 The ecosystem within the Bill Williams River National Wildlife Refuge is situated in an ecotone (transition zone) between Mojave Desert and Sonoran Desert ecoregions, increasing diversity of plant species present within it, There are few places in the Arizona deserts where one can view saguaro cacti forests, wetland broadleaf cattail ("Typha latifolia") stands, and cottonwood woodlands in a single viewshed.The refuge protects the largest remaining stand of the cottonwood-willow forests plant community along the lower Colorado River, with Fremont's cottonwood ("Populus fremontii") and Goodding's willow ("Salix gooddingii") the primary tree species.The rare desert context riparian habitat of Bill Williams River NWR supports diverse resident fauna, and draws a variety of neotropical migratory birds, from Central and South America en route to their breeding grounds in the north.