Tennessee Valley Authority
Reservoir releases improvements
Maintaining a Wetted Riverbed
The problem
When dams on tributary rivers use water from the reservoir upstream to generate power, there is plenty of water in the riverbed downstream. But when the demand for electricity drops off for a few hours each day, power generation—and water flowing through the dam—can stop. When this happens, large areas of previously wetted riverbed are exposed for hours at a time, until the next release from the upstream dam. The result of prolonged exposure is the loss of aquatic habitat and a decrease in the abundance and availability of food, conditions that affect the health and fitness of the fish that live in the water below the dam, or tailwater.
The
solutions
TVA uses three different technologies to maintain water flow in the riverbed below tributary dams when hydroelectric generation is shut off:
Turbine pulsing
At some dams, TVA releases water through the dams at regular intervals throughout the day, called pulsing. This helps to create an essentially steady flow of water within a few miles, maintaining a more constant wetted habitat downstream of the dams.
Weirs
At other dams, TVA has built downstream weirs. These operate like small dams, holding back some of the water when power is being generated, then slowly releasing it when generation stops.
Small hydroelectric units
At two TVA hydropower plants (Blue Ridge and Nottley), small hydroelectric units run when the main turbine is not operating so that water is continuously released downstream.