TVA River System Update
River Neighbors
Rain and runoff
Although rain in the eastern Valley was 3.7 inches above normal for the last three months of 2009, it seems to be tapering off in 2010. Rainfall was normal in January, and 1.6 inches below normal in February.
Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) also is decreasing due to the drier weather, but it remains above normal. After reaching a high point of 4.65 inches in December 2009, runoff slowed to 3.89 inches in January and 3.65 inches in February. Normal runoff for February is 3.06 inches.
The higher runoff levels are the result of a combination of factors, including saturated ground conditions and the occurrence of several hard downpours, which tend to produce more runoff than extended periods of light rain.
Flood guide levels show the amount of storage allocated for flood damage reduction during different times of the year. During the summer, TVA's goal is to meet downstream flow requirements while keeping the reservoir level at the dam as close to the flood guide level as possible to support reservoir recreation. From June 1 through Labor Day, reservoir levels fall below the flood guide only when rain and runoff are insufficient to meet flow requirements. During the rest of the year, the primary objective is to keep the reservoir level at or below the flood guide to ensure there is enough space in the reservoir to store the rain and runoff from flood events.
Historical reservoir elevations
Ever wondered what’s the highest your reservoir has been – or the lowest? The table below shows the maximum and minimum elevations since initial filling of the reservoirs listed. Keep in mind, though, that TVA operated the Tennessee River system differently when all but one of the minimum reservoir elevations shown occurred. TVA’s current operating policy targets higher winter elevations on most of the tributary reservoir system, which makes it unlikely—barring an extreme drought or a deep drawdown for maintenance—that the minimum levels shown below will occur in the future.
Reservoir operations
“We are right where we want to be.” That's what Chuck Bach, TVA General Manager of River Scheduling had to say about the status of the TVA reservoir system on March 1.
“We started the year with a lot of water in storage due to all the rain in late 2009, and then we had a fairly significant rain or snow event about every week from January through early February. In order to maintain flood storage space, we had to spill at most main-river dams and at some tributary dams through much of February. High flows also forced the closure of locks on the upper end of the river system, and barge traffic through the gorge below Chattanooga had to be suspended on several occasions.”
By the end of February, reservoirs were back to seasonal levels, according to Bach.
“Right now, we’ve got an optimal amount of water flowing through the reservoir system. We've got the flood storage space we need this time of year. But we still have plenty of water for other benefits. We've got the flow we need for hydroelectric power generation, but not so much that we have to spill water. And we've got the flow needed for commercial navigation, but not the high water levels that can restrict barge traffic through the locks on the river system or through the Tennessee River Gorge below Chattanooga.
TVA's operating policy this time of year is to maintain reservoir elevations at or below seasonal flood guide levels, which is exactly what TVA is doing, says Bach. “If we get a heavy rain, affected reservoirs could go above their flood guides temporarily to reduce downstream flooding. But we'll draw those reservoirs back down to seasonal levels as soon as that can be done without increasing downstream flood damage.
“Around the middle of March, flood guide levels start going up on tributary reservoirs, so depending on rain, reservoir users can expect to see water levels start coming up soon. The forecast is for a wet spring, so we anticipate filling tributary reservoirs on schedule.
To see your reservoir's flood guide and track its elevation, go to TVA's Reservoir Information Web page. Choose your reservoir from the pull-down menu on the right-hand side of the page and then select Operating Guide.
Questions and answers about the spring fill
When will the water level in my reservoir start going up?
TVA begins aggressively storing water to fill tributary reservoirs in mid-March. This is because less flood storage space is needed in the spring when the roots of growing plants begin to intercept more of the rain that would otherwise run off into the reservoir system. In addition, weather patterns begin to change with less chance for larger, more organized storm systems.
Main-river reservoirs are kept at lower levels until mid-April or May because they have limited flood storage space compared to tributary reservoirs due to topography and the requirement in the TVA Act to provide a nine-foot waterway for commercial navigation on the main Tennessee River. However, since there is not as much difference between winter and summer levels on main-river reservoirs as on tributary reservoirs, it doesn’t take long to bring main-river reservoirs to summer elevations.
When will my reservoir reach “full pool”?
TVA tries to fill tributary reservoirs to summer levels by Memorial Day weekend. Most main-river reservoirs are targeted to fill by mid-April. Watts Bar, Chickamauga, and Fort Loudoun Reservoirs are targeted to fill by mid-May to help reduce the risk of flooding at Chattanooga, Tennessee.
When reservoirs actually reach targeted levels depends on rain and runoff. Beginning in mid-March, tributary reservoirs are allowed to fill as quickly as possible, as long as reservoir levels do not significantly exceed flood guide elevations (which allow for higher reservoir levels in the spring). If low rainfall prevents reservoirs from filling at the desired rate, TVA stores as much water as possible, allowing only minimum releases to meet downstream flow requirements.
What is full pool on my reservoir?
To see the targeted summer pool levels for TVA-managed reservoirs, go to TVA’s Reservoir Information web page. Choose your reservoir from the pull-down menu and then select Operating Guide.
For main-river reservoirs, you'll see a gray band showing the normal operating zone, which is highest during the summer months. For tributary reservoirs, you'll see a blue line representing the flood guide elevation, which is at the highest level on June 1. This line is set to support reservoir recreation from June 1 through Labor Day, while still preserving a small amount of flood storage capacity as a protection against a potential flood-producing summer storm.
Why isn’t my reservoir filling as fast as other reservoirs?
If your reservoir doesn’t seem to be filling as fast as a neighboring reservoir, it could be because your area isn’t getting as much rain. Rain amounts often vary widely even over adjacent watersheds, with direct impacts on the rate that reservoirs in each watershed fill.
Fill rates also can vary significantly from one reservoir to another due to differences in the size of the land area draining into each reservoir, the amount of ground cover, soil characteristics, and reservoir shape and surface area.
Special reservoir operations
Little Bear Creek – TVA is continuing to adjust th
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