The Bill Williams River National Wildlife Refuge, the IBA, a nine-mile corridor and delta, is the last extensive naturally flood-regenerated riparian and wetlands habitat on the Lower Colorado River. This riparian zone is an area of year-around available water and riparian vegetation creating cooler, more humid conditions than the surrounding desert where rainfall averages only 3 inches per year and summer temperature often exceed 120 degrees. In the winter the warmth allows many plants to keep their leaves, others to bloom in December and January, and the insects to remain active, all of which makes it a vital stopover for migration and wintering of birds on the Colorado River Flyway. Habitats consist of extensive mesquite bosque and desert uplands, as well as cottonwood-willow, bulrush, cattail, and seep willow.
As a National Wildlife Refuge, conservation of species and their required habitats is the most important consideration in management of this IBA. The refuge was established primarily to provide habitat for migratory birds, especially neotropical migrants, and to protect native riparian habitats. Land use is focused on wildlife orientated recreation and education. Although only 6,105 acres in size the refuge encompasses a wide diversity of habitats including open lake waters; a year around stream with beaver dams; cattail and bulrush marshes; a naturally flood-regenerated cottonwood-willow forest; mesquite bosque; desert washes with ironwood, smoketree, and palo verde; uplands dominated by creosote/white bursage, ocotillo and cacti including saguaro; extensive cliffs; and long- abandoned agricultural fields.
The Bill Williams River National Wildlife Refuge is the steward for this site through their NWR Friends program. Information about the refuge can be found at http://www.fws.gov/southwest/refuges/arizona/billwill.html
The Bill Williams River National Wildlife Refuge is the southern most extensive riparian and wetlands habitat on Lower Colorado River in the United States. This IBA is a stopover for neotropical migrants using the Colorado River flyway as well as being important wintering habitat for many northern species. The refuge provides breeding habitat for 25 species of conservation concern in Arizona. These species nesting in the IBA include federally listed endangered and threatened species, i.e., Southwestern Willow Flycatcher and Yuma Clapper Rail; Arizona Partners in Flight High Priority species, i.e., Southwestern Yellow-billed Cuckoo, American Bittern, and Lucy's Warbler; and Audubon WatchList, Red Listed species, i.e., California Black Rail, Bell's Vireo and Bendire's Thrasher. Bell's Vireo estimated numbers averaged over the past ten years is 2700 individual birds.
Two other breeders within the IBA, Clark's Grebe and Yellow Warbler (D. petechia sonorana), are also listed as species of concern by the state of Arizona (Threatened 1996) and US Fish and Wildlife Service (Birds of Conservation Concern), respectively. This IBA is one of the few areas left on the Lower Colorado River with large breeding populations of warblers, particularly Yellow and Lucy's and Yellow-breasted Chats, as well as other riparian obligate species. Peregrines breeding in the vicinity of the refuge use habitats within the refuge after they fledge. Crissal, Bendire's and Curve Billed Thrashers, (USFWS and Audubon Yellow Listed, birds of conservation concern) breed in the riparian edges and mesquite bosque as well. Overall 343 species have been recorded on the refuge.
Other Flora and Fauna
Due to the unique natural regeneration of the complex riparian plant communities and its position on the ecotone between the Sonoran and Mohave deserts, an extensive and diverse invertebrate fauna is also present on the Bill Williams River. For example, 11 species of butterflies are present that were once common on the Lower Colorado River before the dams but are now found only on the refuge, two more species are common here but rare otherwise. Other native species of invertebrates are doubtless equally well represented. Virtually the entire complex of species native to the Lower Colorado River historically are still present on the Bill Williams River NWR including 53 mammals from Desert Shrews to Desert Bighorns and from Pocket mice to Mountain Lions. The bats are particularly well represented with several rare forest dependent species present. Likewise the reptile and amphibian diversity is equally high (30 species) including the rare Lowland Leopard Frog, desert Tortoise and numerous species of lizards and snakes. The endangered Razorback Sucker and Bonytail Chub are also present in the delta along with many introduced fish species.
Threats may include upstream water diversions, wildfire, invasive exotic species spreading into riparian habitat along river, particularly Tamarix ramosissima, and excessive or inappropriate public recreation.
The Threat Importance of animal and plant introductions is high.
U.S. Fish and Wildlife Service Refuge with an emphasis on conservation of neotropical migrant birds, marsh birds, riparian, and marshland habitats.
A nine-mile corridor and delta, the Bill Willliams River NWR is the last extensive naturally flood-regenerated riparian and wetlands habitat on the Lower Colorado River. This riparian zone is an area of year-around available water and riparian vegetation creating cooler, more humid conditions than the surrounding desert where rainfall averages only 3 inches per year and summer temperature often exceed 120 degrees. This is a unique ecosystem where you can look at cattails, cottonwoods and Saguaros from the same spot.
Fremont cottonwood and Gooding's willow are the dominant trees in the gallery riparian forest. Mesquite bosques are on the higher floodplain terraces that are surrounded by desert scrub habitat that is transitional between the Sonoran and Mohave deserts. The marsh habitat is at the confluence of the Bill Williams and Colorado Rivers.
As a National Wildlife Refuge, conservation of species and their required habitats is the most important consideration in management of this IBA. The refuge was established primarily to provide habitat for migratory birds, especially neotropical migrants, and to protect native riparian habitats. Land use is focused on wildlife orientated recreation and education.
Research has been a long-term commitment of the Bill Williams River NWR. Many of the extensive and intensive studies by R.D. Ohmart and B.W. Anderson included the riparian areas of the refuge. Several long-term avian research projects are also ongoing including Southwestern Willow flycatchers, Yellow-billed Cuckoos, and Brown-headed Cowbirds. As the refuge is the only naturally regenerating forest left on the Lower Colorado River, it is and has been used as the baseline for many studies of birds as well as riparian plant ecology. The refuge maintains a series of long-term monitoring transects in six habitats for birds, mammals, reptiles and amphibians. It also surveys yearly for Yuma Clapper Rails, waterfowl, and other marsh and water birds, and carries out an Audubon Christmas Bird Count. The refuge promotes research wherever possible to better understand the ecological complexity and interactions of these habitats and species, especially for improving the restoration of native habitats and their associated communities elsewhere on the Lower Colorado River.
Protected as a U.S. Fish and Wildlife Service National Wildlife Refuge. The refuge designates a no-wake zone for boats using the Lake Havasu bay within the refuge and for the Bill Williams River delta and marsh area.