Hiwassee Reservoir

Ecological Health Ratings

The overall ecological health condition of Hiwassee Reservoir rated “fair” in 2021, similar to all previous years. Throughout the years, common issues that reduce the overall rating for the reservoir have been elevated chlorophyll concentrations, low dissolved oxygen concentrations in the lower water column, and “poor” ratings for bottom life.

The ecological health of Hiwassee 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 fish assemblage. TVA monitors two locations on Hiwassee Reservoir—the deep, still water near the dam called the forebay (Hiwassee River Mile 77.5), and the middle part of the reservoir (Hiwassee River Mile 85.0).

Ecological Health Ratings for Hiwassee Reservoir 1994-2021

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 Hiwassee Reservoir - 2021

*Sediment quality was assessed based on monitoring results from 1994 through 2018.

Dissolved oxygen

Dissolved oxygen rated “poor” at the forebay and “fair” at the mid-reservoir. Oxygen concentrations were below two milligrams per liter in portions of the lower water column during summer and autumn at both monitoring locations. Because the low concentrations encompassed a substantial portion of the bottom of the reservoir in the forebay, dissolved oxygen was assessed a “poor” rating at this location. Dissolved oxygen has rated “poor” most years at the forebay, but ratings at the mid-reservoir have varied between “good”, “fair” and “poor”.

Hydro turbine intakes typically draw water from the lower water column of the upstream reservoir, which can create low-oxygen conditions immediately downstream of dams. To address this problem, TVA installed aeration equipment at Hiwassee Dam to improve the quality of water released from the dam. 

Chlorophyll

Chlorophyll in 2021 rated “fair” at both monitoring locations because average concentrations were slightly elevated relative to the expected range. Annual average chlorophyll concentrations have fluctuated through time at both locations, resulting in “good”, “fair” and “poor” ratings.

It should be noted that chlorophyll concentrations in Hiwassee Reservoir are assessed relative to expectations for the Blue Ridge Ecoregion, which has naturally low nutrient concentrations. Therefore, chlorophyll concentrations are expected to be much lower in Hiwassee than in other Tennessee Valley reservoirs located outside the Blue Ridge Ecoregion. 

Fish

As in previous years, the fish community in 2021 rated “good” at both monitoring locations. The number and variety of fish observed at each location were consistent with long-term averages, and fish health was assessed a “good” rating with low incidences of disease and parasites. Spotted bass was the most abundant top carnivore observed at both locations. Bluegill and green sunfish were the most prevalent forge fish. Several sicklefin redhorse, which are being considered for the federal endangered species list, were observed at the mid-reservoir site.

Bottom life

Bottom life rated “poor” at the forebay and “fair” at the mid-reservoir. This is comparable to the “poor” to “low fair” ratings in most previous years. The benthic communities at both monitoring locations are characterized by low density and diversity of organisms. Benthic organisms consisted mostly of midges and worms, which are more tolerant of the low dissolved oxygen conditions that develop in the lower water column during summer. 

Sediment

Sediment samples were not collected from Hiwassee Reservoir in 2021. Sediment quality was assessed a “good” rating at both locations based on monitoring results from 1994 through 2018. In 2015 and 2018, sediment quality rated “good” at both monitoring locations because no PCBs or pesticides were detected in the sediment samples, and the concentrations of metals were within suggested background levels. A “good” rating is typical for sediment quality at both locations. However, low levels of PCBs were detected in the forebay sediment sample in 2012, and chlordane was detected in samples from both locations in 1993; no other contaminants have recorded concentrations above background levels.

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 are 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 Hiwassee Reservoir, visit the Occupational and Environmental Epidemiology Branch, North Carolina Division of Public Health.