Columbia Unglaciated




Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA. Additional text provided by J. Hales.


United States

Major Habitat Type

Temperate floodplain rivers and wetlands

Drainages flowing into

Most of the major rivers rise to the west of the Continental Divide and flow westward across the areas of uplift toward the Pacific Ocean (McPhail & Lindsey 1986).

Main rivers to other water bodies

The major river in this ecoregion is the Columbia, from just above the point where it becomes the Washington-Oregon border to its confluence in the Pacific. Other important rivers include the lower half of the Snake, its tributary the Salmon River in Idaho, and the John Day and Deschutes rivers in Oregon.



The Columbia River is the second largest river in the United States. The Columbia Unglaciated ecoregion, together with the Upper Snake [122], marks the portion of the larger Columbia River basin that was never glaciated. This ecoregion covers most of eastern and northern Oregon, reaches just into Washington along the southern border of the state, spans central and southeastern Idaho, and covers small portions of northern Nevada and western Montana.  


The ecoregion is dominated by rugged mountain ranges separated by a series of lowlands and plateaus (McPhail & Lindsey 1986). Elevations range from 70 - 1,500 m (McNab & Avers 1994).

Freshwater habitats

The ecoregion hosts a rich variety of freshwater habitats, including lakes, ponds, swamps and rivers. These unglaciated areas served as ice-free refuges for species during the Pleistocene period of glaciation (McPhail & Lindsey 1986). 

Terrestrial habitats

Characteristic vegetation types in the ecoregion range from temperate coniferous forests to grasslands and shrub steppe (Ricketts et al. 1999).

Description of endemic fishes

The only endemic species within the ecoregion is the Oregon chub (Oregonichthys crameri).

Ecological phenomena

The Columbia basin provides critical spawning grounds for anadromous fish, including numerous native salmon runs (chinook, coho, sockeye).

Justification for delineation

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


  • Frest, T. J.;Johannes, E. J. (1995). "Interior Columbia Basin mollusk species of special concern" Seattle, WA: Deixis.
  • McPhail, J. D. and Lindsey, C. C. (1986). "Zoogeography of the freshwater fishes of Cascadia (the Columbia system and rivers north to the Stikine)" C. H. Hocutt and E. O. Wiley (Ed.) The zoogeography of North American freshwater fishes ( pp. 615-637 ) New York: John Wiley & Sons.
  • 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.
  • 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.
  • 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.
  • McNab, H. and Avers, P. (1994) \Ecological Subregions of the United States: Section Descriptions\ Washington DC. U.S. Department of Agriculture, Forest Service.