TVA monitors ecological conditions at 69 sites on 31 reservoirs. Each site is monitored every other year unless a substantial change in the ecological health score occurs during a two-year cycle. If that occurs, the site is monitored the next year to confirm that the change was not temporary. Roughly half the sites are sampled each year on an alternating basis.

The overall health ratings of TVA reservoirs are based on five ecological indicators:

Dissolved oxygen. A good rating means there is enough oxygen dissolved in the water to support a healthy population of fish and other aquatic life. Oxygen is as important to aquatic life as it is to life on land.
Chlorophyll. Chlorophyll is a measure of the amount of algae in the water. A good rating means that algal growth is within the expected range. If levels of algae are too low, the reservoir’s food web can be affected. If levels are too high, water treatment costs may increase, and oxygen supplies in the bottom layer of water may be depleted by decaying algae. Algal growth depends primarily on the amounts of nitrogen, phosphorus, and other nutrients in the water.
Fish. A good rating means there is a large number and variety of healthy fish.
Bottom life. A good rating means that a variety of animals live on the reservoir bottom (worms, insects, and snails, for example).
Sediment. A good rating means that the reservoir bottom is free of pesticides and PCBs and that concentrations of metals are within expected background levels.

When monitoring ecological conditions at each reservoir, TVA takes samples from up to four locations, depending on the reservoir’s size. These sites are classified as:

Forebay. The deep, still water near a dam.
Mid-reservoir. The middle part of a reservoir, where a transition occurs from a river-like environment to a reservoir-like environment.
Embayment. A very large slough or cove. (TVA monitors only four embayments: Hiwassee River on Chickamauga Reservoir; Big Sandy River on Kentucky; Bear Creek on Pickwick; and Elk River on Wheeler.)
Inflow. The riverlike area at the extreme upper end of a reservoir.

Stream ecological health

TVA Watershed Teams, made up of water-resource professionals and education specialists, work throughout the Tennessee Valley to improve water-resource conditions. These teams sample conditions at nearly 200 stream sites each year to help identify key issues and potential improvement projects.

Fish consumption advisories

State agencies are responsible for advising the public of health risks from eating contaminated fish. Each state uses its own criteria for deciding whether an advisory is necessary. TVA assists the states by collecting fish from TVA reservoirs and checking the tissue for metals, pesticides, PCBs, and other chemicals that could affect human health. Check the fishing regulations published by your state for specific advice on fish consumption (available wherever fishing licenses are sold).

Two national advisories related to mercury in fish are also in effect. In January 2001, the Environmental Protection Agency (EPA) advised pregnant women, nursing mothers, and young children to limit fish consumption to one meal per week. EPA’s advice is for freshwater fish caught by friends and family from local waters. The Food and Drug Administration issued a companion advisory for the same groups of people recommending against consumption of shark, swordfish, king mackerel, and tilefish purchased in stores and restaurants.

To view fish consumption advisories for a particular reservoir, please use the reservoir selection box above.

Chemicals responsible for these advisories

Five chemicals—PCBs, chlordane, DDT, dioxins, and mercury—are primarily responsible for contaminating fish in the Tennessee River system.

PCBs, chlordane, and DDT are no longer manufactured because they have been linked to a variety of health concerns. PCBs were used in many products, from electrical transformers to hydraulic fluid for farm equipment. Chlordane was used mostly to control termites, and DDT was used to control mosquitoes, flies, boll weevils, and many other insects.

Dioxins have also been linked to a variety of health concerns. They are unintended by-products of the industrial processes used to make white paper products and some herbicides. The incomplete combustion of certain organic materials, like plastics, also produces dioxins.

Too much mercury in the diet can cause brain and kidney damage in humans. The presence of mercury in reservoirs and streams is usually due to past industrial activities, and levels of the substance have declined as industries have stopped discharging their wastes into waterways.

These five chemicals do not dissolve well in water, so they are found mostly in the mud on the bottom of reservoirs and rivers. They can build up in the fatty tissue of fish, particularly bottom-feeders.

Reducing health risks

If you eat fish often, there are ways to significantly reduce your risk from pollutants. Eat smaller, younger fish. Discard the skin and fatty part of fish fillets, and broil, bake, or grill your fish. It’s also a good idea to vary the kinds of fish you eat. Substitute a few meals of crappie, sunfish, and perch for fish that tend to accumulate contaminants more rapidly. Bottom-dwelling fish like catfish and carp, for example, tend to accumulate more PCBs and other organic pollutants, and large predators such as bass tend to accumulate more mercury.