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APRIL 5, 2010

In this issue:
Rain and runoff
Reservoir elevations
Reservoir operations
In answer to your question about reservoir elevations
Fish spawning
When a "lake" isn't a lake - and why it matters
More TVA information

Instead of publishing TVA River Neighbors, a quarterly electronic newsletter, TVA is now providing monthly updates on the operation of TVA-managed reservoirs by e-mail

To sign-up for future updates, provide feedback, change your e-mail address, or opt out of receiving future e-mails, please send an e-mail request to reservoirupdate@tva.com.

 

Rain and runoff
For the past two months, rain in the eastern part of the Tennessee Valley has been about 60 percent of normal.

Runoff (the amount of water that reaches the river system when it rains instead of being absorbed into the ground) was slightly above normal in February, but only 66 percent of normal in March.

 

Reservoir elevations
Although both rain and runoff were below normal in March, all main-river reservoirs were within their seasonal operating ranges and tributary reservoirs were at or very close to their targeted seasonal elevations as of midnight, April 4. A combination of factors enabled TVA to keep tributary reservoirs near target elevations despite the dry conditions. These included above-normal rain in the eastern Valley during the last three months of 2009 and TVA efforts to conserve water in the tributary reservoir system by only releasing enough water to meet downstream flow commitments.

Tributary reservoir elevations¹

Reservoir

April 4, 2010
Observed Elevation

April 4
Flood Guide²

June 1
Flood Guide²

South Holston

1720.1

1720.0

1729

Watauga

1955.6

1955.0

1959

Cherokee

1053.8

1052.9

1070.9

Douglas

973.5

972.8

994

Fontana

1671.8

1669.4

1703

Norris

1007.4

1008.7

1020

Chatuge

1922.1

1920.3

1926

Nottely

1767.0

1766.6

1777

Hiwassee

1493.7

1493.6

1521

Blue Ridge

1679.1

1678.6

1687

Tims Ford

883.3

882.0

888

Normandy

871.3

870.3

875

¹Elevations above mean sea level
²Flood 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.

 

Reservoir operations
TVA reservoir operations are currently focused on filling tributary storage reservoirs to June 1 target elevations while making sure there is enough water flowing through the river system to meet other needs.

“January rainfall was normal, but February and March have been drier,” said Chuck Bach, TVA General Manager for River Scheduling, “so we are trying to be conservative with the water in the tributary reservoir system.”

Below-normal rainfall in March, April, and May is a concern because of the effects on the spring fill. Not only does it mean lower rainfall totals; it means less runoff. Even when it does rain, not as much of the water reaches the reservoir system when the ground is dry. Plus, more of the water is absorbed by the roots of growing plants during the spring.

Being conservative with the water means only releasing the minimum amount needed to meet downstream flow commitments, said Bach.

Flow commitments ensure that water is available for a variety of purposes, including navigation, water supply, and aquatic habitat. If there isn't enough rain to provide the water to meet these flow commitments, the water has to come from upstream reservoirs. In that case, we are careful to release only the minimum amount of water needed, and we try to draw the needed water from the upstream reservoirs that are closest to their target levels versus reservoirs that are lower because they’ve had less rain. We hold on to the rest to increase the chances of reaching June 1 elevation targets.”

What happens if the weather is wet?

There isn't much rain in the forecast for the next few weeks, according to Bach. But, if the forecast is wrong and rainfall is above average, TVA could have to increase water releases. “Flood damage reduction has priority. If we get a lot of rain and tributary reservoirs go above their flood guide levels, we're obligated to bring them back down even if that means releasing water during the spring fill. This time of year, we want to keep the reservoirs right at flood guide to give us the best chance of filling. But we've got to keep them from going above flood guide to ensure that we have room to store the water if we do get hit by a major spring storm.”

A lot depends on rain and runoff in April and May, said Bach. “We’ve positioned ourselves to fill the reservoir system on schedule assuming near-normal rainfall and runoff. But we're dependent on rain. We’ll need an average of about an inch of rain a week in the eastern Valley to fill on schedule.”

To see a reservoir's flood guide and track its elevation, go to TVA's Reservoir Information site. Choose a TVA-managed reservoir from the pull-down menu on the right-hand side of the page. Then, from the pull-down menu for “Type of Information,” select “Operating Guide,” then “View Info.”

