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Dams and Water Quality

When people build a dam on a river, it changes the water quality. Some of these changes are good, but others can cause problems.

At dams that produce hydroelectric power, these problems can really affect the fish and other animals that live in the “tailwater” area. That’s the stretch of river on the downstream side of the dam, where the water comes out when electricity is being produced.

Keeping water in the riverbed

When water is flowing through the dam to make electricity, there is plenty of water in the tailwater area. But when the equipment that makes the electricity is shut off, the riverbed can dry out, sometimes for several miles below the dam, which is bad news for fish—as you can imagine!

photo of small waterfalls created by weir
The weir at South Holston Dam keeps water in the riverbed and adds oxygen to it so that fish and other aquatic animals and plants can survive.

In 1991 TVA decided to do something about this problem. At some dams, TVA built small dams, called “weirs,” which hold back some of the water when electricity is being generated, then slowly empty when generation stops.

At other dams, TVA turns the equipment used to make electricity on and off throughout the day to make sure that there is always enough water flowing through the dam to keep the tailwater area from drying out.

do you know…
...what fish need, besides water, to stay alive? Click here for the answer

What is stratification?

Oxygen is another big problem in the tailwater area below some dams, especially dams that form very deep reservoirs. That’s because the water used to spin the turbines at these dams comes from the lower part of the upstream reservoir. This water usually doesn’t have much oxygen in it during late summer and autumn because of a natural process called “stratification.”

Maybe you’ve never heard this word before, but if you’ve ever jumped into a reservoir on a hot summer day, you’ve probably felt the effects of stratification. The deeper you go, the colder the water feels; then you pop back up to the warmer surface water that’s been heated by the sun’s rays.

Because of the difference in temperature, the surface water and the bottom water of deep reservoirs don’t mix, so the bottom water becomes trapped. It doesn’t have any contact with the air, which means there’s no way to replace the oxygen that is used up as plants die, settle to the bottom, and decay. By late summer or fall, sometimes there’s no oxygen in the water near bottom at all.

You’ve probably figured out the problem already: when this low-oxygen water passes through the dam in the process of producing electricity, it affects the amount of oxygen in the tailwater area and the health of the animals that live there.

photo of pumps on surface of reservoir
These pumps at Douglas Dam operate like big fans to help push water that’s higher in oxygen down to the bottom.

TVA tackled this problem at the same time it decided to do something to keep the riverbed from drying out. The solution involved a lot of research. TVA installed different kinds of equipment at 16 dams to add oxygen to the water—from huge fans that push the oxygen-rich surface water down to the reservoir bottom to hoses that hang just above the reservoir bottom with holes that create millions of tiny bubbles as oxygen is pumped into the hoses from a tank on the land.

Want to know more? Read about tailwater improvements on the TVA Web site.