Okanagan Ecoregion

Description & Physiography

The Okanagan Ecoregion lies east of the crest of the Coast and Cascade Mountain ranges and west of the Columbia and Selkirk Mountains. The ecoregion is characterized by long, rounded ridges, rolling plateaus, wide valleys, and large lakes with the Thompson-Okanagan Plateau in the northeast and the Okanagan Highlands in the southeast. In the northwest and southwest portions of the ecoregion, the Chilcotin, Interior Transition, and Okanagan Ranges are characterized by rugged mountains and deep valleys. To the east, the mountains are more rounded, particularly the Kettle Range and Huckleberry Mountains in. The south-central portion of the ecoregion contains the northern extent of Palouse grasslands—an area characterized by rolling, highly fertile loess hills, and scattered wetlands. The Sawtooth Ridge northeast of Lake Chelan marks the southwestern border of the ecoregion. In Washington, the ecoregion includes the Methow and Okanogan valleys and the Okanogan Highlands east to the Colville and Spokane valleys.

Elevations within the ecoregion range from below 1,000 ft to peaks in the Interior Transition Ranges that are over 10,000 ft. Glaciation has left its imprint in the form of hummocky moraines, drumlinoid features, terraces, esker complexes, and glacial lake deposits. Major water bodies in the western and northern portions of the ecoregion in British Columbia include the Thompson River and its lakes and tributaries which join the Fraser River at Lytton. To the east and south lie Okanagan Lake and the Similkameen River, which flows south into Washington State.

Climate

The ecoregion has both the coldest climate in Washington and some of the hottest and driest weather recorded in British Columbia. The ecoregion is influenced by the extremes of hot, dry air from the Columbia Basin in the summer and cold, dense arctic air in the winter. The western part of the ecoregion is dry because it is within the rain shadow of the Coast and Cascade Mountains; however, precipitation increases to the east as air masses rise, cool, and drop moisture over the Rocky Mountains. Annual precipitation varies from less than 31 cm (12 in) in the greater Okanogan valley of Washington and British Columbia to 127–229 cm (50–90 in) in the Cascades. Most of the ecoregion lies within a 36–61 cm (14 to 24 in) precipitation zone. Throughout the region, fairly steep temperature and precipitation gradients occur from the mountains to the valleys.

Plants & Animals

Endemic species found within this ecoregion include the night snake (Hypsiglena torquata) and pygmy short-horned lizard (Phrynosoma douglasii). The ecoregion contains most of the remaining grasslands, shrub-steppe, and low-elevation dry forests in British Columbia. The low elevations of the Okanogan and Similkameen River valleys, where dry climate and desertlike habitats are northern extensions of the Great Basin, are particularly important for shrub-steppe species. This area is a critically important movement corridor into the mountainous areas of the western United States for wide-ranging carnivores such as grizzly bears (Ursus arctos), grey wolves (Canis lupus), lynx (Lynx canadensis) and wolverines (Gulo gulo). 

Conifer forests dominate mountain ridges and low hills in the ecoregion, while valleys and lowlands are often non-forested. The conifer forests are more open and less continuous, consisting of smaller stands, than are forests west of the Cascade crest and in the Canadian Rocky Mountains. Douglas-fir–ponderosa pine (Pseudotsuga menziesii–Pinus ponderosa) forests characterize the ecoregion and grade to shrub-steppe in the low broad valleys in the eastern part of the ecoregion and to grasslands in the western part. Whitebark pine (Pinus albicaulis), lodgepole pine (Pinus contorta var. latifolia), and subalpine larch (Larix lyallii) form parklands in the highest elevations of the ecoregion and are often associated with dry alpine or subalpine meadows. Moister forests are dominated by Douglas-fir, with western larch (Larix occidentalis), western white pine (Pinus monticola) or trembling aspen (Populus tremuloides) as common components.

Humans & History

Continental and alpine glaciers played a major role in shaping the landforms of the Okanagan Ecoregion. The entire area was glaciated during the Pleistocene epoch. Extensive surficial moraines were deposited as the glaciers retreated, and lakes, such as Kamloops and Okanagan Lake, formed in the ice-carved depressions. Streams and rivers cut through the surficial moraines and created steeply incised gullies with exposed bedrock in transition areas between the headwaters and the lower-lying valleys.

Historically, stand replacement fires occurred at irregular intervals from 10 years in the lowland foothills to 150 years or more at high elevations. Decades of fire suppression have resulted in a landscape composed of dense, fire-prone forests.

Today

The Okanagan Ecoregion is a biologically rich area consisting of numerous convergent ecological habitat types. The climate and abundant natural resources of the ecoregion have supported a rapidly expanding human population and agricultural industry; however, intensive land use threatens the region’s biodiversity. Conservation organizations and government agencies are increasing their protection and restoration efforts in the region, but their limited resources make careful coordination of conservation efforts a necessity.

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Links to Completed Ecoregional Assessments

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