Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA. Information was also provided by Salvador Contreras Balde


United States

Major Habitat Type

Xeric freshwaters and endorheic (closed) basins

Drainages flowing into

The Colorado once emptied into the Gulf of California through the Colorado River Delta, an expansive and complex system of wetlands. Irrigations projects within the last century have significantly reduced freshwater flows to a trickle.

Main rivers to other water bodies

The drainage area of the Colorado River, which flows 2,282 kilometers, is typically divided into an upper and lower basin. The major tributaries to the Upper Colorado River are the Green, Gunnison, Dolores, and San Juan rivers, and those in the lower basin are the Little Colorado, Virgin, Bill Williams, and Gila rivers (Williams et al. 1985). The Gila River contains a biota distinct enough to warrant a separate ecoregion [131], as does the Virgin River [129], included with what is the now disjunct White River.



Stretching from southwestern Wyoming to the northeastern tip of Baja California in Mexico, this ecoregion covers a portion of southeastern Nevada, parts of western and northern Arizona, the northwestern corner of New Mexico, most of eastern Utah, and western Colorado. The ecoregion primarily lies within the physical province of the Colorado Basin and includes portions of the Wyoming Basin and the Sonoran Desert, making it one of the most arid ecoregions in North America (Minckley et al. 1986).


From its headwaters in the Rocky Mountains to its terminus, the Colorado River experiences a vertical elevation drop of more than 2700 m. The upper basin is an area of high relief through erosive, swift-flowing rivers that have incised the landscape; the lower basin is comparatively low gradient, flowing historically through a broad alluvial valley.

Freshwater habitats

Historically, this ecoregion’s freshwater habitats were dominated by warm, silt-laden rivers with highly variable flows. These large rivers were fed by cold, clear montane creeks as well as springs. Mountain lakes have also supplied coldwater habitats. At the other extreme, habitats of the Colorado Delta were characterized by extremely high and variable tides, temperatures, salinities, and siltation loads, with vast marshes, riparian forests and backwaters (Minckley et al. 1986).

Terrestrial habitats

The variety of soils and elevation gradients throughout this ecoregion support a wide range of vegetation from alpine conifer forest complexes to desert shrubs and grasses (Ricketts et al. 1999).

Description of endemic fishes

The assemblage of large river fish species historically found in the Colorado and its main tributaries (including, in some cases, the Gila) is truly extraordinary. The humpback chub (Gila cypha), bonytail (G. elegans) and razorback sucker (Xyrauchen texanus) all display morphological adaptations for life in turbid, fast-flowing habitats. The Little Colorado spinedace (Lepidomeda vittata), found only in creeks and small rivers of eastern Arizona, shares its genus with just two other species in the neighboring Vegas-Virgin ecoregion [129] (Page and Burr 1991). 

Other noteworthy fishes

The Colorado pikeminnow (Ptychocheilus lucius) is a near-endemic of particular interest as a top carnivore and the largest cyprinid in North America. Other large-river near-endemic fishes include the flannelmouth sucker (Catostomus latipinnis), Little Colorado River sucker (C. sp.), Sonora sucker (C. insignis), and desert sucker (C. clarki), as well as endemic subspecies of cutthroat trout (Oncorhynchus clarki stomias, O. c. pleuriticus) and bluehead sucker (C. discobolus yarrowi) (Page and Burr 1991; Sigler and Sigler 1994).

In contrast to the relatively large-bodied minnows and suckers, this ecoregion’s streams and creeks support a suite of fishes and other taxa adapted to small freshwater habitats. The Apache trout (Oncorhynchus apache) was historically found in the clear, cool mountain headwaters and lakes in the upper Salt River and Little Colorado River systems (Page and Burr 1991). The Las Vegas dace (Rhinichthys deaconi), now extinct, was historically found in springs along Las Vegas Creek, a small tributary to the Colorado River in Nevada. Like the ecoregion’s large river habitats, these springs and associated small freshwater habitats support a number of endemic subspecies, such as the Kendall Warm Springs dace (Rhinichthys osculus thermalis), found only in a small tributary to the Green River (Williams et al. 1985). 

Justification for delineation

Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995).

Level of taxonomic exploration

Generally well-known


  • Abell, R.,Olson, D.,Dinerstein, E.,Hurley, P. T.,Diggs, J. T.,Eichbaum, W.,Walters, S.,Wettengel, W.,Allnutt, T.,Loucks, C. J.;Hedao, P. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.
  • Behnke, R. J. and Benson, D. E. (1983) \Endangered and threatened fishes of the Upper Colorado River Basin\ Washington, DC, USA. U.S. Fish and Wildlife Service.
  • Ricketts, T. H.,E. Dinerstein,D.M Olson;C.J. Loucks (1999). "Terrestrial ecoregions of North America: A conservation assessment" Washington, D.C.: World Wildlife Fund.
  • Minckley, W. L., Hendrickson, D. A. and Bond, C. E. (1986). "Geography of western North American freshwater fishes: Description and relationships to intracontinental tectonism" C. H. Hocutt and E. O. Wiley (Ed.) The zoogeography of North American freshwater fishes New York: John Wiley.
  • Sigler, J. W. and Sigler, W. F. (1994). "Fishes of the Great Basin and Colorado Plateau: Past and present forms" K. T. Harper, L. L. St. Clair, K. H. Thornes and W. M. Hess (Ed.) Natural history of the Colorado Plateau and Great Basin Niwot: University of Colorado Press.
  • Williams, J. E., Bowman, D. B., Brooks, J. E., et al. (1985). "Endangered aquatic ecosystems in North American deserts with a list of vanishing fishes of the region" Journal of the Arizona-Nevada Academy of Sciences 20 pp. 1-62.
  • Maxwell, J. R., Edwards, C. J., Jensen, M. E., et al. (1995) \A hierarchical framework of aquatic ecological units in North America (Nearctic Zone)\ St. Paul, MN. North Central Forest Experiment Station, USDA Forest Service.
  • Page, L. M., and B. M. Burr (1991). "A field guide to freshwater fishes: North America north of Mexico" New York, NY: Houghton Mifflin Co..