Willamette Valley (ecoregion)

Temperate broadleaf and mixed forests ecoregion of the United States

This article is about the ecoregion designated by the EPA. For the river valley it contains, see Willamette Valley.

Willamette Valley

Wetlands near Eugene, Oregon

Willamette Valley ecoregion (3)

Ecology
Realm| Nearctic
Biome| Temperate broadleaf and mixed forests
Borders|

* Cascades
* Coast Range
* Klamath Mountains
* Puget lowland forests

Bird species| 177[1]
Mammal species| 77[1]
Geography
Area| 14,900 km2 (5,800 sq mi)
Country| United States
States|

* Washington
* Oregon

Coordinates| 44°48′N 123°12′W﻿ / ﻿44.8°N 123.2°W﻿ / 44.8; -123.2
Conservation
Habitat loss| 65.679%[1]
Protected| 4.07%[1]

The Willamette Valley ecoregion is a Level III ecoregion designated by the United States Environmental Protection Agency in the U.S. states of Oregon and Washington. Slightly larger than the Willamette Valley for which it is named, the ecoregion contains fluvial 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.[2][3]

## Setting

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The ecoregion covers an area of 14,900 square kilometers (5,800 sq mi),[4] lying mostly in Oregon, with a small portion lying across the Columbia River in southern Washington. The ecoregion lies in the Willamette Valley, which runs from south to north between the Oregon Coast Range to the west and the Cascades Range to the east. The ecoregion is drained mostly by the Willamette River and its tributaries, which flows into the Columbia River straddled by Portland, Oregon.

## Landscape and threats

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Grove of Oregon white oaks near Hillsboro Fall colors in Wilsonville

Historically, the Willamette Valley forests were mostly an oak savanna, tall grasslands with scattered Oregon white oaks (Quercus garryana), and groves of coast Douglas-fir (Pseudotsuga menziesii subsp. menziesii). The river floodplains contained extensive wetlands, stands of willow, alder, and cottonwood, and gallery forests.

This landscape was maintained by the Native American inhabitants of the valley who set frequent fires which encouraged the open grasslands and killed young trees. The American settlers of the region, since the 19th century, suppressed fires and converted much of the valley to agriculture, which has caused much of the former grassland and savanna to revert to closed-canopy forest.

Less than one-tenth of one percent of the original savanna vegetation remains. The remaining enclaves include a section of Oregon white oak savanna preserved at Mount Pisgah Arboretum in Eugene.

## Level IV ecoregions

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Level IV ecoregions in the Willamette Valley. (Full map).

The Willamette Valley ecoregion is made up of four Level IV ecoregions:

### Portland/Vancouver Basin (3a)

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The Portland/Vancouver Basin ecoregion (named for the cities of Portland and Vancouver) is a geological depression at the base of the Portland Hills 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 90 m), with buttes as high as 650 feet (200 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 from the Columbia River Gorge periodically bring continental temperature extremes. The region covers 305 square miles (790 km2) in Washington and 269 square miles (697 km2) in Oregon, including the northern and eastern suburbs of the Portland metropolitan area. It contains several National Wildlife Refuges within the Ridgefield National Wildlife Refuge Complex.[2][3]

### Willamette River and Tributaries Gallery Forest (3b)

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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 150 m). The region includes the historic floodplains of the Willamette River system, which rarely function today due to flood control dams in the upper 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 Floodplain, has been protected within the William L. Finley National Wildlife Refuge. 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 675 square miles (1,748 km2) in Oregon in a narrow band rarely more than 5 miles (8 km) wide that extends along nearly the entire length of the Willamette River and the lower reaches of the McKenzie, Long Tom, Santiam, Yamhill, Molalla, Clackamas, and Tualatin rivers, including the Tualatin River National Wildlife Refuge.[2][3]

### Prairie Terraces (3c)

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Ankeny National Wildlife Refuge in the Prairie Terraces ecoregion

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 (50 to 150 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 1,971 square miles (5,105 km2) in Oregon along the length of the valley and includes the Baskett Slough and Ankeny national wildlife refuges.[2][3]

### Valley Foothills (3d)

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The Valley Foothills ecoregion is a transitional zone between the agricultural Willamette Valley and the more heavily forested 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 1,500 feet (457 m). 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 wild 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 2,415 square miles (6,255 km2) in Oregon and 112 square miles (290 km2) in Washington.[2][3]

## See also

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* 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
* The conservation group World Wildlife Fund maintains an alternate classification system:
* List of ecoregions (WWF)
* List of ecoregions in the United States (WWF)

## References

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1. ^ a b c d Hoekstra, J. M.; Molnar, J. L.; Jennings, M.; Revenga, C.; Spalding, M. D.; Boucher, T. M.; Robertson, J. C.; Heibel, T. J.; Ellison, K. (2010). Molnar, J. L. (ed.). The Atlas of Global Conservation: Changes, Challenges, and Opportunities to Make a Difference. University of California Press. ISBN 978-0-520-26256-0.
2. ^ a b c d e  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).
3. ^ a b c d e  This article incorporates public domain material from Pater, D; Bryce, SA; Kagan, J; et al. Ecoregions of Western Washington and Oregon (PDF). United States Geological Survey. (color poster with map, descriptive text, summary tables, and photographs; with a Reverse side).
4. ^ "Willamette Valley forests". Terrestrial Ecoregions. World Wildlife Fund.

## External links

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* Willamette Valley forests (Vanderbilt University)

* 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]
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