Habitats for fish in the American Southwest include springs, seeps, perennial and ephemeral streams, rivers, playa lakes, and reservoirs. Freshwater fish are the most imperiled of the vertebrate groups in the U.S., and in the southwest the causes of imperilment include the construction of dams, loss and degradation of habitat, pollution, over-exploitation, and introduction of non-indigenous species. Aquatic ecosystems in the American Southwest also show some of the highest rates of endemism on the continent. A surprising number of endemic fish occur in the Chihuahuan Desert ecoregion, for example, where nearly half of the species are either endemic or of limited distribution. In the Colorado River, 64% of the fish species are endemic, with many being threatened or endangered.