Willamette Valley

VIEW DATA LAYERS IN COMPASS
Finley Muddy Creek Area
Photo Credit: George Gentry, USFWS

Description

Bounded on the west by the Coast Range and on the east by the Cascade Range, this ecoregion encompasses 5,308 square miles and includes the Willamette Valley and adjacent foothills. Twenty to 40 miles wide and 120 miles long, the Willamette Valley is an elongated, level alluvial plain with scattered groups of low basalt hills. Elevations on the valley floor are about 400 feet at the southern end near Eugene, dropping gently to near sea-level in Portland. The climate is characterized by mild, wet winters and warm, dry summers. Fertile soil and abundant rainfall make the valley the most important agricultural region in the state.

Societally, the Willamette Valley is a land of contrasts. Bustling urban areas are nestled within rural farmlands. A wide diversity of industries contribute to the vibrant economy. It includes more than half of the state’s agricultural-producing counties, and the vast majority of the largest private sector employers (e.g., manufacturing, technology, forest products, agriculture, and services). Interstate 5 runs the length of the ecoregion, contributing to the transportation system and flow of goods while connecting communities. The Willamette Valley is the most urban ecoregion in the state, containing 8 of the 10 largest cities in Oregon. It is also a fast-growing ecoregion. Pressure on valley ecosystems from population growth, land use conversion, recreation, and pollution is likely to increase as population centers expand.

Characteristics

Important Industries

Agriculture, manufacturing, technology, forest products, construction, retail, services, government, academia, health care, tourism

Major Crops

Nursery and greenhouse plants, grass seed, wine grapes, Christmas trees, poultry, dairy, vegetables, small fruits and berries, nuts, grains, hops, cannabis

Important Nature-based Recreational Areas

Smith and Bybee Wetlands Natural Area, Henry Hagg Lake, Fall Creek Lake, Milo McIver State Park, Willamette River Greenway and Water Trail, Willamette Valley National Wildlife Refuge Complex, Fern Ridge Reservoir, McDonald and Dunn Research Forests, Rivers to Ridges parks network, Howard Buford Recreation Area, Tualatin and William L. Finley National Wildlife Refuges, Sauvie Island Wildlife Area, E. E. Wilson Wildlife Area

Elevation

4 feet (Columbia River) to over 2,000 feet in the south

Important Rivers

Willamette, McKenzie, Santiam, Sandy, Molalla, Clackamas, Tualatin, Yamhill, Calapooia, Marys, Muddy, Pudding, Luckiamute, Long Tom

Conservation Issues and Priorities

The Willamette Valley is a steadily growing and densely populated ecoregion, containing the state’s three largest metropolitan areas, Portland, Eugene, and Salem. Since the 1850s, much of the ecoregion has been altered by development (agricultural and urban), particularly affecting oak woodland, oak savanna, grassland, riverine, and wetland habitats. The Willamette River has been disconnected from its floodplain, and much of the historical habitat has been fragmented. Most of the Willamette Valley ecoregion is privately-owned, presenting challenges to conservation efforts. Conservation strategies that focus on the needs of individual at-risk species and key sites are particularly critical in this ecoregion.

Historical accounts indicate that prior to European settlement, much of the Willamette Valley was covered by native grasses, forbs, and oak savanna. The Kalapuya people regularly set fires to improve hunting and travel. The fires helped to maintain the Valley’s mosaic of grasslands, oak savannas, wet prairies, and other open habitats. The cessation of fire management by indigenous peoples and subsequent fire suppression efforts have led to denser vegetation and encroachment of trees like Douglas-fir, impacting the health of oak woodlands and reducing the size of grasslands and prairies.

Key Conservation Issues of particular concern in the Willamette Valley ecoregion include Land Use Changes, Disruption of Disturbance Regimes (both fire and floodplain function), and Invasive Species. In addition to the statewide factors, specific hazards to wildlife in urban areas and habitat fragmentation are of conservation concern here.

Climate

The Willamette Valley is mostly a low elevation valley with terraces and floodplains encompassed by rolling hills. The climate is Mediterranean, with mild, wet winters and warm, dry summers. Mean annual precipitation varies by elevation, with more occurring in the mountainous foothills. Many tributaries join the Willamette River, entering this ecoregion from the Cascades to the east and Coast Range to the west.

Warming is projected in all seasons across the region, with the largest increases projected for summer. Summer average temperature is projected to increase as much as 6°F for the 2040-2069 period, and as much as 10°F for the 2070-2099 period for a high emissions scenario (SSP585; Figure 1). Warming temperatures will lead to wetter winters and drier summers. Late summer precipitation is projected to decrease as much as 20% by the end of the century (Figure 2). Models generally agree on late summer precipitation projections, so confidence in these projections is high. Projections of cool season precipitation show an increase of 6% for the same period under a high emissions scenario (SSP585), although there is less confidence in predicted changes in cool season precipitation. Under a high emissions scenario (SSP585), growing degree days are projected to double for the 2070-2099 period, while frost days are projected to decrease from 50 per year on average to fewer than 20 per year on average.



