Columbia Plateau ecoregion
Ice Age floods cut coulees into the basalt of the Channeled Scablands (ecoregion 10a)
Level III ecoregions of the Pacific Northwest. The Columbia Plateau ecoregion is marked with (10).
Ecology
Realm	Nearctic
Borders	List Eastern Cascades Slopes and Foothills (9) Blue Mountains (11) Northern Rockies (15) Idaho Batholith (16) North Cascades (77)
Geography
Countries	United States Canada
States/Provinces	Washington Oregon Idaho British Columbia
Coordinates	47°N 119°W / 47°N 119°W / 47; -119

The Columbia Plateau ecoregion is a Level III ecoregion designated by the United States Environmental Protection Agency (EPA) encompassing approximately 32,100 square miles (83,139 km²)^[1] of land within the U.S. states of Washington, Oregon, and Idaho. The ecoregion extends across a wide swath of the Columbia River Basin from The Dalles, Oregon to Lewiston, Idaho to Okanogan, Washington near the Canada–U.S. border. It includes nearly 500 miles (800 km) of the Columbia River, as well as the lower reaches of major tributaries such as the Snake and Yakima rivers and the associated drainage basins. It is named for the Columbia Plateau, a flood basalt plateau formed by the Columbia River Basalt Group during the late Miocene and early Pliocene. The arid sagebrush steppe and grasslands of the region are flanked by moister, predominantly forested, mountainous ecoregions on all sides. The underlying basalt is up to 2 miles (3 km) thick and partially covered by thick loess deposits.^[2] Where precipitation amounts are sufficient, the deep loess soils have been extensively cultivated for wheat. Water from the Columbia River is subject to resource allocation debates involving fisheries, navigation, hydropower, recreation, and irrigation, and the Columbia Basin Project has dramatically converted much of the region to agricultural use.^[2]^[3]^[4]

The Columbia Plateau ecoregion has been subdivided into at least fourteen Level IV ecoregions, as described below. The EPA has not yet published an Ecoregions of Washington poster similar to the informational posters for Oregon and Idaho, and the Washington information presented here is not as complete as the information about the other two states.

Level IV ecoregions

Channeled Scablands (10a)

The Channeled Scablands ecoregion contains the coulees and Channeled Scablands of Washington carved out by the cataclysmic Missoula floods, from Wenatchee to Spokane, including public land within the Lake Roosevelt National Recreation Area and Wenatchee National Forest.^[4]

Loess Islands (10b)

The Loess Islands ecoregion consists of large pockets of thick loess deposits surrounded by the Channeled Scablands throughout Eastern Washington, including public land within the Turnbull National Wildlife Refuge and the Juniper Dunes Wilderness.^[4]

Umatilla Plateau (10c)

The Umatilla Plateau ecoregion, named for the Umatilla who originally inhabited the area, is characterized by a nearly level to rolling, treeless plateau, underlain by basalt and veneered with loess deposits. Elevation varies from 1,000 to 3,200 feet (300 to 980 m). Glacial features such as patterned ground are common. Areas with thick loess deposits are farmed for dryland winter wheat or irrigated alfalfa and barley. Rangeland dominates more rugged areas where loess deposits are thinner or nonexistent. Mean annual precipitation is 9 to 15 inches (230 to 380 mm) and increases with elevation. In uncultivated areas, moisture levels are generally high enough to support grasslands of bluebunch wheatgrass, Sandberg bluegrass, and Idaho fescue, without associated sagebrush, which is more common in the Pleistocene Lake Basins. Stiff sagebrush may be found on very shallow soils. Alien cheatgrass covers broad areas. The region covers 3,712 square miles (9,614 km²) in Eastern Oregon, including the Umatilla Indian Reservation and agricultural areas south of the Columbia River.^[2]

Okanogan Drift Hills (10d)

The Okanogan Drift Hills ecoregion is located in Douglas and Okanogan Counties, Washington, including part of the Colville Indian Reservation.^[4]

Pleistocene Lake Basins (10e)

