New reservoir health information on TVA.com
TVA scientists recently completed their analysis of reservoir monitoring conducted in summer 2008. TVA’s Tyler Baker, an environmental scientist who has been involved in monitoring the condition of TVA-managed reservoirs for the past 20 years, discusses the results:
“Our 2008 monitoring results were what we’d expect after another year of drought-like conditions. Reservoirs on the main Tennessee River tended to have lower overall ecological health ratings than during years with more normal flow conditions, and most tributary reservoirs rated about the same or slightly better.
“We did a full health checkup on 15 TVA-managed reservoirs in 2008, including 11 tributary reservoirs and four main-river reservoirs. Three of the four main-river reservoirs rated lower compared to their long-term average ratings, but the majority of the tributary reservoirs rated the same.
Reservoir Health Ratings
|*Average ratings are calculated using the overall scores for each reservoir for all years monitored.|
“Main-river and tributary reservoirs respond differently to drought conditions mainly because of the difference in residence time — the length of time that water is held in the reservoir before it is released. Main-river reservoirs typically have short residence times. The water usually flows through the reservoir within a week or two. In contrast, it can take several months for water to flow through most tributary reservoirs. This is because the inflow — the amount of water coming in to these reservoirs from tributary rivers and rainfall events — is so small compared to their total volume.
“During dry, low-flow conditions, the residence time increases in main-river reservoirs. There’s more time for organic material to settle to the bottom and decompose and more time for thermal stratification of the water column, which limits the mixing needed to replenish oxygen supplies in the bottom water. Algae and other microscopic plant populations also have more time to grow. The result is an increase in the volume of water with low dissolved oxygen, higher chlorophyll concentrations, and often lower ratings for bottom life compared to years with more normal flow conditions. That’s exactly what we observed in 2008.
“Conditions in tributary reservoirs tend to be fairly stable. Dry conditions have less influence on most tributary reservoirs because residence times are usually long enough to allow for oxygen depletion and algal growth every year. In fact, dry conditions often result in improved ratings. The less it rains, the fewer nutrients and less organic material washes from the land into the water, which keeps the growth of algae in check. That, in turn, means less oxygen demand due to the natural process of decomposition and better oxygen conditions, especially in the deep, still water near tributary dams.
“It’s too early to predict the results of our 2009 monitoring. The region is experiencing an interesting combination of near normal rainfall and below normal runoff, indicating that groundwater supplies are still being replenished after several years of drought conditions. But I wouldn’t be surprised to see some improvement in main-river reservoir health scores. We’ve had more water available this year to help maintain higher flows, so residence times will be closer to normal. That should reduce algal growth and oxygen depletion.
“Runoff in the eastern Valley is substantially better this year than it was in 2007 and 2008, which could have a negative effect on chlorophyll and dissolved oxygen concentrations in tributary reservoirs. But it is still below normal, so I doubt if we’ll see much change in overall health ratings for those reservoirs.”
For detailed information on the health of your reservoir, visit TVA’s Reservoir Ratings web page.
About TVA’s reservoir monitoring program
TVA’s reservoir monitoring program provides basic information on the health of the aquatic ecosystem in TVA-managed reservoirs. That information helps to guide day-to-day operations. If problems are found, it’s also used to target more detailed studies.
Physical and chemical indicators — water temperature, acidity, and dissolved oxygen, chlorophyll, and nutrient concentrations — are monitored on an annual basis. Sediment contaminants and biological indicators, such as fish and bottom life, are monitored every other year unless a substantial change is detected, in which case that reservoir is monitored again the next year to determine if the change was temporary. Sediment and biological sampling is conducted on roughly half the reservoirs each year on an alternating basis. Reservoir health scores are updated every two years when TVA conducts full monitoring on a reservoir.
Depending on the reservoir’s size, TVA scientists take samples from three locations: the deep, still water near the dam, called the forebay; the middle part of the reservoir, where a transition occurs from a river-like environment to a reservoir-like environment; and the river-like area at the extreme upper end of a reservoir, called the inflow. TVA also monitors four large coves, called embayments: the Hiwassee River embayment on Chickamauga Reservoir; the Big Sandy River embayment on Kentucky; the Bear Creek embayment on Pickwick; and the Elk River embayment on Wheeler.