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) from summer through early autumn.

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

Chlorophyll rated good at the two mid-reservoir monitoring locations and at the upper end of the fair range at the forebay. For most years, chlorophyll has rated good at all locations on Norris. Chlorophyll occasionally has rated fair at the forebay because of lower- than-expected concentrations.  A poor rating for the forebay in 2007 was also the result of lower-than-expected chlorophyll concentrations, which was likely due to extremely dry conditions that year. Fewer nutrients and less organic material are washed into the reservoir when rainfall and runoff are low, and 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 location. Monitoring typically shows good species diversity and balanced population characteristics at the mid-reservoir locations. The forebay rates fair because of collection of fewer fish species than expected.

Bottom life

Bottom life rated fair at all three monitoring 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 the forebay and good at the mid-reservoir monitoring locations. As in previous years, the forebay sediments had an elevated concentration of arsenic. However, the concentration of lead, which is typically elevated in the forebay, was within expected background levels. Sediment quality has rated good at the mid-reservoir locations all years monitored, except in 2005 when lows levels of chlordane, a pesticide previously used to control termites and crop pests, were detected in the sediments at both locations.

See definitions of ecological health indicators and monitoring locations.