The Bill Williams River National Wildlife Refuge protects the lower course of the river, to its mouth at Lake Havasu reservoir. The Sonoran Desert rare riparian habitat of Bill Williams River NWR draws a variety of Neotropical migratory birds, from Central and South America en route to their breeding grounds in the north. It has one of the last stands of the natural 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.