The Pleistocene Lake Basins ecoregion is a nearly level to undulating lake plain that contained vast temporary Pleistocene lakes that were created by flood waters from glacial lakes Missoula and Columbia. In Oregon, the flood waters accumulated from the eastern entrance of the Columbia River Gorge upstream to the Wallula Gap to form ancient Lake Condon. Elevation varies from 300 to 1,200 feet (90 to 370 m). Today, the region is the driest and warmest part of the Columbia Plateau, with mean annual precipitation varying from 7 to 10 inches (180 to 250 mm). Major irrigation projects provide Columbia River water to the region, allowing the conversion of large areas into agriculture. Where present, native vegetation consists of needle-and-thread, Indian ricegrass, bluebunch wheatgrass, Sandberg bluegrass, and basin big sagebrush. Alien cheatgrass covers broad areas. The largest of the Columbia Plateau subregions, the Pleistocene Lake Basins covers 1,407 square miles (3,644 km²) in Eastern Oregon and larger areas in Washington, encompassing lower elevations throughout the Mid-Columbia Basin. It includes part of the Yakama Indian Reservation, as well as public lands within the Umatilla, Cold Springs, McNary, and Columbia national wildlife refuges, the Hanford Nuclear Site, and the Hanford Reach National Monument.^[2]^[4]

Dissected Loess Uplands (10f)

The Dissected Loess Uplands ecoregion consists of disjunct rolling hills and flat plateau remnants cut by the Lower Snake and Clearwater Canyons. Elevation varies from 1,500 to 3,600 feet (460 to 1,100 m). Pure grasslands dominate lower elevations, with bluebunch wheatgrass, Idaho fescue, and Sandberg bluegrass. Mountain brush grows on north facing slopes and higher, moister sites, with snowberry and wild rose. Livestock grazing and farming have eliminated much of the original plant cover. Nevertheless, the Dissected Loess Uplands is not as suited to farming as neighboring regions because it has thinner soils. The Dissected Loess Uplands covers 224 square miles (580 km²) in western Idaho and larger areas in southeastern Washington, south of the Snake River between Dayton and Lewiston, including part of the Nez Perce Indian Reservation.^[3]^[4]

Yakima Folds (10g)

Palouse Hills (10h)

The Palouse Hills ecoregion contains the unglaciated Western foothills of the Northern Rocky Mountains known as the Palouse Hills, after the Palouse River that runs through them and the Palus tribe who originally inhabited the area. Elevation varies from 2,500 to 3,000 feet (760 to 910 m). Mountain-fed perennial streams occur, and intermittent, loess-bottomed streams rise within the region. The soils are rich in organic matter and very productive, supporting extensive wheat farming, but they are easily eroded. Dry stream channels may be tilled. The region supports fescue-wheatgrass, fescue-snowberry, and fescue-hawthorn associations, as well as ponderosa pine savanna and open ponderosa pine and Douglas-fir forests. The region covers 453 square miles (1,173 km²) along the Idaho border, including parts of the St. Joe National Forest and the Coeur d'Alene Indian Reservation, and larger areas in Whitman and Spokane Counties, Washington.^[3]^[4]

Deep Loess Foothills (10i)

The Deep Loess Foothills ecoregion contains the lower elevation, northwest-facing slopes of the eastern Blue Mountains, with perennial streams fed by snow melt from the adjacent high mountains. Elevation varies from 1,500 to 3,000 feet (460 to 910 m). Moisture levels are high enough to support grasslands of Idaho fescue, Sandberg bluegrass, and bluebunch wheatgrass, but the region is dominated by non-irrigated winter wheat, barley, alfalfa, and green pea farming on the highly productive, loess-rich soils. The region covers 146 square miles (378 km²) in Oregon and a contiguous area in Washington, in a narrow band from Pendleton to Dayton, including part of the Umatilla Indian Reservation.^[2]^[4]

Nez Perce Prairie (10j)

The Nez Perce Prairie is a loess-covered plateau named after the Nez Perce tribe who originally inhabited the area and whose reservation is located here today. It is higher, cooler, less hilly, and has shallower soils than the Palouse Hills. Elevation varies from 2,000 to 4,100 feet (600 to 1250 m), with buttes up to 5,700 feet (1,700 m). Idaho fescue and bluebunch wheatgrass are native, with Douglas-fir and ponderosa pine on north-facing slopes. Cropland is now extensive and grows wheat, barley, peas, and hay. The headwaters of many perennial streams are impacted by agricultural land use, negatively impacting the water quality of downstream canyon reaches. The region covers 725 square miles (1,878 km²) in Idaho, on the Nez Perce Indian Reservation and surrounding areas.^[3]

Deschutes/John Day Canyons (10k)