 

In answer to your question about reservoir elevations
Why isn't my reservoir filling as fast as some other reservoirs?
Some Nottely Reservoir users have asked TVA this question lately. At the end of March, Nottely was right at its flood guide level, but nearby Chatuge Reservoir was nearly two feet higher than its flood guide level.

In this case, the difference was due to a relatively unique situation. Normally, the water released in the process of generating hydroelectric power at Chatuge Dam is used to meet downstream flow requirements. But the power plant at Chatuge is scheduled to be taken out of service for maintenance in mid-April. TVA is trying to bring the reservoir level up to the spillway crest elevation (1923.5 feet) so the spillway gates can be used to maintain a minimum water flow during the maintenance outage.

Typically, however, differences in reservoir elevations are due to other factors. For example, if your reservoir seems lower than a neighboring reservoir, it could be because there was more rain in the neighboring watershed. It may not have rained at your house, but it might have rained a lot just a short distance away. Reservoir elevations are highly dependent on local rainfall and runoff.

Another possibility is that you are comparing two different types of reservoirs. Large tributary reservoirs fluctuate the most because they do the bulk of the work in controlling floods. They have more storage capacity than other reservoirs and must be drawn down more aggressively—which, of course, means they take longer to fill. Main-river reservoirs don’t fluctuate nearly as much because they have less storage space and because of navigation requirements. Other reservoirs are maintained at near-steady levels year around. These reservoirs are operated primarily to maximize power production or provide local benefits. How your reservoir is operated depends on how it was designed and its purpose within the entire system.

TVA uses a tool called a balancing guide to ensure that tributary reservoirs are treated equitably during the spring fill and summer recreation season. During this time, when we have to release water to meet downstream flow requirements, we are careful to keep the elevation of all reservoirs similar relative to this elevation guide. In other words, if one reservoir is well above its balancing guide level and another reservoir is right at the guide level, we’ll try to draw the needed water from the reservoir with the higher level relative to the guide.

Another thing to keep in mind in comparing the elevation of different reservoirs is their size and shape. Differences in size and shape may cause one reservoir to look full compared to another even though their elevations are in balance. Withdrawing the same amount of water from a deep, bowl-shaped reservoir, for example, will expose more of the bank than withdrawing the same amount of water from a reservoir shaped more like a shallow serving dish. Also, reservoirs have different individual minimum flow requirements to meet water supply and other downstream needs. TVA has to release the water needed to meet these requirements regardless of the reservoir’s elevation relative to its balancing guide.

 

Fish Spawning
Fish spawning is an important consideration in spring reservoir operations. In April, for example, TVA tries to provide a flow of 8,000 cubic feet of water per second at Watts Bar Dam at least through the middle of the month—and longer if possible—for downstream sauger spawning.

Sometime between mid-March and mid-April, priority is given to holding reservoir levels steady for a two-week period to benefit the bass and crappie spawn.  Peak spawning for these species typically occurs when the water temperature at a five-foot depth reaches 65 degrees Fahrenheit. If reservoir levels are reduced during this period, fish nests and eggs may be stranded above the water line or fish may abandon nests if water becomes too shallow.

Rising water levels affect fish spawning success less than falling levels. But TVA also tries to avoid more than a one-foot-per-week increase in reservoir elevations during the peak spawning period.

Stabilizing reservoir levels aids fish spawning success for these species, but it can conflict with other reservoir operating objectives. If a heavy rain occurs during the spawning period, for example, TVA may have to store the water temporarily to reduce downstream flood damage (resulting in rising reservoir levels), and then release water to recover flood storage space when the rain stops and the danger of flooding is over (causing reservoir levels to drop).

Also, the period to maintain constant tributary reservoir levels for fish spawning coincides with the period for filling reservoirs to reach their target summer elevations. If reservoirs are not filling on schedule due to lack of rain and runoff, TVA’s operating policy requires that TVA release only enough water from tributary reservoirs to meet downstream flow requirements. In this case, TVA may have to store the water from a large storm event to assist with the spring fill even if it occurs during the spawning period.

 

When a “lake” isn't a lake — and why it matters
When you talk about spending the day “on the lake” in the Tennessee Valley, chances are you will actually be enjoying an outing on one of the 49 reservoirs in the TVA system.