Photo Credit: ODFW.
Figure 1. Model median historical (1950-2010) and future projections of summer maximum temperature change for three future periods (2010-2039, 2040-2069, 2070-2099) under a high emission scenario (SSP585) for the Willamette Valley. Future projections show increasing temperatures over the century, with slightly less warming in the southern Willamette Valley (Raymond and Fusco, 2024).


Photo Credit: ODFW.
Figure 2. Model median historical (1950-2010) total late summer precipitation and projected percent change for three future periods (2010-2039, 2040-2069, 2070-2099) under a high emission scenario (SSP585) for the Willamette Valley. Future projections show decreasing late summer precipitation across the Willamette Valley ecoregion, with the greatest decrease in the 2040-2069 period under a high (SSP585) scenario (Raymond and Fusco, 2024).

Limiting Factors and Recommended Approaches

Limiting Factor:

Land Use Conversion and Urbanization
CMP Direct Threats 1, 2.1, 2.3, 7.2

Of all the ecoregions in Oregon, the Willamette Valley has been most impacted by development and conversion of native habitats to other uses. The fragmented nature of remaining habitat patches in the Willamette Valley creates barriers to movement and dispersal for many species, particularly amphibians, reptiles, invertebrates, and small mammals. Continued fragmentation of remnant habitats, including grasslands and oak woodlands, has resulted in small and disjunct populations that are at an increased risk of genetic isolation and extirpation.
In addition to rural and urban residential and commercial development, conversion of habitat for vineyards is also increasing. Application of fungicides and insecticides applied to protect wine grapes and other crops, including grasses and hazelnuts, may impact the health, survival, and reproduction of pollinators, birds, and fish. The extensive development of the Willamette Valley ecoregion also means the potential for conflict between people and wildlife is high. For example, agricultural production may cause conflicts with wildlife that are attracted to feed on plants, nuts, or ripened fruit. Lack of suitable habitat may cause bats and birds to nest or roost in houses and other human structures. High road densities and road traffic volumes increase the potential for collisions with wildlife, with hundreds of thousands of animals lost each year to roadkill.

Recommended Approach

Landscape Scale: Because so much of the Willamette Valley ecoregion is privately-owned, voluntary cooperative approaches are the key to long-term conservation using tools such as financial incentives, Candidate Conservation Agreements with Assurances, and conservation easements. Careful land use planning is also essential. Work with local governments to support and implement existing land use regulations to preserve farmland, open spaces, recreation areas, and natural habitats. Monitor changes in land uses across the landscape and in land use plans and policies.
Within Urban Areas: Parks and natural areas, wildlife corridors, and green infrastructure can contribute to conservation, connect people to the natural environment, and enhance the quality of life in communities. Mitigate the impacts of development to the greatest extent possible. Many Priority Wildlife Connectivity Areas exist throughout the Willamette Valley that can help maintain important movement corridors for wildlife. Promote programs such as the Backyard Habitat Certification Program to increase wildlife habitat and connectivity within urban spaces. Support and promote innovative campaigns and programs to reduce wildlife hazards. Work with municipalities to develop policies, such as wildlife-friendly building guidelines, wildlife-friendly lighting strategies, and integration of fish and wildlife crossings into transportation plans to reduce hazards. Engage in community outreach and education efforts to promote habitat protection and coexistence with wildlife.

Limiting Factor:

Altered Fire Regimes
CMP Direct Threats 7.1, 11.3, 11.4

Past forest practices and fire suppression have resulted in young, dense, conifer-dominated forests where open-structured habitats like grasslands, oak savannas, and wet prairies once dominated. Maintenance of these Key Habitats is dependent, in part, on periodic burning. Forest crowding is a particularly significant issue in oak habitat, as large legacy oaks have become surrounded by dense stands of younger trees. The dense stands that have replaced open-structured habitats are at increased risk of high-intensity fires, disease, and damage by insects. Wildfire risk is further exacerbated by warming climate conditions and changes to patterns of precipitation, and more frequent, moderate to severe wildfires are becoming more common, particularly along the urban/rural interface surrounding many Willamette Valley communities. Reintroduction of fire poses significant management problems in many areas of the ecoregion. These problems include conflicts with surrounding land use, smoke management, air quality, and safety.

Recommended Approach

Use multiple tools, including mowing and controlled grazing, to maintain open-structured habitats. Ensure that tools are site-appropriate and implemented to minimize impacts to native species. Reintroduce fire at locations where conflicts, such as smoke and safety concerns, can be minimized. Work with communities to ensure that air quality and other local concerns are addressed. Support the formation of Prescribed Burn Associations and other community partnerships to promote and implement controlled burns.

