Norris Reservoir

As in previous years, the ecological health of Norris Reservoir rated “fair” in 2017.

The ecological health of Norris Reservoir has been monitored using the same methodology since 1994.  Ecological health evaluations focus on five indicators: dissolved oxygen, chlorophyll, sediment quality, benthic macroinvertebrate community (bottom life), and the fish assemblage.  TVA monitors three locations on Norris Reservoir—the deep, still water near the dam, called the forebay (Clinch River Mile (CRM) 80.0), and two mid-reservoir locations (Clinch River Mile 125.0 and Powell River Mile 30.0).

Ecological Health Ratings for Norris Reservoir, 1994-2017

To view or export the data, click on the menu in the top-right corner of the chart.

 >72 = Good

 59-72 = Fair

 <59 = Poor

Ecological Health Indicators for Norris Reservoir, 2017

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 a portion of summer and 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 heats 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.

As part of TVA’s Reservoir Release Improvement Program, TVA has installed aeration equipment at Norris Dam to improve the quality of water downstream from the dam in the Clinch River. 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 given the geology of the watershed.  Lower ratings are 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 assemblage rated “good” at all three monitoring locations.  Historically, ratings for the fish assemblage have fluctuated between “good” and “fair” at the forebay, while the fish assemblages at both mid-reservoir locations have consistently rated “good”.  In 2017, the number and variety of fish observed at each location were consistent with long-term averages.  A total of thirty-four species was observed reservoir-wide.  Top carnivores, benthic invertivores (species that feed primarily on bottom-dwelling insects), and intolerant species (species known to require good water quality conditions) were well represented at each location.  Fish health was assessed a “good” rating with low incidences of disease and parasites.

Bottom life

Bottom life rated “good” at the Clinch mid-reservoir location and “fair” at the forebay and Powell 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.  The higher rating at the Clinch mid-reservoir in 2017 was largely because samples contained a greater number and variety of organisms including the occurrence of a less tolerant taxon of caddisfly.

Sediment

Sediment quality rated “good” at the two mid-reservoir monitoring locations and “fair” at the forebay.  “Good” ratings indicate no pesticides or PCBs were detected and the concentrations of metals were within suggested background levels.  Concentrations of arsenic and lead exceeded suggested background concentrations in the sediment sample collected at the forebay, resulting in a “fair” rating.  The forebay sediment samples typically have elevated concentrations of arsenic and lead.  PCBs were detected in the sediment samples collected at each location in 2011.  Lows levels of the pesticide chlordane also were detected in the sediments at each site in some previous years. 

Fish consumption advisories

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.