A closer look at reservoir system flow requirements

In 2004, TVA adopted a flow-based approach to operating the reservoir system—and, according to River Scheduling manager Chuck Bach, that has made all the difference.

“In the process of reviewing our operating policy, we asked the public and state and federal agencies to tell us what they valued about the reservoir system and what they wanted from it in the future. The answer was loud and clear. Reservoir users wanted us to look at operating alternatives that enhanced recreation, protected water quality, and increased economic development.

“After additional public review, data collection, statistical analysis, and computer modeling, we determined that the best way to accomplish that objective was by adopting a flow-based approach that would help us make the best use of the available water.”

TVA already was operating the reservoir system to meet specific flow requirements at individual dams. Bach explains: “Reservoir-specific flow requirements serve a variety of purposes â€” from improvement of downstream aquatic habitat to waste assimilation, water quality, and water supply â€” and vary from dam to dam depending on reservoir size, turbine generating capacity, the water needs of downstream industries, and other factors. We’re committed to releasing at least the amount of water specified to maintain these benefits. But, during dry periods, we may have to release more water than required to meet the flow requirement at a particular dam in order to provide a minimum flow at a downstream dam.”

The system-wide flow requirement, on the other hand, is both a minimum and a maximum. The river flow at Chickamauga Dam cannot be lower than the amount specified in the table below to ensure an adequate flow through the system, and it cannot be higher in order to conserve as much water as possible in tributary reservoirs.

System Flow Requirements at Chattanooga, June 1 – Labor Day

Ensuring an adequate flow through the system is critical to protecting water quality and aquatic habitat. It also enables TVA to maintain the channel depths for navigation on the main Tennessee River that are mandated by federal law in the TVA Act.

The system-wide flow is measured at Chickamauga Dam, located near Chattanooga, Tennessee, because this location provides the best indication of the flow for the upper half of the Tennessee River system.

The system-wide flow requirement depends on the total volume of water stored in the tributary reservoir system. It is highest in August because warmer air temperatures result in warmer river temperatures, which means more flow is needed to protect aquatic habitat and water quality. The higher August flow requirement also helps to control power rates by allowing TVA to generate more hydroelectric power to meet the high power demand which typically occurs in late summer due to the higher temperatures.

The minimum flow requirement drops after Labor Day because air and river temperatures start to cool in autumn. However, in normal years, TVA begins releasing water from tributary storage reservoirs at a faster rate following Labor Day weekend to make room for rainfall from the flood-producing storms that are most likely to hit the Tennessee Valley in the winter and early spring.

In dry years, the system flow requirement can have a significant impact on tributary reservoir levels, says Bach. “If it doesn’t rain, the only way we can meet the Chickamauga flow requirement is by releasing the water stored in tributary reservoirs â€” and that means lower water levels for those reservoirs. As folks on tributary reservoirs saw last year and again this year, the effects can be especially apparent beginning August 1, when the flow requirement at Chickamauga increases from 13,000 cubic feet per second to 25,000 cubic feet per second.”

Bach says policy guidelines are in place to ensure that water is drawn from tributary reservoirs equitably. “We follow our balancing guide procedures to make sure that no reservoir loses a disproportionate amount of water.”

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