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.