February 2011 Reservoir System Update

Rain and runoff

The Eastern Valley received 2.55 inches of rain in January, which is about two inches below normal.

Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) was just over an inch below normal.

The cumulative deficits for the past two months are 4.64 inches of rainfall and 1.38 inches of runoff.

Reservoir elevations

As the chart below shows, most of the large tributary storage reservoirs were very close to their seasonal flood guide elevations on Feb. 1.

Flood-guide elevations show the amount of storage allocated for water from flood-producing storms during different times of the year. TVA releases water as needed to keep reservoirs at or below their flood guide elevations to be ready for these storms. In summer, the goal is to keep reservoirs as close to their flood guide elevations as possible to support recreation while still meeting minimum flow commitments. But, in winter, reservoirs may be lower than their flood guide elevations as water in storage is used to meet winter power demands and other needs.

¹ Water elevation at the dam in feet above mean sea level
² Flood-guide elevations show the amount of storage allocated for flood-damage reduction during different times of the year. The amount of storage varies with the potential flood threat. Flood-guide elevations are lowest from Jan. 1 through mid March because winter storms are generally larger, occur more frequently, and produce more runoff. Flood-guide elevations increase between mid-March and June 1 as the risk of flooding decreases. They are highest from June 1 through Labor Day to support summer reservoir recreation. After Labor Day, TVA begins the unrestricted drawdown to winter flood-damage reduction levels.

Blue Ridge Reservoir is a special case. TVA began a deep drawdown on Blue Ridge in mid-July 2010. The reservoir is being held at an elevation between 1620 and 1630 feet above sea level—compared to its normal winter flood-damage-reduction level of 1668—as part of a project to rehabilitate the 79-year-old dam. Get an update on this project below.

Reservoirs along the main Tennessee River—Fort Loudoun, Watts Bar, Chickamauga, Nickajack, Guntersville, Wheeler, Wilson, Pickwick, and Kentucky—were all within or close to their normal operating zones at midnight on Feb. 1.

Hydroelectric power generation

Conventional hydroelectric power generation was 74 percent of normal in January due to the continued dry conditions. Dry weather impacts hydroelectric power generation because hydro plants are “fueled” by water flowing through the dam.

Answers to frequent questions about winter reservoir operations

When will my reservoir start going back up?
The spring fill typically begins in mid-March. Most reservoirs fill to their highest level by June 1. You can track your reservoir’s elevation from TVA’s Reservoir Information page and see how it compares with the expected operating range and actual pool levels for the same date last year.

Why does TVA lower reservoir levels so far in winter?
The risk of flood-producing storms in the TVA service area is highest in winter and early spring. Summer storms typically affect only a portion of the region, but winter storms can cover the entire region for several days, with one storm followed by another even larger storm three to five days later. Plus, winter storms produce more runoff than summer storms. In summer, a lot of the rain is absorbed by the dry ground and vegetation instead of flowing into the reservoir system. But in winter and early spring, most of the rain ends up in the reservoir system.

History provides clear evidence of this pattern. During the past 140 years, the largest flood events along the Tennessee River occurred in March 1867, February-March 1875, April 1886, March 1897, March 1917, January-February 1957, March 1963, March 1973, April 1977, May 1984, February-March 1994, April 1998 and May 2003.

Reservoirs are drawn to their lowest level by Jan. 1 to provide the water-storage capability needed to reduce flood damage from such storms.

Wouldn’t it be a good idea to hold on to some of the rain we get in winter to help fill reservoirs to summer levels?
Although holding on to extra rain could help the spring fill, TVA

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