Tennessee Valley Authority
Reservoir releases improvements
Improving Conditions Below Main-River Dams
Differences in conditions below tributary and main-river dams require different approaches to improving tailwater quality.
Keeping water in the river channel isn’t an issue below main-river dams because these dams were built for navigation as well as flood control and hydropower. They were designed so that, even at the lowest reservoir level, the water behind one dam backs up to the downstream side of the next dam, meeting the requirement to provide at least a nine-foot navigable depth.
Dissolved
oxygen is also
less
of a problem below main-river reservoirs because thermal
stratification is weaker than in tributary reservoirs. That’s
because the water flows through the main-river reservoirs more quickly — usually
in a matter of days or weeks — and the depths are shallower.
Main-river reservoirs and embayments do experience reductions in dissolved oxygen, however, especially during low-flow conditions.
The solutions
At Fort Loudoun Dam, TVA uses an oxygen-injection system to help maintain adequate dissolved-oxygen levels. Perforated hoses suspended above the reservoir bottom bubble gaseous oxygen into the upstream water before it’s pulled through the dam.
At Watts Bar Dam, two different methods help meet aeration targets. The first is called unit preference. Since turbines nearest the banks typically draw in reservoir water that is higher in dissolved oxygen, TVA operates those units first during periods when oxygen levels are low. If additional aeration is needed, an oxygen-injection system is also available. As at Fort Loudoun, oxygen can be bubbled into the water through perforated hoses suspended above the reservoir bottom.
At the seven dams on the lower part of the river — Chickamauga, Nickajack, Guntersville, Wheeler, Wilson, Pickwick, and Kentucky — TVA provides minimum flows to help maintain adequate dissolved-oxygen levels downstream. This is done by releasing a specified amount of water per second at three key locations — Chickamauga, Pickwick, and Kentucky — during different seasons of the year, using a 14-day running average.
Maintaining minimum flows at these locations ensures adequate water levels in other main-river reservoirs because storage limitations demand the tight coordination of flows between these reservoirs. (Aeration systems are used at Fort Loudoun and Watts Bar Dams because flow maintenance would have a negative impact on reservoir levels in upstream tributaries.)
The biweekly seasonal flows that TVA provides below main-river dams also produce other benefits. Besides maintaining dissolved oxygen levels, they provide water for navigation and for industrial and municipal use. At some main-river dams, these flows also help improve conditions for sauger spawning.