencies.
Managers also agreed to conduct annual surveys.
The surveys tracked how fish and other aquatic organisms responded to the different flow regimes.
The results would later be used to compare the low, medium, and high-flow settings.
The three flow regimes were implemented in stages during the first 20 years of the license.
Results would determine how the bypassed reach would be managed in the future by Dominion Energy under the license.
What twenty years of monitoring revealed
Major evaluations were done in 2024 & 2025.
Researchers studied how fish and mussel populations grew under each of the three operational modes.
When comparing the three options, researchers determined that low-flow mode resulted in poor results compared to the other two operational modes.
Populations increased under both medium-flow and high-flow modes.
The surprise came when researchers compared the two higher settings.
The monitoring data indicated that there were minimal additional ecological benefits provided by running at maximum capacity versus at the intermediate level.
All three species improved their population numbers when compared to low-flow mode.
Maximum flow rate resulted in similar ecological outcomes to those observed under moderate flow.
Much of the same ecological function was accomplished under medium-flow conditions.
What the findings mean for future operations
Dominion Energy plans to operate at medium-flow for the remainder of its current license.
If Dominion decides to use more water in the powerhouse to meet growing electrical demands, they will have more water available to do so.
The monitoring studies show that even though less water is released into the bypassed reach under medium-flow conditions, sufficient amounts of water remain in the river to maintain healthy aquatic habitat conditions.
The benefits of the high-flow pattern were not significantly different from the medium-flow pattern.
Much of what is accomplished ecologically is achieved by operating at a lower flow rate.
And that means more water can go toward generating electricity while maintaining healthy aquatic habitats for wildlife.
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