When you’re talking about the U.S. Army Corps of Engineers’ six mainstem dams on the Missouri River, the word small is a relative term.
While the dams and their powerhouses vary in size, they are all imposing structures. For instance, Gavins Point Dam, near Yankton, South Dakota, is the smallest of the six, yet it took 7 million cubic yards of earth to build and its three Kaplan generators are capable of generating electricity for 68,000 homes.
This makes it that much more ironic that something as small as a zebra mussel could give it such big problems.
Zebra mussels grow to a maximum length of 1.9 inches and according to the U.S. Fish and Wildlife Service are native to the Caspian, Black, and Azov seas. They were first documented in the United States in 1988 in Lake St. Clair between Lake Huron and Lake Erie. The mussels are believed to have arrived via ships discharging ballast water containing their larvae. The United States Geological Survey theorized barge traffic helped them spread throughout interconnected waterways, but if conditions are right, mussels can also temporarily survive out of water meaning they can spread overland if attached to watercraft or equipment.
Zebra mussels reproduce at incredibly high rates. A single female is capable of releasing up to one million eggs per year. Once a population is established, the potential for staggering population growth can be met. As an invasive species, they alter local ecosystems and do not have many predators that can keep their population in check.
The first mussel was discovered at Gavins Point in 2014 at Midway Recreation Area attached to the underside of a dock being removed for winter. Since then they have been spotted at the Lake Sharpe Reservoir at Big Bend Dam.
“The dams weren’t engineered to deal with this nuisance,” Michael Schnetzer, senior power plant mechanic at Gavins Point Dam said. “When these dams were designed in the thirties, forties, and fifties zebra mussels were never heard of. They weren’t put into the design of the dams.”
In a reservoir, millions of mussels reproduce and release their larvae, known as veligers, downstream where they get sucked into the dam’s piping system. These veligers then mature and attach themselves throughout the piping where they reproduce and the cycle continues. As the population within the piping grows, problems arise when the mussels begin to block off water flow reducing the efficiency of effected systems.
“For a hydropower facility like Gavins Point it’s a perfect storm situation,” said Jonas Grundman, natural resources specialist Omaha District U.S. Army Corps of Engineers.
Zebra mussels cause trouble by causing the inside pipe diameter to decrease overtime if not blocking them entirely. Even if the flow of water is not stopped completely, the mussels can die and detach sending a ball of shells through the pipes and creating blockages elsewhere in the system. The worst thing a mussel can do is shut down a generator, which is not an unheard of occurrence at Gavins Point.
Gavins Point has three generators but once or twice a month a generator will be shut down because of mussel infestation. When mussels cause the shutdown of a generator it is an unplanned outage, which means electricity is not being produced. In addition to shutting down generators there is also a fear that the infestation could get so bad the mussels could block intake valves or even prevent the dam’s tail race gates from being opened.
“Anything the lake water touches requires more maintenance than it did in the past,” Schnetzer said.
Schnetzer estimates that between 2014 and December 2018, 2,500 man- hours have been spent on mussel related issues instead of other maintenance. By the numbers, that is 62.5 40-hour workweeks spent on zebra mussels alone.
Glenn Cunningham and Zorinsky lakes in Omaha, Nebraska, are two potential examples of mussel population eradication. Each lake was drained and their mussel populations were exposed to freezing conditions over a winter. However, these are small lakes whereas the reservoir at Gavins Point is not only much larger but also supplies water to an entire region.
Potentially successful measures have been taken to mitigate some of the mussels’ impact on the facility. In 2019, Gavins Point installed an ultraviolet light system which kills mussel larvae floating through the system, which would otherwise attach somewhere and grow to adulthood. The system works by disrupting protein chains within their cell structure. Gavins Point also installed self-cleaning strainers, which block out anything larger than one eighth of an inch.
Piping and the cooling systems will be replaced in the future. The new cooling system’s design was done with mussel mitigation in mind.
While preventing the spread of invasive species is difficult, you do not need an industrial grade filtration system to do the job. To prevent the spread of mussels between bodies of water all equipment (whether recreational or industrial) should be cleaned, drained, and dried before being moved. Zebra mussels negatively impact hydropower infrastructure but also litter recreational beaches with sharp shells, foul up ecosystems, and disrupt irrigation systems.