Tennessee Valley Authority
May rain helps fill tributary reservoirs
The eastern Tennessee Valley got some much-needed rain in May, ensuring that most — if not all — tributary reservoirs will reach June 1 summer recreation levels on schedule.
Some areas in the western Tennessee Valley received more than nine inches of rain in early May, forcing TVA to spill water at Pickwick and other dams on the main Tennessee River.
In fact, heavy downpours caused some tributary reservoirs to rise above flood guide levels, forcing TVA to release water to provide the storage space needed to prevent downstream flooding in the event of more heavy rain.
“From May 1 to 10, the Valley got almost five inches of rain, about as much as we would normally expect for the entire month,” says Chuck Bach, manager of River Scheduling at TVA. “We went from operating in a water conservation mode to a flood risk reduction operation almost overnight.”
Some portions of the Valley received more than nine inches of rain during the 10-day period.
Tributary reservoir elevations increased about 5.5 feet on average, with the largest increase at Fontana (12 feet).
Tributary reservoir elevations
Reservoir |
Observed Elevation* |
Flood Guide Elevation* |
||
May 15, 2008 |
May 15, 2009 |
May 15 |
June 1 |
|
South Holston |
1718.5 |
1730.2 |
1729 |
1729 |
Watauga |
1952.9 |
1959.8 |
1959 |
1959 |
Cherokee |
1062.1 |
1067.2 |
1069 |
1070.9 |
Douglas |
986.2 |
994.2 |
993 |
994 |
Fontana |
1693.4 |
1705.0 |
1703 |
1703 |
Norris |
1016.6 |
1022.5 |
1020 |
1020 |
Chatuge |
1924.5 |
1925.1 |
1925 |
1926 |
Nottely |
1774.3 |
1774.8 |
1776 |
1777 |
Hiwassee |
1512.7 |
1520.5 |
1520 |
1521 |
Blue Ridge |
1687.5 |
1686.4 |
1686 |
1687 |
*In feet
“When it rains in the spring, people naturally want us to hold on to the extra water, especially if the weather has been dry,” says Bach. “But this is a good example of why we have to be so careful. Rainfall records show that the Valley is prone to large storms in the spring, and if we aren’t ready for them, the risk of flood damage increases.”
Once tributary reservoirs return to flood-guide levels, Bach says TVA will go back to operating in water conservation mode.
“We’ll only release enough water from the tributary system to meet downstream flow requirements. We won’t use any extra water for hydropower generation.”
That will help keep tributary reservoirs at summer levels. But there are no guarantees, according to Bach.
“Rainfall for 2007 and 2008 was 30 inches below normal. That’s a pretty deep hole to fill up, and so far we aren’t making any progress. Rainfall for the calendar year to date is still only 94 percent of normal. We need to make up some of that 30-inch deficit in order to recharge groundwater supplies and return runoff to normal levels. Until that happens, a lot of the rain will soak into the ground or be used up by growing plants before it ever reaches the river system, which will reduce stream flows and make it more difficult to keep reservoirs at summer elevations.”
Runoff for the year through mid May was about 79 percent of normal in the eastern Valley.
No matter how dry it gets, says Bach, TVA has to release a minimum amount of water from tributary reservoirs. “We’ve got to provide enough flow to keep the downstream riverbed from drying out and to protect water quality, water supply, and other needs. If we don’t get enough inflow to replace that water, reservoir levels can drop during the summer even though we’re operating in a conservation mode.”
For the time being, though, Bach is glad to have some water in the tributary reservoir system. “Compared to the last two years, things are looking really good. Higher pool levels make TVA’s job — balancing the use of the water for multiple purposes — a lot easier.”
Stay up to date online
To track your reservoir’s 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. You’ll see a graph with a red line labeled 2009 Midnight Elevations.
Read biweekly reservoir system updates.