A conversation with TVA’s Tyler Baker
Region’s reservoirs prove drought resilient
In our last issue, we took a look back at how the 2007 drought impacted the broad range of benefits provided by the Tennessee River system—from commercial navigation and hydropower generation to recreation, water supply, water quality, and aquatic habitat. Given the interest expressed by our readers in the health of Valley reservoirs, we decided to take a closer look at water quality impacts in this issue. For insight, we turned to TVA’s Tyler Baker, an environmental scientist who has been involved in monitoring the condition of TVA-managed reservoirs for the past 20 years.
How would you sum up the impact of the 2007 drought on the health of TVA-managed reservoirs?
Considering the ecological conditions that could have developed because of the drought, it was a good year.
The dry weather had an impact, especially on reservoirs on the main stem of the Tennessee River. But, overall, the conditions we observed were consistent with ratings in previous dry years, which is good news given that 2007 was the driest in 118 years of record.
TVA sampling showed an increase in the volume of water with low dissolved oxygen and in chlorophyll concentrations at several main-river locations. But our monitoring gave us an early warning, and we were able to minimize the impacts of oxygen conditions at most locations by increasing flow enough to mix the water column. Both oxygen and chlorophyll concentrations improved with the onset of cooler weather and increased flows.
How did the dry weather impact main-stem reservoirs?
Typically, the residence time for main-stem reservoirs—the time the water is held in a reservoir before it is released through the dam—is relatively short: from 4 to 24 days. But in 2007, there was almost no local inflow. Essentially, the only water coming in to main-stem reservoirs was water released from upstream tributary reservoirs, and TVA was conserving this water as much as possible. As a result, residence times in main-stem reservoirs roughly doubled. This allowed more time for phytoplankton productivity—as evidenced by elevated chlorophyll concentrations—and contributed to increased thermal stratification. Seven main-stem reservoirs had the highest summer average chlorophyll concentrations that we’ve seen since we began monitoring, and several sites had an increase in the volume of water affected by low dissolved oxygen.
Phytoplankton are microscopic plants, such as algae, that live in lakes, reservoirs, and other bodies of water. Although some level of phytoplankton production is essential to maintain a healthy aquatic community, adverse impacts can occur as concentrations increase. These include reduced water clarity, more frequent algal blooms, lower dissolved oxygen concentrations, and more frequent water treatment problems.
Thermal stratification refers to the temperature layering effect that occurs in water. Stratification is common in most tributary reservoirs because they have much longer retention times—from 75 to over 300 days. As the days get longer and hotter, the temperature of the surface water rises. Warm water is less dense than cold water, so it literally floats on top of the cooler water. This density difference inhibits mixing, resulting in thermal stratification—the separation of water into horizontal layers due to temperature differences.
The problem with thermal stratification is that it traps the bottom layer of water. The oxygen in the bottom water is depleted as organic material settles to the bottom and decays, and it isn’t replenished because there isn’t any mixing with the oxygen-rich surface water. This results in low dissolved oxygen concentrations in the lower layers of the water column, which can adversely impact aquatic life and water quality.
Main-stem reservoirs occasionally experience some thermal stratification, particularly during late spring when TVA holds the water in order to fill tributary reservoirs. But any stratification is typically destabilized as TVA releases more water from tributary dams in June, July, and August to meet downstream needs. Last summer was different in that TVA continued to conserve water in tributary reservoirs through August due to the dry conditions. This resulted in lower-than-normal flows through main-stem reservoirs and poorer dissolved oxygen conditions.
What about tributary reservoirs?
Dry conditions often result in improved ratings for chlorophyll in tributary reservoirs, and that’s exactly what happened in 2007. With so little rain, fewer nutrients and less organic material were washed from the land into the water, which kept the growth of algae in check. As a result, chlorophyll concentrations were lower than we’d expect in a year with more normal rainfall, especially in the lower reaches of most tributary reservoirs (the area closer to the dam).
