Across the nation, the scenario repeats. Atlantic sturgeon, once a hallmark of the eastern seaboard, can reach only about half of their historic spawning grounds. Some 40 percent of the 800 or so varieties of freshwater fish in the US, and more than two-thirds of native mussels, are rare or endangered, in part because man-made barriers have altered their ecosystems. Reservoirs disrupt currents, altering water’s velocity and temperature. That can harm its quality and interrupt the reproductive cycles of aquatic creatures. Stanching a river stops the distribution of sediment and the formation of logjams, two things critical to creating healthy habitat. It also eliminates floodplains and natural meanders, both of which prevent the banks from overflowing.