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The Willamette Valley ecoregion is a Level III ecoregion designated by the United States Environmental Protection Agency (EPA) in the U.S. states of Oregonmarker and Washingtonmarker. Slightly larger than the Willamette Valley for which it is named, the ecoregion contains terraces and floodplains of the Willamette River system, scattered hills, buttes, and adjacent foothills. It is distinguished from the neighboring Coast Range, Cascades, and Klamath Mountains ecoregions by lower precipitation, lower elevation, less relief, and a different mosaic of vegetation. Mean annual rainfall is 37 to 60 inches (96 to 152 cm), and summers are generally dry. Historically, the region was covered by rolling prairies, oak savanna, coniferous forests, extensive wetlands, and deciduous riparian forests. Today, it contains the bulk of Oregon’s population, industry, commerce, and agriculture. Productive soils and a temperate climate make it one of the most important agricultural areas in Oregon.

The Willamette Valley ecoregion has been subdivided into four Level IV ecoregions, as described below.

Level IV ecoregions

Portland/Vancouver Basin (3a)

The Portland/Vancouver Basin ecoregion (named for the cities of Portlandmarker and Vancouvermarker) is a geological depression at the base of the Portland Hillsmarker fault block. It contains the confluence of the Willamette and Columbia Rivers and is composed of deltaic sands and gravels deposited by Pleistocene floods, notably the Missoula Floods. Elevation varies from 0 to 300 feet (0 to 91 m), with buttes as high as 650 feet (198 m). Historically, the basin was characterized by Oregon white oak groves and Douglas-fir forests on the uplands; black cottonwood groves on riverbanks and islands; Oregon ash, red alder, and western redcedar in riparian areas; and prairie openings maintained by Native American burning, with camas, sedges, tufted hairgrass, fescue, and California oatgrass. Numerous wetlands, oxbow lakes, and ponds can still be found, but today the region is dominated by urban and suburban development, pastures, cropland, and tree farms. The climate is usually marine-influenced, but easterly winds entering via the Columbia River Gorgemarker periodically bring continental temperature extremes. The region covers in Washington and in Oregon, including the northern and eastern suburbs of the Portland metropolitan areamarker. It contains several National Wildlife Refuges within the Ridgefield National Wildlife Refuge Complexmarker.

Willamette River and Tributaries Gallery Forest (3b)

The Willamette River and Tributaries Gallery Forest ecoregion includes low-gradient, meandering river channels, oxbow lakes, and meander scars incised into the broad floodplains of the Willamette River and its tributaries. Elevation varies from 40 to 500 feet (12 to 152 m). The region includes the historic floodplains of the Willamette River system, which rarely function today due to flood control dams in the upper Willamette Basin that have reduced the frequency and volume of floods and contributed to the decline of the endemic, endangered Oregon chub. A small section, designated as the Willamette Floodplainmarker, has been protected within the William L.marker Finley National Wildlife Refugemarker. Historically, riparian gallery forests containing ash, black cottonwood, alder, and bigleaf maple grew on fertile, alluvial soils. Today, most of the forests have been replaced by agriculture and residential development. The region covers in Oregon in a narrow band rarely more than wide that extends along nearly the entire length of the Willamette River and the lower reaches of the McKenziemarker, Long Tom, Santiam, Yamhill, Molalla, Clackamas, and Tualatin rivers, including the Tualatin River National Wildlife Refugemarker.

Prairie Terraces (3c)

The Prairie Terraces ecoregion includes all terraces of the Willamette River upstream of the Portland/Vancouver Basin. It is dissected by low-gradient, meandering streams and rivers. Elevation varies from 160 to 500 feet (49 to 152 m). The region's broad fluvial terraces once supported oak savanna and prairies, featuring Oregon white oak, camas, sedges, tufted hairgrass, fescue, and California oatgrass. Wetter areas supported Oregon ash, Douglas-fir, bigleaf maple, black cottonwood, and an understory of poison-oak, hazel, and Indian plum, with some Ponderosa pine to the south. Today, only relict native prairie remains. The poorly drained soils derived from glacial lake deposits are extensively farmed for grass seed and small grains, as grasses tolerate poor drainage and poor rooting conditions better than other crops. Historically, seasonal wetlands and ponds were common, but many streams are now channelized, and the wetlands have been reclaimed for grain crops. In addition to agriculture, the Prairie Terraces have experienced the brunt of urban and suburban development along the Interstate 5 corridor. The region covers in Oregon along the length of the valley and includes the Baskett Sloughmarker and Ankenymarker national wildlife refuges.

Valley Foothills (3d)

The Valley Foothills ecoregion is a transitional zone between the agricultural Willamette Valley and the more heavilyforested Cascade and Coast ranges. It contains rolling foothills with medium gradient, sinuous streams, and a few buttes and low mountains, rising to an elevation of approximately . The region receives less rainfall than its more mountainous neighbors, and consequently the potential natural vegetation is distinct. The eastern foothills are wetter than those that lie on the western side of the valley in the lee of the Coast Range. Historically, the drier areas supported Oregon white oak and madrone woodlands and prairies, with California oatgrass, fescue, blue wildrye, brodiaea, and other prairie forbs; while the moister areas supported Douglas-fir forests, with sword fern, oceanspray, hazel, baldhip rose, poison oak, and alien Himalayan blackberry. Today, the valley foothills are characterized by rural residential development, pastures, timberland, vineyards, Christmas tree farms, and orchards. The largest of the Willamette Valley subregions, it covers in Oregon and in Washington.

See also


  1. (and the Reverse side) Many sentences in this article are copied verbatim from the source, which is in the public domain.
  2. (and the Reverse side) Many sentences in this article are copied verbatim from the source, which is in the public domain.

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