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Williamette Valley

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NameColor on MapEPA Code‡
Portland/Vancouver Basin3a
Willamette River and Tributaries Gallery Forest3b
Prairie Terraces3c
Valley Foothills3d

† Status: ✓ = Complete ○ = Needs Image … = Incomplete ∅ = Stub Only

This code refers to the US EPA's Level 4 ecoregion codes for the continental U.S., see here.


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About the Williamette Valley

The Williamette Valley is a north-south valley located mostly in northwestern Oregon and extending barely into Washington state. The Williamette river is the central feature of this valley, which is surrounded by mountains to the west, east, and south. At the north of this region, it flows into the Columbia river, which for the most part represents the northern border of this region, although we define the region so that it includes a small, ecologically-similar area on the north bank of the Columbia river, around Vancouver Washington.

The climate here is transitional between a marine west coast climate and Mediterranean climate. Winters are cool and wet and summers warm and dry, with a relatively smooth transition through spring and autumn seasons with more average temperatures and rainfall. Overall, temperatures are moderate relative to other climates of this high a latitude, but relative to the Puget Lowlands to the north, and especially to the broader Marine West Coast Forest region, this area is drier and has slightly more variable weather, including greater potential for heat waves in summer. Severe weather, such as tornadoes and thunderstorms, is rare, and snowfall tends to be ephemeral.

Most of this region consists of broad, flat bottomlands, terraces, and rolling hills. Elevations range from 20ft (6m) along the Columbia river to about 1970ft (600m) around the edges of this region. The region is mostly underlain by volcanic basalt and marine sandstone, but in most places the bedrock is deeply covered in alluvial deposits, colluvium, and deposits that formed under glacial lake Allison around 15,000 years ago. This lake formed when glacial melt was temporarily blocked by a narrow strait near where Kalama, Washington is now located. As the water drained, it left fine sediments in this area. Soils here tend to be diverse in texture, but tend to be nutrient-rich and highly fertile, unlike most of the soils in the broader surroundings.

Several large rivers and numerous streams flow through this area, most of which originate outside the region, in mountains where rainfall is higher. Some of the streams originate in the hillier areas around the edges of this region. There are numerous ponds and seasonal wetlands.

Before European colonization, this area was covered by a mix of vegetation types. Like the areas to the north, there was significant coniferous forest, wetlands, and deciduous forests in riparian areas, but there were also more open habitats including prairies and oak savannahs. Dominant trees include oregon white oak (Quercus garryana), Douglas-fir (Pseudotsuga menziesii), pacific madrone (Arbutus menziesii), and ponderosa pine (Pinus ponderosa). Bottomlands featured black cottonwood (Populus tristis), Oregon ash (Fraxinus latifolia), bigleaf maple (Acer macrophyllum), Douglas-fir, western redcedar (Thuja plicata), and a variety of shrubs.

This area is heavily utilized for cropland, industry, and urban development. Almost no prairies remain. The bulk of Oregon's population lives here, mostly in the metro area of Portland, with some in the Salem metro area as well, and the population, and residential land use, has been increasing significantly in recent years. This is one of the primary agricultural areas in Oregon, producing vegetables, fruits, nuts, nursery products, and grass seed. The foothills support vineyard, Christmas tree farms, and there is some sheep and cattle grazing. Humans have altered the hydrology somewhat, creating a few large reservoirs maintained by dams.

This region is bordered to the west by the Coast Range, a higher-elevation, more rugged region with greater rainfall and a more consistently-humid climate. It is bordered to the east and north by the Cascades, also a more rugged region, which has higher rainfall than this region but less so than the Coast Range, and has a more variable climate as well. To the south this region is bordered by the northernmost end of the Klamath Mountains/California High North Coast Range, yet another mountain range that, near the border with this region, tends to be slightly lower and less rugged than the ranges to the east and west, although farther south they become much more rugged.

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1. Wiken, E., Griffith, G. "North American Terrestrial Ecoregions - Level III", Commission for Environmental Cooperation, (2011) Web.