If you live near a TVA reservoir, you’ve seen it yourself...
The water level can go up and down during the summer, and by Labor Day it can start to drop quickly. Before long you’ve got to cross a mudflat to get from the shoreline to the water.
Nobody likes mudflats—including the people at TVA who let the water out of the reservoirs. But they can’t be avoided if the dams are going to do the job they were built to do. Before the dams in the TVA system were built, just about every major storm resulted in serious flood damage to homes and businesses along the river. Now, most of this damage can be avoided by closing upstream dams and holding back the stormwaters until the danger of flooding is over. But for this system to work there has to be room in the reservoirs to hold the extra water.
How does TVA make this room? You guessed it: by lowering the water level before the “flood season” begins. Just as you let water out of the bathtub by opening the drain, TVA can open gates in the dams to let water out of the reservoirs.
TVA tries to keep reservoir levels as high as possible during the summertime so people can enjoy water sports. Keeping reservoir levels up even longer would make a lot of people happy, especially those who have homes on the lakefront, boaters and people who make a living from recreation, such as marina owners. But TVA also has to think about some problems that might happen.
TVA’s plan for lowering the reservoirs is based on rainfall records that have been kept for many years. These records show that big storms that produce floods are most likely to hit the Tennessee Valley in the winter and early spring. So TVA makes sure the reservoirs are lowered to “flood-control levels” by January 1. Those levels are low enough to leave room in the reservoir for rainwater to flow in.
TVA starts lowering reservoirs around Labor Day because it wouldn’t be a good idea to let out a whole lot of water very fast. A certain amount of water has to be kept in the reservoirs for other things like running the turbines in the dams that make electricity, giving fish enough water to live in and keeping the water deep enough to float the big barges that travel up and down the Tennessee, Ohio, and Mississippi rivers. So what would happen if TVA waited to start lowering reservoir levels?
A lot depends on the weather! If a major storm hits before TVA has time to make room to store the extra water, more flooding would be likely. Also, the price people pay for electricity could be affected. To make room for the stormwater, TVA would have to let the water out of its reservoirs fast—so fast, in fact, that it couldn’t be released through the turbines. That would mean less water for making electricity.
In very dry years, waiting to lower reservoir levels also could affect conditions for fish and other living things in the water and reduce water depths for barges on the Ohio and Mississippi Rivers.