Norris Reservoir

Dissolved oxygen

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 (less than two milligrams per liter) 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.

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.

See definitions of ecological health indicators and monitoring locations.