Norris Reservoir

The most significant ecological health issue on Norris is low dissolved oxygen concentrations. Dissolved oxygen rated poor at all three monitoring locations because the lower half of the water column contained little oxygen (< 2mg/L) during the summer.

This issue is mostly the result of the reservoir’s basic characteristics. Norris is a deep tributary storage reservoir with a long summer retention time; it can take more than 200 days for water to move through the reservoir. As the summer sun beats down on the surface of the reservoir, a warmer layer of water forms on top of a cooler layer. The layers do not mix, so the bottom layer becomes devoid of oxygen as the oxygen is used up by decaying plants and other materials that settle to the bottom.

TVA has installed equipment to add oxygen to the water as it is flows through Norris Dam. Learn more about these improvements.

Ecological Health Indicators at Norris Reservoir — 2011

Monitoring locationDissolved
oxygen
ChlorophyllFishBottom
life

Sediment
Forebay Poor Good Fair Fair Fair
Mid-reservoir, Clinch River Poor Good Good Fair Fair
Mid-reservoir, Powell River Poor Good Good Good Fair

Dissolved oxygen

As in previous years, dissolved oxygen rated poor due to low concentrations in the lower water column near Norris Dam. However, the volume of water affected from 1998 to 2004 was smaller than in other years because the sampling site was in the immediate area of the diffusers. Since the site was moved upstream in 2006, the volume of low-oxygen water has increased but remains less than pre-1998 levels.

Chlorophyll

As in most years, chlorophyll rated good at all three monitoring locations. Chlorophyll occasionally has rated fair or poor at the forebay because of lower-than-expected concentrations. This is more likely to occur during extremely dry conditions because fewer nutrients and less organic material are washed into the reservoir when rainfall and runoff are low. Likewise, the low-flow conditions limit the dispersion of nutrients within the reservoir.

Fish

The fish community received good ratings at both mid-reservoir monitoring locations and a “high fair” rating at the forebay. Monitoring typically shows good species diversity and balanced population characteristics at the mid-reservoir locations. The forebay has rated fair each year monitored due largely to the collection of fewer fish species than expected.

Bottom life

Bottom life rated good at the Powell mid-reservoir location and fair at the forebay and Clinch mid-reservoir locations. Bottom life typically rates poor or fair at the forebay and fair or at the lower end of the good range at the mid-reservoir sites.

Sediment

Sediment quality rated fair at all three monitoring locations. Low levels of PCBs were detected in the sediment samples at each location, and the concentrations of arsenic were above suggested background levels at the forebay and Powell mid-reservoir locations. The forebay sediments typically have elevated concentrations of arsenic and lead. Lows levels of the pesticide chlordane were detected in the sediments at each site in some previous years.

Fish consumption advisories

Norris Reservoir—TVA maintains a program to examine contaminants in fish fillets from TVA reservoirs and their major tributary streams on a rotational basis. The data collected from this program is distributed to the state officials who are responsible for placing or removing fish tissue consumption advisories on those bodies of water. For information on advisories currently in effect for Norris Reservoir, visit the Tennessee Wildlife Resource Agency's Web page.