Washington Biodiversity Project
 
Washington Biodiversity Project

Columbia Plateau Ecoregion: Biodiversity

map showing columbia plateau ecoregion

Map: WA Department of Natural Resouces

Two great rivers, the Columbia and the Snake, dominate the dramatic dry landscape of Washington’s largest ecoregion—home to an inland sea of sagebrush and the state’s fertile agricultural heartland.

Location

Lying between the Cascades and the Rockies, the semi-arid Columbia Plateau occupies nearly one-third of the state.

It is a region bordered by the Cascades to the west, the Okanogan Highlands to the north, the Rockies to the east, and the Blue Mountains to the southeast.

In Washington, the ecoregion is bisected by the Columbia River itself. The plateau that makes up this ecoregion tilts upward and southward into the Great Basin of eastern Oregon, western Idaho, and northern Nevada.

Interstate Highway 90 runs east-west through the Columbia Plateau cities of Spokane, Moses Lake, and Ellensburg. Southeast from Ellensburg, Interstate Highway 82 runs the length of the Yakima River Valley, linking Yakima with the Tri-Cities (Kennewick, Pasco, and Richland). Further north, U.S. Route 2 connects Wenatchee with the former railroad towns of Coulee City, Wilbur, and Davenport, as well as Spokane. Southward, Pullman lies close to the Idaho border, in the rolling hills of the Palouse.

Outstanding Biodiversity Features

  • Dramatic geological history led to diverse habitats. Millions of years ago, vast lava flows covered the region in basalt. In more recent millennia, epic glacial floods carved away the deep rock, leaving the coulees and Channeled Scablands of today.
  • Shrub-steppe and grasslands: home to unique plants and iconic birds. The biologically rich Columbia Plateau supports 18 endemic plant species and numerous at-risk birds, among them the sharp-tailed grouse, the sage thrasher, and the sandhill crane.
  • The Palouse Hills: Washington’s breadbasket. The region’s dryland grain and legume farming is vital to our food security. However, the native grasslands that once carpeted the Palouse have shrunk to just 1% of their original expanse.
  • Powerful rivers: shaping—and shaped by—regional economic development. Dams and hydropower development helped build the Northwest’s economy. A cost has been the inundation of fertile floodplains and alteration of riparian habitats. Salmon, sturgeon, and lampreys—once abundant—struggle with the changed waterways.

Landforms

In Washington, the Columbia Plateau’s landscape ranges from an elevation of 160 feet along the Columbia River to nearly 4,000 feet above sea level in the Badger and Tekoa mountains. Prominent landforms in the ecoregion include the Palouse Hills, the Channeled Scablands, the Pasco Basin, and the Yakima Fold Hills.

Two dramatic events shaped the geology of the Columbia Plateau. First a great flood of lava started 17 million years ago and continued for roughly ten million years. It left the ecoregion covered in basalt—6,000 feet deep in some places.

Layered atop that basaltic bedrock are wind-driven silts and volcanic ash from the Cascades. These formed the rich loess soils of the Palouse region, which can be up to 200 feet thick.

In more recent geologic history—a mere 12,700 years ago—vast floods associated with Glacial Lake Missoula carved out the Channeled Scablands of eastern Washington and sculpted the coulee landscape of today.

Climate

The Cascade Mountains cast a long rain shadow over the Columbia Plateau. In the state’s semi-arid interior, summers are hot and dry and winters are cold and gray.

Precipitation occurs mainly between late fall and early spring. Annual averages vary from six inches along the Columbia River near Hanford Reach to 25 inches in the Palouse Hills on the ecoregion’s eastern edge. Snow falls in the ecoregion, though it rarely lasts long before melting.

With its low elevations, and a moderating maritime effect, annual temperatures average 40 to 57 degrees, though extremes can range from subzero to over 100 degrees. Drought and fire are not uncommon.

Vegetation

The Columbia Plateau is dominated by shrub-steppe, like much of the Great Basin. This is a once expansive habitat of shrubs, forbs and bunchgrass. Aromatic shrubs such as sagebrush and bitterbrush offer good browsing to a wide range of wildlife, from sage grouse and pygmy rabbits to mule deer and Rocky Mountain elk.