The Deschutes/John Day Canyons ecoregion consists of very steep to precipitous canyonlands containing the Deschutes and John Day Rivers. Deeply cut into the basalt, the canyons fragment a lightly populated portion of the Umatilla Plateau. Canyon depths up to 2,000 feet (600 m) create drier conditions than on the plateau above. Sparse bunchgrasses, Wyoming big sagebrush, and cheatgrass grow on the rocky, colluvial soil. Riparian vegetation in narrow reaches is often limited to a band of white alder at the water line, with some mockorange, western clematis, and chokecherry; broader floodplains and gravel bars are dominated by introduced species, such as reed canarygrass, sweetclover, and teasel. The rivers support chinook salmon and steelhead runs. The region covers 674 square miles (1,746 km²) in Oregon, along the lower reaches of the Deschutes and John Day rivers and their tributaries.^[2]

Lower Snake and Clearwater Canyons (10l)

The Lower Snake and Clearwater Canyons ecoregion consists of deep canyons cut through the basalts of the Columbia Plateau by the Snake and Clearwater Rivers. Canyon depths exceed 1,400 feet (430 m) and create drier conditions than in neighboring regions; mean annual precipitation is only 12 to 25 inches (300 to 640 mm). Outside of towns and transportation corridors, the canyons provide good habitat for bighorn sheep and game birds, with grasslands of bluebunch wheatgrass, Idaho fescue, bluegrass, and Wyoming big sagebrush. The region covers 80 square miles (207 km²) in Idaho and slightly larger areas in Washington, along the lower reaches of the Snake and Clearwater rivers and their tributaries.^[3]^[4]

Okanogan Valley (10m)

The Okanogan Valley ecoregion is located along the lower reaches of the Okanogan and Methow Rivers and their tributaries in northeastern Washington, including land managed by the Okanogan–Wenatchee National Forest.^[4] This ecoregion extends northward into the southern Okanagan Valley of British Columbia up to the southern shores of Skaha Lake. This includes towns such as Osoyoos, Oliver, and Okanagan Falls.

Umatilla Dissected Uplands (10n)

The Umatilla Dissected Uplands ecoregion consists of dissected, hilly uplands with a terrace-like appearance, where the dry grasslands of the Columbia Plateau meet the forested Blue Mountains ecoregion. Slopes are rolling to very steep. Elevation varies from 1,600 to 4,400 feet (490 to 1,340 m). The hills and uplands are covered with Idaho fescue, bluebunch wheatgrass, and Sandberg bluegrass. Near the Blue Mountains, some north-facing slopes have Douglas-fir and ponderosa pine, with snowberry, pinegrass, and ninebark. The region is mostly used as rangeland because it lacks the thick, arable loess deposits that cover the agricultural Umatilla Plateau. Scablands, composed of arrays of earth mounds surrounded by rock polygons, are relics of Pleistocene glacial periods. The region covers 743 square miles (1,924 km²) in Eastern Oregon, in a narrow band along the northern edge of the Umatilla National Forest, mostly outside the national forest's boundaries.^[2]

Ecoregions defined by the EPA and the Commission for Environmental Cooperation:
- List of ecoregions in North America (CEC)
- List of ecoregions in the United States (EPA)
- List of ecoregions in Oregon
- List of ecoregions in Idaho
- List of ecoregions in Washington
The conservation group World Wildlife Fund maintains an alternate classification system:
- List of ecoregions (WWF)
- List of ecoregions in the United States (WWF)

v t e Level III Ecoregions in Oregon
Marine West Coast Forest	1 Coast Range 3 Willamette Valley
Northwestern Forested Mountains	4 Cascades 9 Eastern Cascades Slopes and Foothills 11 Blue Mountains 78 Klamath Mountains
North American Deserts	10 Columbia Plateau 12 Snake River Plain 80 Northern Basin and Range
as defined by the United States Environmental Protection Agency [1]

