Understanding the annual drawdown
TVA reservoirs are operated on an annual cycle that includes three key dates:
January 1: Reservoirs are drawn to their lowest level to get ready for flood-producing storms. These storms typically occur in winter and early spring, when vegetation is dormant and runoff is the highest.
June 1: The spring fill, which typically begins in mid-March, is complete, and reservoirs are filled to their highest level. If water is limited, releases are restricted to the amount of water needed to meet reservoir-specific and system flow commitments.
Labor Day: Drawdown restrictions are lifted. Water is released as needed to meet power demands and provide other benefits.
Average Monthly Rain and Runoff
So what changes after Labor Day? For the answer, we turned to Chuck Bach, manager of TVA River Scheduling.
“From June 1 through Labor Day, TVA restricts the drawdown of tributary storage reservoirs to provide higher levels for recreation. During this period, under normal operations, just enough water is released from these reservoirs to meet downstream flow requirements. We generate hydroelectric power with the water released to meet those requirements, but we don’t release any extra water solely for the purpose of hydro generation. That doesn’t mean that reservoir levels stay steady. Obviously, if there isn’t enough rain to replace the water released to meet minimum flow requirements, levels will drop. But the restrictions on releases help to provide the higher pool levels that everyone wants to see in the summer.
“After Labor Day, flow restrictions are lifted. We start releasing water from tributary storage reservoirs at a faster rate with the goal of lowering water levels to January 1 flood damage reduction levels as efficiently as possible. Beginning the drawdown after Labor Day gives us enough time to release the water gradually as it is needed to meet other demands, such as navigation, power, and reservoir and river quality.
“A later drawdown would benefit boaters and the recreation industry, but it also would jeopardize other river system benefits. Delaying the drawdown could impact power rates since it would limit hydropower generation — our least expensive power source — for an even longer period of time. But other benefits could be affected, as well.
“If we held reservoirs levels up into October and then got hit by the remnants of a tropical storm, for example, we’d be faced with higher spill rates and the possibility of flooding low-lying land. If the weather were abnormally dry, there would be less water available for discharge below Kentucky Dam on the lower Ohio and Mississippi Rivers, so industries that rely on low-cost river transportation could be impacted. Water quality also could be affected because holding water behind tributary dams longer means higher surface water temperatures, lower dissolved oxygen concentrations, and more algae—all of which can impact fish and other aquatic life.
“A lot of people don’t understand why we can’t wait to start releasing water until a big storm is in the forecast. Weather-forecasting technology has improved tremendously in recent years, but it still isn’t capable of providing reliable precipitation forecasts. Doppler radar allows us to watch weather systems as they form and move across the region. But when it comes to predicting when and where a front may stall and how many inches of rain it will produce before it starts moving again, it’s basically educated guesswork. Plus, it takes time to move water through the system efficiently. If we waited until the three-to-five day forecast calls for heavy rain, we’d have to draw the water level down very quickly, which could mean flooding low-lying land downstream. If the predicted rain doesn’t materialize — or we don’t get as much rain as predicted — we’d have released water needlessly. That means less water for recreation, navigation, water supply, and other purposes. Also, if dry weather sets in, refilling the affected reservoirs could be difficult.
“We are well aware of the importance of keeping reservoir levels up as long as possible. That was a key reason why we reevaluated our reservoir operating policy a few years ago. Based on extensive computer modeling and statistical analyses, we were able to delay the start of the annual unrestricted drawdown from August 1 to after Labor Day and to increase winter pool levels on 11 tributary storage reservoirs. Unfortunately, that review also showed that we cannot keep reservoir elevations up longer without jeopardizing flood damage reduction and other benefits that TVA’s system of dams and reservoirs was built to provide.”