Corrective measures are needed early again this year and probably point to another bad low DO season. Vents on turbines at Bull Shoals and Norfork dams were opened on July 14th this summer as dissolved oxygen readings fell below the state standard of 6.0 parts per million for trout waters. This is an annual occurrence, although it is typically a more severe problem during high water years. This is the third back-to-back high water year for the White River reservoirs, which is historically rare. The Corps started taking corrective action about the same time the previous two summers.
As the lakes stratify in the summer months, decaying matter in the water on the bottom of the lake uses up oxygen through microbial action. As it becomes depleted of O2, and as the powerhouse runs heavy to meet hydropower demands, that dead water gets pulled through the turbines and into the tailwaters endangering trout and other fish and aquatic life. This has resulted in numerous fish kills over the years. And it is particularly troublesome as the low DO condition worsens into the late fall of the year just as brown trout are attempting to migrate to gravel beds to spawn below the dams.
Even though a dissolved oxygen committee was formed by Governor Bill Clinton in the early 1990s to correct the problem, a decision on a remedy has been elusive and no money has come forward to fix it. The state argues this is a continued mitigation problem endemic to the building of the dams, and the Corps of Engineers counters the dams were authorized by Congress for flood control and hydropower, and the fishery is mitigated through the federally funded hatchery system which is run by the U.S. Fish and Wildlife Service. The Corps says it can’t be held responsible for the fact that low DO, which falls below the state water quality standards for trout of 6.0 ppm, limits successful spawning of trout in the river system and occasionally results in fish kills. The low DO is a problem with watershed quality, it argues.
If the trout tailwaters were only a put-and-take fishery, then occasion fish kills might be acceptable. But impacting as it does just at the time of the brown trout spawn makes this a worse case scenario. The smallmouth fishery that was here before the construction of the dams was a natural, not a put-and-take fishery. Further, the tailwater trout fisheries have proven they can successfully compete with well established wild trout fisheries as far as producing several trout species which grow to rival trout in their native ranges and even produce world record fish. That successful natural reproduction of brown trout in the tailwater system is not only possible but likely is now without question.
The biology textbook I have states a DO level of 8.0 ppm is necessary for healthy naturally reproducing trout and in order to have a successful spawn. A DO level of 6.0 falls short of this, although Game and Fish biologists say it is a realistic number to strive for given the annual problem with the anoxic water from the dams. The Corps would like to see the state drop the water quality standard for trout to 5.0 ppm as corrective measures are expensive.
According to readings from USGS gages today, DO below Norfork dam today dropped to 4.3 milligrams per liter (this is the same as parts per million or ppm) under full generation. When this happens, the Corps usually implements a maximum load restriction which helps keep the DO level higher. As the problem worsens maximum load restrictions are further reduced, since heavy generation pulls more anoxic water off the bottom of the lake and the vents on the turbines which siphon air into the system become less effective at higher generation loads. The Corps has yet to implement those restrictions this summer. But I would look for it to happen soon.
Arkansas Game and Fish Commission biologists started using stocking restrictions on Norfork and Bull Shoals tailwaters a few years ago when DO readings consistently stay low. As the reading worsens look for these measures to be put into effect this fall. It looks to be another bad DO season on the tailwaters. The DO problem goes away as soon as the lakes finish turning over usually by December 15th.
© 2010, Scott Branyan