v t e Level III Ecoregions in the United States
Marine West Coast Forest	1 Coast Range 2 Puget Lowland 3 Willamette Valley 111 Ahklun and Kilbuck Mountains 113 Alaska Peninsula Mountains 115 Cook Inlet 119 Pacific Coastal Mountains 120 Coastal Western Hemlock-Sitka Spruce Forests
Northwestern Forested Mountains	4 Cascades 5 Sierra Nevada 9 Eastern Cascades Slopes and Foothills 11 Blue Mountains 15 Northern Rockies 16 Idaho Batholith 17 Middle Rockies 19 Wasatch and Uinta Mountains 21 Southern Rockies 41 Canadian Rockies 77 North Cascades 78 Klamath Mountains 105 Interior Highlands 116 Alaska Range 117 Copper Plateau 118 Wrangell Mountains
Mediterranean California	6 Southern and Central California Chaparral and Oak Woodlands 7 Central California Valley 8 Southern California Mountains
North American Deserts	10 Columbia Plateau 12 Snake River Plain 13 Central Basin and Range 14 Mojave Basin and Range 18 Wyoming Basin 20 Colorado Plateaus 22 Arizona/New Mexico Plateau 24 Chihuahuan Deserts 80 Northern Basin and Range 81 Sonoran Basin and Range
Temperate Sierras	23 Arizona/New Mexico Mountains
Great Plains	25 Western High Plains 26 Southwestern Tablelands 27 Central Great Plains 28 Flint Hills 29 Central Oklahoma/Texas Plains (Cross Timbers) 30 Edwards Plateau 31 Southern Texas Plains 34 Western Gulf Coastal Plain 40 Central Irregular Plains 42 Northwestern Glaciated Plains 43 Northwestern Great Plains 44 Nebraska Sand Hills 45 Piedmont 46 Northern Glaciated Plains 47 Western Corn Belt Plains 48 Lake Agassiz Plain
Eastern Temperate Forest	32 Texas Blackland Prairies 33 East Central Texas Plains 35 South Central Plains 36 Ouachita Mountains 37 Arkansas Valley 38 Boston Mountains 39 Ozark Highlands 51 North Central Hardwood Forests 52 Driftless Area 53 Southeastern Wisconsin Till Plains 54 Central Corn Belt Plains 55 Eastern Corn Belt Plains 56 Southern Michigan/Northern Indiana Drift Plains 57 Huron/Erie Lake Plains 59 Northeastern Coastal Zone 60 Northern Appalachian Plateau and Uplands 61 Erie Drift Plain 63 Middle Atlantic Coastal Plain 64 Northern Piedmont 65 Southeastern Plains 66 Blue Ridge 67 Ridge and Valley 68 Southwestern Appalachians 69 Central Appalachians 70 Western Allegheny Plateau 71 Interior Plateau 72 Interior River Valleys and Hills 73 Mississippi Alluvial Plain 74 Mississippi Valley Loess Plains 75 Southern Coastal Plain 82 Laurentian Plains and Hills 83 Eastern Great Lakes and Hudson Lowlands 84 Atlantic Coastal Pine Barrens
Northern Forests	49 Northern Minnesota Wetlands 50 Northern Lakes and Forests 58 Northeastern Highlands 62 North Central Appalachians
Tropical Wet Forests	76 Southern Florida Coastal Plain
Southern Semi-Arid Highlands	79 Madrean Archipelago
Taiga	101 Arctic Coastal Plain 102 Arctic Foothills 103 Brooks Range 104 Interior Forested Lowlands and Uplands 106 Interior Bottomlands 107 Yukon Flats 108 Ogilvie Mountains
Tundra	109 Subarctic Coastal Plains 110 Seward Peninsula 112 Bristol Bay-Nushagak Lowlands 114 Aleutian Islands
as defined by the United States Environmental Protection Agency [2]

References

^ Final Programmatic Environmental Impact Statement on Wind Energy Development on BLM-Administered Lands in the Western United States. United States Department of the Interior; Bureau of Land Management. p. 6. Retrieved October 24, 2018. columbia plateau 32100.
^ ^a ^b ^c ^d ^e ^f ^g ^h This article incorporates public domain material from Thorson, TD; Bryce, SA; Lammers, DA; et al. Ecoregions of Oregon (PDF). United States Geological Survey. (color poster with map, descriptive text, summary tables, and photographs; with a Reverse side).
^ ^a ^b ^c ^d ^e This article incorporates public domain material from McGrath, CL; Woods, AJ; Omernik, JM; et al. Ecoregions of Idaho (PDF). United States Geological Survey. (color poster with map, descriptive text, summary tables, and photographs; with a Reverse side).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k U.S. Environmental Protection Agency, Draft: Level III and IV Ecoregions of the Northwestern United States (PDF), archived from the original (PDF) on 2019-04-12 Cited for Washington information instead of an Ecoregions of Washington poster, which has not yet been located.