Other plant communities can be found in this semi-arid region, including salt desert scrub and native grasslands.

Native grasslands remain in scattered pockets of the Palouse and in some canyons. In other areas, grasslands have been created by fire and agricultural operations. Throughout the ecoregion, cheatgrass and other invasive species have displaced native forbs and bunchgrasses.

Forests of ponderosa pine and Douglas-fir grow where the foothills of the Columbia Plateau meet the surrounding mountain ranges.

In much of the Columbia Plateau, a living cryptobiotic crust of blue green algae, lichens, and mosses protects and enriches the soil. The importance of this layer, which shields soils from erosion and fixes nitrogen, is becoming more appreciated. Cryptobiotic crusts face threats from livestock trampling, off-road vehicle traffic, and invasive weeds.

Reardon Lake, near Spokane

Photo: Howard Ferguson, WDFW

Herbaceous wetlands, such as potholes, marshes, and wet meadows, are found throughout the Columbia Plateau. Their aquatic plants, rushes, and thickets of shrubs constitute another conservation priority.

The plateau’s riparian areas and wetlands are other habitats high on the conservation radar. Floodplain cottonwood groves; shrubby stream banks of willow, red-osier dogwood, and mock orange; potholes; marshes; and gravelly river channels offer habitat to a great range of fish and wildlife while occupying only a small area. Invasive species such as Russian olive and purple loosestrife alter their function and habitat value.

Overall, the ecoregion is richly endowed with flora, hosting 46 plant community alliances and roughly 450 plant community associations. The Washington Natural Heritage Program regards more than 20% of these associations as vulnerable. Shrub-steppe and the interior grasslands are considered by many to be among the highest conservation priorities in the ecoregion.

According to NatureServe, eighteen endemic plant species occur with the Columbia Plateau ecoregion.

The floristic richness of the Columbia Plateau is still being discovered. Species new to science, like Umtanum desert buckwheat and White Bluffs bladderpod, were found as recently as the mid-1990s.

Wildlife

From short-horned lizards to sharp-tailed grouse, roughly 45% of Washington’s 829 species of vertebrates are found in the Columbia Plateau ecoregion.

Animal Group

Approx. number of species
Mammals
82
Reptiles and amphibians
27
Birds
192
Fish
73
Butterflies
113
Dragonflies and damselflies
59
Other insects
Yet to be determined
Other invertebrates
Yet to be determined

The Columbia Plateau affords significant habitat for migratory waterfowl and wetlands-dependent birds. While dams of the Columbia Basin Project inundated many wetlands, new ones were created from irrigation runoff. These wetlands, combined with the region’s grain fields and reservoirs, provide food and shelter for birds such as sandhill cranes, wigeons, and buffleheads.

Sandhill cranes

Photo: Howard Ferguson, WDFW

Raptors nest in high densities here. Introduced game birds, such as chukars and ring-necked pheasants, make use of the varied terrain. At-risk bird species in the ecoregion include the upland sandpiper and the sage grouse.

Salmon make spawning runs up rivers in the inland Northwest, though in far fewer numbers than they once did. Fish species of conservation concern include the bull trout and the mid-Columbia coho. Invasive animals such as bullfrogs and brook trout jeopardize the Columbia’s Plateau’s natural aquatic heritage.

The region has dozens of wildlife species considered of greatest conservation concern by the Washington Department of Fish and Wildlife. Mammals like the pygmy rabbit and American badger; reptiles and amphibians like the striped whipsnake and the tiger salamander; and invertebrates like the Juniper hairstreak butterfly and Mann’s mollusk-eating ground beetle are vulnerable to the ongoing changes in the Columbia Plateau’s environment.

People in the Ecoregion

Human history in the Columbia Plateau dates back at least 13,000 years and possibly even earlier. For at least 5,000 years, the native peoples of the region lived in villages along the rivers. They relied on salmon fishing, root and berry harvesting, and hunting. They burned large areas to promote good berry and game habitat, and after the introduction of the horse in the mid-1700s, to improve grazing.

