engineers working

Running Hot and Cold

Tims Ford Dam is operated in such a way as to provide tailwater temperatures cold enough to support a thriving trout fishery, yet warm enough to support the endangered boulder darter.

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 federally listed species is a small fish called a boulder darter. The only known naturally occurring population of boulder darters exists in the Elk River drainage basin. The Elk River is also home to a regionally important trout fishery. Because this is the only natural location of the darter, it falls to TVA to ensure it can coexist with trout.

Adjusting Operations

The trout fishery, which is stocked by the Tennessee Wildlife Resources Agency (TWRA), is a put-and-take fishery that’s maintained during the summer months by the water released through Tims Ford Dam. When TVA releases water from the dam, it is very cold due to the depth of the reservoir. Trout thrive in the cold water, but it is too cold for the boulder darter, several endangered mussels that inhabit the tailwater and warm-water sportfish.

The cold water released when TVA runs the hydropower turbines at the dam for long periods of time can cause a drop in water temperatures many miles downstream of the dam. This change in temperature can stress boulder darters and other warm-water fish such as smallmouth bass and other important sportfish.

After careful study, TVA began adjusting its hydropower operations at Tims Ford Dam to provide temperatures suitable for boulder darters, listed mussels and other warm-water species downstream of the cold-water trout fishery. Water temperatures are being monitored along the length of the tailwater with the goal of adjusting the operation of Tims Ford Dam to keep water temperatures at the Farris Creek stream gage (Elk River Mile 124) within the targeted temperature range for trout while keeping water temperatures near the Fayetteville stream gage (Elk River Mile 93) similar to seasonal temperatures in free-flowing streams in the region, which can reach the mid-80s (F) during late summer.

More on the Boulder Darter

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, Tenn., and Lauderdale County, Ala.

Adult boulder darters are about three inches long and olive to gray in color. As the 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 (ERM 93) and downstream in the Elk River to ERM 30 or 31.

Because boulder darters live only two or three years, successful annual reproduction is critical. The darters prefer temperatures between 60 and 70 degrees Fahrenheit (16 and 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

Coldwater releases from Tims Ford Dam allow a put-and-take fishery for two non-native trout species—rainbow and brown trout—to be maintained in the Tims Ford tailwater. 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; however, historically, water temperatures in the area between Old Dam Ford (ERM 120) and Fayetteville increase above the temperatures trout prefer during summer months. Water temperature conditions suitable for trout survival and growth in the Elk River are typically restricted to areas upstream of Old Dam Ford beginning in July. TWRA identifies only the area from Old Dam Ford upstream to Tims Ford Dam as a trout tailwater in its Stocked Trout Program.

Over the years, TVA has worked with several local groups to improve trout habitat in the Elk River and 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 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.