American shad ( Alosa sapidissima ) are native to the east coast of North America from St. Johns River, Florida, Lawrence River region in Canada. Since 1800s, dams have reduced access spawning habitat. To assess impact dams, we estimated historically accessed habitat coastal rivers (485,618 river segments with 21,113 current dams) based on (i) width, (ii) distance seawater, and (iii) slope (to ...