Reservoir System Update - August 2010

Rain and runoff

Eastern Valley rainfall for the year to date is 76 percent of normal.

Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) is currently 91 percent of normal in the eastern Valley.

Reservoir elevations

Most tributary storage reservoirs in the TVA system were within about two feet of their flood guide levels at the end of July, with the exception of South Holston, Watauga, and Blue Ridge.

South Holston and Watauga were 4.1 and 4.9 feet below their flood guide levels, respectively, due to a lack of local rainfall. TVA is conserving water in these reservoirs to bring them into balance with other tributary reservoirs.

TVA began a special deep drawdown on Blue Ridge Reservoir in mid-July. The reservoir will be lowered to an elevation between 1620 and 1630 feet above sea level by November as part of a project to rehabilitate the 79-year-old dam. Get an update on this project below.

¹Elevations above mean sea level
²Flood guide elevations show the amount of storage allocated for flood damage reduction during different times of the year. From June 1 through Labor Day, TVA's goal is to meet downstream flow requirements while keeping the reservoir elevation at the dam as close to the flood guide level as possible to support reservoir recreation. During this period, reservoir elevations fall below the flood guide only when rainfall and runoff are insufficient to meet flow requirements. The rest of the year, the primary objective is to keep the reservoir elevations at or below the flood guide to ensure there is enough space in the reservoir to store the rain and runoff from flood events.

Reservoir operations

Reservoir levels typically drop in August, and this year will be no exception. In fact, the drop is likely to be even more noticeable than usual, says Chuck Bach, TVA General Manager for River Scheduling.

“Our system flow requirement, which is measured at Chickamauga Dam, goes up each Aug. 1 because river temperatures gradually increase as the summer progresses. Providing more flow in late summer helps to keep the water from becoming too warm, which in turn helps to protect aquatic habitat and water quality. In years with normal rain and runoff, the reservoirs receive enough inflow to make up for much of the water that’s released to meet the higher flow requirement. As a result, there’s less impact on reservoir elevations in those years.

“However, this year we started August with more than a seven-inch rainfall deficit, and with no significant rain in the forecast. That means tributary reservoir elevations will be impacted as we release the minimum amount of water needed to meet our seasonal flow requirement.”

Bach notes that TVA carefully schedules releases to meet two objectiv