Limiting Factor:

Water
CMP Direct Threats 7.2, 11.4

Water quantity is a limiting factor for fish and wildlife. Changing climate conditions are leading to rising temperatures and altered patterns of precipitation, which affects water availability across different times of year. In streams, seasonal low flows can limit habitat suitability and reproductive success for many fish and wildlife species. In much of the Willamette Valley, the demand for water for supporting population growth and agricultural production is putting increasing pressure on the supply of groundwater. This decreases groundwater discharge of cold water to rivers and streams, subsequently reducing the availability of both cold water refugia and suitable habitat for cold-water dependent species.

Water quality can also limit species and habitats. Runoff from agricultural areas can contaminate waterways. Warming temperatures, combined with higher nutrient levels due to agricultural runoff, is increasing the prevalence of toxic cyanobacterial blooms, sometimes called “harmful algal blooms” or “HABs”, leading to fish and wildlife mortality and safety issues for people and pets. Stormwater runoff from urban areas and from historic industrial activities, such as the Portland Harbor Superfund site, has introduced a variety of pollutants into wetland and aquatic habitats.

Recommended Approach

Provide incentives and information about water usage and sharing during low flow conditions (e.g., late summer). Promote water management actions that enable climate resilience and adaptation. Invest in watershed-scale projects for cold water and flow protection. Identify and protect cold water rearing and refugia habitat for aquatic species. Increase awareness and manage timing of applications of potential aquatic contaminants. Improve compliance with water quality standards and pesticide use labels administered by the DEQ and EPA. Work on implementing Senate Bill 1010 (Oregon Department of Agriculture) and DEQ Total Maximum Daily Load water quality plans.

Limiting Factor:

Habitat Fragmentation
CMP Direct Threats 1, 2.1, 2.3, 3.3, 8.1

Habitats for at-risk native plant and animal species are largely confined to small and often isolated fragments, such as roadsides and sloughs. Habitat fragmentation also limits species’ ability to move across the landscape to fulfill life history needs. Opportunities for large-scale protection or restoration of native landscapes are limited. Barriers to large-scale ecosystem restoration include existing development, growth pressures, high land costs, and fragmented land ownerships. Remnant habitat patches in the otherwise highly developed Willamette Valley ecoregion are critical to protect, maintain, and restore. Even small patches of intact habitat can serve as important steppingstones for wildlife movement and migration.

Roadways are also a significant contributor to habitat fragmentation in this ecoregion. Interstate 5 bisects the ecoregion, running north to south. The number of lanes, traffic speeds, and volume of freight and motorist traffic make the interstate a near complete barrier to species movement, preventing species dispersal, range expansion, or migration. The Willamette Valley ecoregion has the highest road density of any ecoregion in the state, significantly impeding wildlife connectivity in urban areas.

Recommended Approach

Broad-scale conservation strategies will need to focus on restoring and maintaining more natural ecosystem processes and functions within a landscape that is managed primarily for other values. This may include an emphasis on more “conservation-friendly” management techniques for existing land uses and restoration of some key ecosystem components, such as river-floodplain connections and wetland and riparian habitats. “Fine-filter” conservation strategies that focus on needs of individual Species of Greatest Conservation Need and key sites are particularly critical in this ecoregion. Manage for the full lifecycle of the species to maintain access to breeding, migration, and nonbreeding habitats. Plan for and promote habitat connectivity. Many Priority Wildlife Connectivity Areas exist throughout the Willamette Valley that can help maintain important movement corridors for wildlife. Integration of fish and wildlife crossings into transportation plans will help to reduce hazards.

Limiting Factor:

Invasive Species
CMP Direct Threat 8.1, 8.2

Invasive species disrupt native plant and animal communities and impact populations of at-risk native species. Hundreds of non-native and invasive plants have been introduced in this ecoregion for use as livestock forage, erosion control, and ornamental purposes. Plants such as reed canary grass and water primrose form dense monocultures, displacing native plants in wetland and riparian areas, choking waterways, and reducing foraging and breeding areas for wildlife. Himalayan blackberry is also widespread in this ecoregion, with significant local impacts to meadows, riparian areas, and grasslands. European hawthorn, black locust, and English ivy, among many others, are also highly invasive, outcompeting and engulfing native trees and shrubs.
Invasive animals have also caused significant issues in the Willamette Valley. American bullfrogs are rapidly expanding, competing with native species for limited resources or preying on native species and/or their eggs or young. Nutria degrade water quality and destabilize stream banks, while competing with native species, such as American beaver and muskrat, for food. Eastern fox squirrel and Eastern gray squirrels prey on native birds and compete with native species, such as Western gray squirrels, for food and nest sites. Common snapping turtles prey on native fish and wildlife species, and red-eared sliders compete directly with Northwestern pond turtles and Western painted turtles. Several mussel species, including zebra, quagga, and golden mussels, pose significant threats to aquatic systems. A variety of non-native fish have been introduced to waterways in the ecoregion, such as Gambusia, bass, and sunfish, which compete with native fish for food and prey directly on native fish, amphibians, and/or their eggs or young.
Emerging threats from invasive invertebrates are also becoming a concern in the Willamette Valley. Japanese beetles can infest and defoliate native hardwood trees like oaks and maples. The non-native emerald ash borer defoliates tree species characteristic of riparian habitats, such as Oregon ash, putting riparian areas, and in-stream habitats that depend on shading from bankside trees, at risk. Mediterranean oak borer targets native oak species, including Oregon white oak and California black oak, which have already experienced significant declines in the ecoregion due to habitat loss and development.

