Understanding the Drawdown
Through careful creation and management of the Tennessee River watershed and its system of dams, TVA can virtually eliminate flooding in the Valley—no small feat.
Prior to the formation of TVA, the Valley was routinely ravaged by flooding, ruining both lives and land. The situation was so dire that beating back floods was a big part of TVA’s original mission. And so TVA set out to form an ingenious system of dams and reservoirs it could operate to keep floods under control by managing water levels differently at different times of the year.
Here’s how it works: TVA prepares for the winter flood season by lowering the level of flood-storage reservoirs to make room to hold the runoff produced by winter storms. When a storm hits, TVA holds the water back by reducing releases from the dams in areas where it is raining. When the rain stops and the danger of flooding is over, TVA gradually lets the water out to get ready for the next storm.
In the summer, when flood risk is lower, TVA keeps lake levels higher to support recreation. To get ready for winter, TVA begins releasing water from tributary storage reservoirs—in “drawdowns”—at a faster rate following Labor Day weekend. This allows TVA to put the stored water to good use during September and October—which are typically hot, dry months—by generating electricity to power air conditioners and supplementing flows for water quality and navigation.
Main-river reservoirs don’t fluctuate as much as the tributaries because of their design and navigation requirements. Their drawdowns are staggered from July through the end of the year to ensure the released water can be used efficiently, generating electricity as it runs through the turbines at as many as nine dams downstream. The seasonal drawdown begins after the 4th of July weekend on Kentucky Reservoir; following the Labor Day weekend on Chickamauga, Guntersville, Wheeler and Pickwick; and on November 1 on Fort Loudoun and Watts Bar.
Ready for the Rains
Reservoirs are typically lowered at least to winter flood-guide levels by Jan. 1 each year. During flood season, these levels may fall below flood guide levels by several feet to satisfy other operating objectives, but flood guide levels will only be exceeded during flood control operations. As soon as the downstream floodwaters begin to recede, the reservoirs are lowered at a controlled rate to recover flood storage space for future storms. If enough water can't be released through the turbines, it is sometimes necessary to let additional water flow through sluiceways or over spillways to speed up the drawdown and regain the storage space needed for future rains.
Aggressive filling of tributary reservoirs to summer levels begins in mid-March, when the chance of flood-producing storms, prolonged wet periods, and multi-storm sequences begins to decline. Main-river reservoirs are kept at lower levels until near the end of the flood season—late April or early May—because flood storage space in these reservoirs is so limited. (For this same reason, however, main-river reservoirs fill more quickly than tributary reservoirs.)
A small amount of flood storage capacity is reserved in all reservoirs through the summer months as a protection against flood-producing storms.