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Ecoregion Description

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Species Richness

# of Endemic Species


121: Columbia Unglaciated

Major Habitat Type:

temperate floodplain rivers and wetlands


Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.


United States


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.  

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 or 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 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).


The climate in the ecoregion is varied, with the Dalles Dam separating what some have termed the lower and middle Columbia Rivers. Above the dam, the climate is arid, with high summer temperatures and periodic droughts; below the dam, the climate is mild and water is abundant (McPhail & Lindsey 1986).

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).

Fish Fauna:

Within the temperate floodplain rivers and wetlands MHT this ecoregion has low species richness, with less than fifty native species of fish. A number of these are found both in the lower and middle Columbia. Notably, the ecoregion supports six species of salmon (Oncorhynchus spp.), whose historic migrations made the Columbia one of the biggest salmon-producing rivers in North America (McPhail & Lindsey 1986).

Description of endemic fishes:

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

Other noteworthy aquatic biotic elements:

Within this ecoregion mollusk endemism is high and often localized; discrete areas of endemism are found in the lower Columbia River, the lower Deschutes River drainage, the Columbia Gorge, the Blue Mountains, the lower Salmon River, the middle Snake River, the Clearwater River drainage, and Hells Canyon. Frest and Johannes  (1995) list 63 freshwater snail and clam species and subspecies of special concern, 25 of which are apparently restricted to single locations.

Ecological phenomena:

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

Evolutionary phenomena:

This ecoregion is noted for its higher taxonomic endemism; the fish genus Oregonichthys is near-endemic to the Columbia basin.

Justification for delineation:

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


Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.

Frest, T. J.,Johannes, E. J. (1995). "Interior Columbia Basin mollusk species of special concern" Seattle, WA: Deixis.

Maxwell, J. R., Edwards, C. J., 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, Henry and Avers, Peter (1994) "Ecological Subregions of the United States: Section Descriptions". Washington DC. U.S. Department of Agriculture, Forest Service.

McPhail, J. D.,Lindsey, C. C. (1986)"Zoogeography of the freshwater fishes of Cascadia (the Columbia system and rivers north to the Stikine)" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. New York: John Wiley.

Ricketts, Taylor H. Dinerstein Eric Olson David M. Loucks Colby J. (1999). "Terrestrial ecoregions of North America: A conservation assessment" Washington, D.C.: World Wildlife Fund.

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