Recommended Approach

Emphasize prevention, risk assessment, early detection, and quick control to prevent new invasive species from becoming fully established. Use multiple site-appropriate tools (e.g., mechanical, chemical, biological) to control the most damaging non-native species. Prioritize efforts that focus on key invasive species in high priority areas, particularly where Key Habitats and Species of Greatest Conservation Need occur. Work with the Oregon Invasive Species Council and other partners to educate people about invasive species issues and to prevent introductions of potentially high-impact species, such as the zebra mussel. Provide technical and financial assistance to landowners interested in controlling invasive species on their properties. Promote the use of native species for restoration and revegetation.

Limiting Factor:

Altered Floodplain
CMP Direct Threat 7.2

The floodplain dynamics of Willamette Valley rivers have been significantly altered. Historically, multiple braided channels dispersed floodwaters, deposited fertile soil, moderated water flow and temperatures, and provided a variety of slow-water habitats, such as sloughs and oxbow lakes. Since Euro-American settlement, however, many of the valley’s major rivers, including the Willamette, Santiam, and McKenzie Rivers, have largely been confined to a single channel and disconnected from their floodplains. Addition of dikes and levees, bank hardening, and channelization to facilitate agricultural irrigation have all had significant impacts to riparian habitats in the ecoregion. Hydroelectric projects located on the major rivers and their tributaries, while reducing downstream flood risk, have altered hydrology significantly, restricting fish passage and acting as barriers to movement of aquatic wildlife.

Recommended Approach

Cooperative efforts are needed to restore floodplain function and critical off-channel habitats. Using green infrastructure and careful planning for development outside of floodplains can help maintain floodplain function. Removing infrastructure such as revetments that channelize rivers can help restore natural stream flows.

Limiting Factor:

Wildlife Hazards
CMP Direct Threats 1.1, 1.2

Urban landscapes can present a variety of hazards for wildlife. High densities of buildings increase risk of bird collisions with windows. Collisions with vehicles and powerlines and entanglements in fencing are also significant hazards for wildlife in urban areas. Outdoor cats, even well-fed, indoor/outdoor cats, prey on a variety of wildlife species, with significant local impacts to populations of birds, small mammals, and reptiles. Similarly, off-leash dogs in natural areas can disturb, injure, or kill wildlife. Noise and light pollution, as well as other disturbances from human presence and activity, can alter wildlife behavior. Food provided to wildlife, either intentionally with devices like bird feeders or unintentionally by leaving garbage unsecured or by leaving pet food outdoors, can increase disease transfer and can cause human-wildlife conflicts. Illegal encampments can degrade or destroy sensitive fish and wildlife habitats, particularly riparian areas, due to loss of vegetation, felling of snags for use as firewood, and unmanaged human refuse and waste. Additional hazards include exposure to pesticides, rodenticides, and other contaminants, harassment, and poaching. These hazards can significantly impact wildlife and undermine habitat conservation efforts.

Recommended Approach

Support and promote innovative campaigns and programs to reduce wildlife hazards. Work with municipalities to develop policies, such as wildlife-friendly building guidelines, wildlife-friendly lighting strategies, feeding ordinances, and integration of wildlife crossings into transportation plans to reduce hazards. Support research into better understanding of urban wildlife hazards and the management strategies to reduce those hazards. Communities can work with local park districts and conservation organizations to support habitat enhancement projects and reduce the need for herbicide use in parks. Communities, local governments, and non-profit organizations can promote bird-friendly building design and outreach efforts about the impacts of cats on wildlife. Support policies that address housing access to alleviate the pressure for illegal camping on public lands.

References

Raymond, C. L. and E. J. Fusco (Eds.). 2024. The State of Climate Adaptation Science for Ecosystems in the Northwest U.S. Prepared by the Climate Impacts Group for the Northwest Climate Adaptation Science Center. https://depts.washington.edu/nwclimateadaptation/socs/

Strategy Species

Conservation Opportunity Areas