Tennessee Valley Authority
Reservoir System Update
Summer thunderstorms and showers have continued to bring relief from the longstanding dry conditions to the eastern Tennessee Valley.
While rainfall amounts are approaching normal, runoff amounts above Chattanooga for the calendar year were 81 percent of normal at the end of last week, indicating that ground water is still recharging after several years of drought conditions.
For the week ending July 31, an average of 2.19 inches of rainfall was recorded above Chattanooga. Runoff was 0.22 inch. (Note: All rainfall and runoff data included here is as of midnight July 30.)
For the calendar year-to-date (Jan. 1-July 31), the Valley has received a total of 29.51 inches of rainfall and 15.03 inches of runoff. Year-to-date, rainfall is 93 percent of normal, and hydro generation is 92 percent of normal.
Hiwassee, South Holston, Watauga, Cherokee, Douglas, Fontana, Norris, Tims Ford, Nottely, Chatuge and Blue Ridge reservoirs were all slightly below their flood-guide elevations at the end of last week and were forecast to stay below their flood-guide elevations this week.
Isolated showers and thunderstorms could develop from the middle to the end of this week. Temperatures are expected to be slightly above average this week before returning to near seasonal for the weekend.
TVA is scheduling flows through the system for economic dispatch, recreation, and to maintain respective flood-storage allocations.
Reservoir Operations during Drought Conditions
During drought conditions, TVA operates the river system in a water conservation mode, releasing only the minimum amount of water from tributary dams required to meet downstream needs.
Water is released to meet reservoir-specific minimum flow requirements, which keep the riverbeds below tributary dams from drying out, and to meet system-wide minimum flow requirements, which are needed to maintain the channel depths for navigation on the main Tennessee River mandated by the TVA Act and to protect water quality and water supply along the length of the river system.
TVA generates as much power as possible with the water released to meet minimum flow requirements, but no additional water is released for the specific purpose of hydropower generation—even though it means relying on more expensive coal-fired and nuclear generation or buying power from outside sources
Beginning June 1, TVA is required to provide increased flow through the river system. The amount of flow depends on the total volume of water stored in the tributary reservoir system and varies depending on the time period.
If the volume of water stored in tributary reservoirs remains below the minimum system operating guide level, TVA is required to provide a weekly average flow of 13,000 cubic feet per second (cfs) at Chickamauga Dam from June 1 to July 31, if the dry conditions persist through that period, and a weekly average flow of 25,000 cfs from August 1 to Labor Day. (System-wide flows are 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.)
If tributary reservoirs are above their minimum operating guide, flows at Chickamauga will increase weekly through the summer until the unrestricted drawdown begins after Labor Day. See what the different flows are here.
If there isn’t enough rain to meet the minimum flow requirement, TVA has to pull the needed water from the tributary reservoir system, which can cause water levels on tributary reservoirs to drop through the summer.
TVA uses balancing guides for each reservoir to ensure that water is withdrawn in an equitable manner. More water is drawn from reservoirs that are higher relative to their balancing guide levels—which means they have more water—and less from reservoirs that are nearer or below their balancing guide levels.
If it rains enough to fill the tributary system above the minimum operating guide level, the flow requirement at Chickamauga increases on a weekly basis.
Track your reservoir’s elevation. Choose your reservoir from the pull-down menu and then select Operating Guide.
Track Valley rainfall here.