Two hundred years ago, Lewis and Clark encountered numerous Columbia Plateau peoples, including the Cayuse, Nez Perce, Palouse, Tenino, Umatilla, Walla Walla, Wanapum, and Yakama. The Yakama Nation remains a large landholder in the ecoregion.

By the mid-nineteenth century, Euro-American settlers began arriving, with in-migration peaking between 1875 and 1925. They put the region’s abundant natural resources to use with timber harvesting, dryland and irrigated agriculture, grazing, and dam construction.

The mid-twentieth century brought tremendous changes. The Columbia Basin Project built Grand Coulee Dam, among others, with the resulting changes to the basin’s hydrology. The Hanford Nuclear Reservation, once a key participant in the nation’s atomic weapons program, introduced radioactive waste to the issues facing the area.

Today the ecoregion is home to 900,000 people. Agriculture, a vital component of Washington’s economy, has had considerable impact on the biodiversity of the Columbia Plateau. More than half the region’s land base has been converted to dryland or irrigated agriculture and urban development.

Despite the numerous changes in the landscape, sizable pieces of the Columbia Plateau’s natural heritage remain. The U.S. Department of Defense’s Yakima Training Center and the Department of Energy’s Hanford Site retain some of the most intact examples of the region’s shrub-steppe.

Human Impact

The Columbia Plateau faces a complex set of challenging conservation issues. These include:

  • Agricultural Conversion. More than half of the shrub-steppe and 70% of the grasslands have been converted to agriculture, from dryland wheat fields to vineyards. This has reduced or fragmented riparian forests, shrub-steppe, and grasslands, resulting in habitat loss for imperiled species such as sage and sharp-tailed grouse, American badgers, and pygmy rabbits.
  • Housing Developments. Conversion of agricultural or undeveloped lands to residential use is another pressing issue. Subdivisions and ranchettes, particularly near riparian areas, threaten critical habitat by dislocating wildlife and blocking migration corridors.
  • Hydropower Impacts. Dams on the Columbia and Snake Rivers submerged floodplain and riparian habitats. Dams also pose significant difficulties to fish such as salmon, sturgeon, and lampreys. Sedimentation, pesticides, and dewatering further complicate fish passage and riverine processes.
  • Changes in Fire Regimes. Changes in the natural fire regime have degraded plant communities in the Columbia Plateau ecoregion. More frequent fires, often fueled by cheatgrass and other alterations to shrub-steppe ecology, have eliminated some sagebrush communities. Fire suppression has encouraged encroachment of shrubs and trees on native Palouse grasslands.
  • Environmental Pollutants. The Columbia Plateau faces contamination from pesticides, herbicides, and other agricultural chemicals; industrial effluents from pulp mills and aluminum plants; and radioactive wastes from the Hanford Reserve.
  • Non-native and Invasive Species. Brook trout and bullfrogs are among the invasive animal species jeopardizing the Columbia’s Plateau’s biological heritage. Noxious weeds, from cheatgrass to knapweed to Russian olive, displace or alter the functioning of native plant communities.
  • Energy Development. Transmission lines and wind turbines can threaten bird habitats and flyways in the region, home to a great number of raptors. Oil and gas development pose additional concerns.

For many, the Columbia Plateau’s biological richness makes it one of the state’s highest conservation priorities. In recent years, a wide range of partnerships among state and federal agencies, universities, and nonprofit organizations have emerged to tackle the ecoregion’s challenges. Ongoing programs work to inventory and monitor priority species; enlarge natural areas; and implement weed control and conservation planning.

Also underway are projects to encourage rural vitality and environmental stewardship. One such effort is the Healthy Lands Initiative, funded in part by the Washington Biodiversity Council.

Definitions:

Coulee: A deep gulch or ravine. >back

Cryptobiotic crust: A highly specialized community of cyanobacteria, mosses, lichens, and their by-products, which create a crust of soil particles bound together by organic materials. >back

Endemic: Native to or limited to a certain region. Species endemic to Washington occur nowhere else. >back

Forb: A broad-leaved herb or forage plant other than a grass. >back

Loess: Wind-deposited sediments made of silt and renowned for their fertility. >back

Shrub-steppe: Grassland with a shrub component. In Washington the shrubs are often, but not exclusively, species of sagebrush. more> (external site)
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