“These days, we are careful to call the body of water behind the dam a “reservoir” instead of a “lake,” said Chuck Bach, General Manager of River Scheduling at TVA. “We don’t use the term “lake” because it implies a natural water formation. Lakes are formed by glaciers, volcanic eruptions, the movement of the earth’s crust, and other natural processes. Reservoirs, on the other hand, are artificial impoundments. They are constructed by people when they build dams along rivers to control the flow of water for a specific purpose.”

TVA-managed reservoirs were created primarily to reduce downstream flooding by holding water back during big storms and to improve the Tennessee River for navigation. The stored water also is used to generate electricity—as much as 14 percent of TVA’s total generation in a rainy year.

While the terms “lake” and “reservoir” may seem interchangeable, said Bach, the difference between the two is critical for TVA. “Unlike natural lakes, water levels on reservoirs are adjusted based on the needs of the system along with natural events such as heavy rains and drought conditions.”

After Labor Day, TVA lowers the water levels on its reservoirs to winter levels in preparation for flood-producing storms in the spring, and then raises them gradually by June 1. Those changes in water levels can be dramatic. On some reservoirs—Fontana, for example—the summer and winter elevations vary by as much as 50 to 60 feet. That’s in comparison to four to five-foot annual fluctuations on the main Tennessee River, where TVA is required to maintain year-round navigation.

Bach said some people, especially newcomers to the Valley, are surprised by the change in water levels. “People will buy a house on the water in summer and not expect to see their dock on the ground in the winter. No one likes to see a ring of mud around their reservoir in winter. But, when you think about the range of objectives involved in managing TVA’s reservoir system, you can better understand the need for the annual drawdown.”

 


Get more information on TVA.com
The links below will take you to reservoir-related information on TVA’s Web site

TVA’s reservoir operating policy:  Learn how TVA manages the flow of water through the Tennessee River system to provide navigation, flood damage reduction, power supply, water quality, water supply, recreation, and other benefits.

Reservoir information:  Get detailed information about individual reservoirs, including observed and predicted elevations and releases at TVA dams, reservoir operating guides, water quality improvements, fish population survey results, and more.  To check reservoir information from your cell phone or other mobile device, go to http://m.tva.com.

Rainfall and stream flows:  Get the latest information on daily rainfall and stream flows across the Valley.

Recreation release schedules:  View the 2010 schedule for water releases for rafting, kayaking, and canoeing below these TVA dams:  Apalachia, Ocoee No. 1, Ocoee No. 2, Ocoee No. 3, Norris, Watauga/Wilbur, and Tims Ford.

Map of TVA reservoirs and power plants:  Our interactive map is your guide to the entire TVA power system, including fossil and nuclear plants, dams and reservoirs, and visitor centers.  You’ll find interesting facts about each facility and learn how they work together for the purposes of power supply, river management, and economic development.

Water supply FAQs:  Get answers to frequently asked questions about obtaining a water intake permit, improving water quality around intakes, inter-basin transfers, and more.

Dangerous areas around TVA dams:  If you like fishing or enjoy swimming and boating on TVA-managed reservoirs, you need to be aware of the possible hazards around dams, locks, and powerhouses.

How to lock through:  Find out what you need to do to safely approach a navigation lock, secure your boat in the lock chamber, and exit the lock.

Reservoir health ratings:  See the latest monitoring results for TVA-managed reservoirs.

Campgrounds and day-use areas:  Get information here about campground fees and amenities as well as picnic pavilion reservations.

TVAkids.com:  TVA’s got a Web site just for kids!  Learn about how TVA makes electricity, reduces flood damage, protects wildlife, and more.  There’s a section for teachers, too.

TVA Heritage:  Read about the people who founded TVA, shaped its purpose, and built its power plants.  TVA Heritage offers fascinating glimpses of the agency’s 76-year history.

Get more information by phone
For the latest information on reservoir elevations and stream flows, call TVA’s Reservoir Information Line from a touch-tone phone:

  • From Knoxville, TN:  865-632-2264
  • From Chattanooga, TN:  423-751-2264
  • From Muscle Shoals, AL:  256-386-2264
  • From all other locations:  800-238-2264 (toll-free)

For answers to questions on how your reservoir is operated, call TVA River Operations at 865-632-6065.

For answers to questions about recreation, permitting procedures, reservoir land management plans, and other environmental issues, call TVA’s Environmental Information Center at 1-800-882-5263.

 
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