Dry conditions can help improve dissolved oxygen concentrations in tributary reservoirs, too, since there’s less organic material settling to the bottom and, consequently, less oxygen demand due to the natural process of decomposition. So, we weren’t surprised to find that oxygen concentrations in the deep, still water near several tributary dams were normal or better than normal in 2007. Nor were we surprised to see an increase in the volume of water with lower-than-normal dissolved oxygen in the upper reaches of tributary reservoirs, where organic matter entering the reservoir from upstream rivers and streams tends to accumulate.
Notable exceptions were Melton Hill, Tellico, Fort Patrick Henry, Apalachia and, to some extent, Boone. These reservoirs aren’t nearly as deep and have shorter retention times than other tributary reservoirs, so dissolved oxygen and chlorophyll concentrations were generally comparable to years with more normal rainfall.
Did the dry conditions affect the fish?
As I mentioned earlier, algal production is essential to the aquatic food web. In fact, relatively high levels are beneficial to largemouth bass and some other fisheries in southeastern reservoirs. But the more algal growth, the more decaying organic matter and the higher the oxygen demand near bottom. Combine increased oxygen demand with the effects of thermal stratification, and you end up with poor conditions for the animals that live on the reservoir bottom since they depend on oxygen to live—the same way we do.
The extent of the impact on aquatic life depends on the duration and magnitude of the low dissolved oxygen conditions. Fortunately, in 2007, the poorest conditions occurred only in the lower water column. Typically, from 60 to 90 percent of the water column had adequate dissolved oxygen concentrations even during the worst months.
Since fish can avoid these low dissolved oxygen areas, they continued to look good in our fall sampling. However, benthic communities—made up of organisms that live on the reservoir bottom—aren’t as mobile and, consequently, ratings for bottom life declined at some sites monitored. The good news is that bottom life at our main-stem inflow monitoring stations, where many native mussels still reside, did not appear to be impacted, rating the same or better than in previous years.
What did TVA do to minimize the water quality impacts of the drought?
In addition to our ongoing monitoring program, we did a lot of sampling to spot check conditions, and we tapped into data collected by state agencies, universities, and municipalities. As a result, we had access to a greater quantity of useful information about reservoir conditions and conditions in the tailwater reaches below dams.
With the benefit of this information—and frequent meetings to discuss changing conditions and different scenarios and strategies for dealing with the ongoing drought—we were better able to protect water quality by carefully using the water in storage. It’s likely, for example, that mussel populations in the main-stem reservoirs would have suffered had we not kept enough water moving through the system to prevent the water column from stagnating.
As the summer wore on, we made adjustments in both the volume and timing of water releases and in the operation of the aeration equipment we’ve installed at many TVA dams to protect downstream aquatic life. We met our minimum flow commitments, which helped to keep the riverbeds below our dams from drying out, and we scheduled flows to avoid having river temperatures exceed environmental limits. As a result, impacts to water quality were minimized.
What is the outlook for this summer?
How reservoirs fare in 2008 will depend not only on how much rain the region receives, but also on the timing, duration, and intensity of the rainfall. Rainfall for the Tennessee Valley from January 1 to May 20 totaled about 17 inches, which is 81 percent of normal. That much-needed rain should help to improve ecological conditions in main-stem reservoirs, but we’re still in a drought situation. It’s hard to predict what will happen later this summer. That’s why we monitor dissolved oxygen, chlorophyll, and other water quality characteristics monthly—even weekly at some locations. We will continue to keep close tabs on ecological conditions so we can adjust flows or step up use of aeration equipment if needed.
Tell us about TVA’s reservoir monitoring program.
The objective of our reservoir monitoring program is to provide basic information on the health of the aquatic ecosystem in each TVA-managed reservoir. That information helps to guide day-to-day operations. If problems are found, it’s also used to target more detailed studies.
We monitor ecological conditions at 69 sites on 31 reservoirs. 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—fish and bottom life—are monitored every other year unless a substantial change is detected, in which case we monitor that reservoir 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 only updated every two years when we conduct full Vital Signs monitoring on a reservoir.
Depending on the reservoir’s size, we take samples from three locations: the deep, still water near a 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. We also monitor several 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.
Monitoring results, including reservoir ecological health ratings, are posted on TVA’s Reservoir Ratings Web page. Our 2007 monitoring results should be available in June.