Balancing temperatures in the Elk River for a diversity of aquatic life
The Elk River below Tims Ford Dam in south-central Tennessee is known for its aquatic diversity. It is home to a wide variety of mussels, snails, and fish, including a number of species protected by the Endangered Species Act. One of these species is a small fish called a boulder darter, known to occur naturally only in the Elk River drainage basin.
The Elk River also is home to a regionally important trout fishery. Because of the cold water released through the dam when TVA generates hydroelectricity, the Tennessee Wildlife Resource Agency (TWRA) is able to stock two non-native trout species — rainbow and brown trout — in the Tims Ford tailwater.
The challenge for TVA and its state partners is to ensure that the darters and the trout can coexist. The issue is water temperature.
The cold water released when TVA runs the hydropower turbines at the dam allows TWRA to maintain the trout fishery. But when power generation continues for long periods of time, it can cause a drop in water temperatures many miles downstream of the dam. This change in temperature can stress boulder darters, affecting spawning success and growth of young boulder darters. Other native warm-water fish in the Elk River such as smallmouth bass also can be impacted.
After careful study, TVA and the U.S. Fish and Wildlife Service agreed that TVA would modify its hydropower operations at Tims Ford Dam to warm up the temperature of the tailwater to help protect and promote recovery of the boulder darter and mussels also listed as endangered species. These modifications should result in improvements to the warm-water recreational fishery in the Elk River.
In 2008, TVA began adjusting releases from Tims Ford Dam. Water temperatures are being monitored along the length of the tailwater to determine the best way to release water from the dam, whether through the spillway, the sluiceway, or the hydropower turbine. The goals is to provide seasonal temperature ranges suitable for trout immediately below Tims Ford Dam, transitioning to ranges suitable for boulder darters, listed mussels, and other warm-water species downstream.
TVA monitors water temperatures in the Elk River closely so that it can adjust the operation of Tims Ford Dam as needed to keep temperatures within the targeted ranges.
More about boulder darters
The boulder darter was listed as endangered in 1988. The only known wild populations of the boulder darter exist in the Elk River drainage basin, although reintroduction efforts are underway in nearby Shoal Creek, which flows through Lawrence County, Tennessee, and Lauderdale County, Alabama. Adult boulder darters are about three inches long and olive to gray in color.
As their name implies, boulder darters are typically found in moderately flowing pools or other appropriate habitat areas with current. They are found among large flat rocks relatively clean of silt or where there is large rubble from old collapsed bridge materials. This highly specific habitat in the Elk River is limited. Boulder darters are present in the vicinity of Fayetteville at Elk River mile 93 and downstream in the Elk River to ERM 30 or 31. Because these darters live only two or three years, successful annual reproduction is critical. The darters prefer temperatures from 60-70 degrees Fahrenheit (16-20 degrees Celsius) for spawning and warmer temperatures for the growth of juveniles. Their breeding season is typically May to June.
Improving conditions for trout
The Tennessee Department of Environment and Conservation has designated the Elk River from Tims Ford Dam downstream to Fayetteville (ERM 93) as a trout stream. This area supports trout during the cooler fall and winter months; historically, however, water temperatures in the area between Old Dam Ford (ERM 120) and Fayetteville increase above the temperatures trout prefer during the summer months. Beginning in July, water temperature conditions suitable for trout survival and growth in the Elk River are typically restricted to the area from Old Dam Ford upstream to Tims Ford Dam, and only this area is included in TWRA’s stocked trout program.
Over the years, TVA has worked with several local groups to improve trout habitat in the Elk River, and the agency has installed equipment at Tims Ford Dam to add oxygen to the water. Two large blowers are used to force air into the water as it passes through the main turbine when TVA generates electricity. If additional aeration is needed, TVA uses an oxygen-injection system in the forebay of Tims Ford Reservoir (the area close to the dam) to help maintain the water quality criterion for dissolved oxygen for waters having an official “trout stream” designation. Water also is released through the sluiceway at the dam to maintain minimum flows when the hydroelectric turbines are not in operation.
For more information, contact TVA's Environmental Information Center.