'Weir' Science

South Holston Dam Weir Fosters World-Class Fishery

Nestled in the northeast corner of Tennessee just a few miles southeast of Bristol – where the South Fork Holston River winds its way through the hills of the Cherokee National Forest to join the South Holston Lake – you’ll find the South Holston Dam. Built for downstream flood control, the dam is one of two TVA dams on the South Fork Holston River. The other is Watauga Dam.

Both projects were started in the early 1940s but were halted in late 1942 by order of the United States War Production Board, which redirected as many resources as possible to support activities central to the war effort. Construction of South Holston resumed in 1947 and was completed Nov. 20, 1950. The dam's single generating unit came online Feb. 13, 1951 – 66 years ago.

Breathe Deep

It wasn’t until 1991 that the dam received what would become its most distinctive feature. That year, as part of TVA’s Lake Improvement Plan, an aerating labyrinth weir was constructed about 1.5 miles below the dam on the South Fork Holston River. The plan aimed to improve dissolved oxygen concentrations in the downstream tailwater and increase minimum flow in the tailwater between periods of generation – both of which benefit aquatic life, as post-generation tailwaters tend to be cold, dense and oxygen-depleted.

In addition to lake improvement goals, the South Holston weir also was designed to reduce backwater on upstream turbines and to minimize the intensity of downstream water recirculation, making it safer for recreational users, including boaters and fishermen. Other types of aerating structures can pose a physical threat to recreation, so the weir’s ability to maintain safe recirculation levels is a key benefit

The beauty of the labyrinth weir lies in its design – it achieves all of this with a single, elegant structure. It’s a powerful blending of utilitarian function and sculptural form. 

Though impressive in its own right, it’s not the weir itself that draws visitors to the South Holston tailwater – it’s the fish that thrive in the oxygen-rich environment that the weir helps create.

Good Fishing

Deep-water releases from hydroelectric dams provide the cold-water habitat trout prefer but often lack sufficient oxygen for healthy fish to thrive. The South Holston weir provides water flow in the river channel during periods of no generation, creating a more consistent, healthy habitat for insects, plant life and fish. 

The weir creates a series of waterfalls that create air pockets, which burst into air bubbles as they plunge downstream. These air bubbles churn the water, allowing oxygen to penetrate the depths of the reservoir, creating optimal conditions for trout to thrive.  

Since the weir’s completion, water quality improvements have supported substantial natural reproduction among the brown trout in the South Fork Holston River. Today, the South Holston tailwater is managed not as a put-and-take or put-and-grow fishery, but as a wild brown trout fishery

The trout are protected during spawning season, both by a protected length range – fish between 16 and 22 inches long must be released unharmed – and by closing spawning areas to fishing. While rainbow trout are still stocked annually, wild brown trout are naturally abundant, making the South Holston tailwater one of the most popular fisheries in the Southeast.