Willamette Valley-Puget Trough-Georgia Basin Ecoregion

Description & Physiography

The Willamette Valley-Puget Trough-Georgia Basin ecoregion is a long ribbon of broad valley lowlands and inland sea flanked by the rugged Cascade and coastal mountain ranges of British Columbia, Washington, and Oregon. It encompasses some 21,431 square miles of Pacific inlet, coastal lowlands, islands, and intermontane lowland, and extends from the Sunshine Coast and eastern lowland of Vancouver Island along Georgia Strait, south through Puget Sound and the extensive plains and river floodplains in the Willamette Valley.

Although the ecoregion's elevation averages 445 feet (maximum 4,203 feet), the effect of the adjacent mountains, ocean intrusions, and glaciation in the region's northern two-thirds have caused dramatic localized differences in climate, soils, and geology. From distinctive combinations of these factors spring an array of
ecological communities ranging from coniferous forests to open prairies, rocky balds, and oak


The Puget Trough’s climate is notorious for high rainfall and gray skies. It’s a temperate maritime climate, with mild, wet winters. Summers are warm and dry—if not particularly sunny. Mean January temperature is 39° F and mean July temperature is 65° F. Precipitation, mostly falling as rain, averages 40 inches per year. The local average can vary greatly in the ecoregion, due to the prevailing moisture-laden winds from the southwest.

Rain shadow areas, affected by the high Olympic Mountains, include the northeast corner of the Olympic Peninsula, Whidbey Island, and the San Juan Islands. Average annual precipitation is 15 to 30 inches. In Seattle or Tacoma, near sea level, average annual precipitation is 35 to 50 inches. Where clouds slam into the Cascade Mountains, in the foothills, average annual precipitation is 60 to 80 inches.

Plants & Animals

Puget Sound’s marine environment is rich and complex. The Sound features a wide variety of deepwater and nearshore habitats. These include coastal lagoons, kelp and seagrass beds, rocky shores, sandy beaches and spits, and salt marsh wetlands.

Steep underwater slopes in many parts of the Sound mean that the narrow strip of shallow water near the shore is all the more valuable. Sunlight allows eelgrass, seaweeds, and plankton to grow—providing food and shelter for myriad creatures. Some of these species are migratory and others reside year-round. Marine mammals include harbor seals, orcas, porpoises, and California sea lions. Marine invertebrates include sea urchins and both native and introduced species of shellfish. Some of the largest octopus and barnacle species in the world live here.

Historically, coniferous forest dominated the vegetation in the Puget Trough ecoregion. Many of the planet’s most impressive stands of trees grew here. Also present were a mix of riparian habitats, oak woodlands, and prairies. The vegetation in most of the ecoregion’s landscapes has now been altered. Cities, suburbs, and industrial lands are common. Managed forests and agricultural lands changed the vegetation, and themselves face pressure from sprawling development. The native forest here is primarily of Douglas fir, western red cedar, and western hemlock. Red alder and big leaf maple grow in riparian areas. Red alder also colonizes areas disturbed by fire or logging. 

Understory plants include sword fern and shrubs such as snowberry, Oregon grape, salmonberry, and many others. In places, the forests now struggle with invasive non-native plants, such as English ivy. Other key trees are Pacific madrone, a frequent feature on dry bluffs, and Oregon ash, common in riparian areas south toward the Columbia River.

Some animal species are adaptable to cities and suburbs, and their populations are holding steady—or even increasing to sometimes alarming numbers. Typical urban wildlife includes raccoons, crows, and coyotes, and introduced species such as opossum, European starlings, and rock pigeons. Other species in the ecoregion have declined significantly over the past 100 years. Their habitats have been altered and fragmented by development and use.

Notable population declines have occurred in the Taylor’s checkerspot butterfly, the Oregon spotted frog, the western pond turtle, the northern spotted owl, the marbled murrelet, and the western gray squirrel (not to be confused with its ubiquitous invasive cousin, the eastern gray squirrel).

History & Humans

The earliest archaeological evidence of people in the Puget Trough ecoregion dates to about 8,000 years before present, at the mouth of the Fraser River in British Columbia and along the lower Columbia River. The ancestors of the Salish or Salishan-speaking peoples flourished in the region and developed into eighteen or more linguistic traditions. These peoples created prosperous maritime cultures that employed the rich biodiversity of the region.

Key resources included:

  • Salmon and shellfish for food and tools
  • Cedar for housing, clothing, and canoes
  • Plants such as nettle, bracken fern, berries, rice root, and camas for food and fiber.

Europeans, and later Americans, also made use of the marine and forest resources of the Puget Trough ecoregion. Land use patterns were established early. 


Human development of the ecoregion has been rapid since the 1850s and continues today, with a 62% growth in population from 1950-2000 and a 16% growth in the past 10 years. Habitat conversion for human uses has been widespread, reflecting the accessibility, rich natural resources, and economic potential of virtually the entire ecoregion. Today, over 40% of the ecoregion has been converted to urban or tilled agricultural uses, and most of the remainder is in production forestry, making this the most highly developed of the Pacific Northwest ecoregions.

As of 1991, more than 50% of the Puget Trough had been converted to urban and agricultural uses. Rural areas in the Puget Trough are managed largely for intensive industrial and private forestry. Pasture and cropland are also dispersed throughout the ecoregion.

In 1999, the ecoregion’s population was nearly 3.9 million—double that of the 1960s. By 2020 the population in the Puget Trough is expected to grow to 5 million. This increasing population is putting pressure on the remaining natural areas and on working lands. Around Vancouver, Clark County’s population is rapidly growing. From 1990 to 1997 it increased 33%—tops for the state during that period.

Puget Sound itself suffers from pollution and other ills. The Endangered Species Act listing of Puget Sound’s wild Chinook salmon was the first to affect such a major urban area. The Puget Sound southern resident orca population has also been listed as endangered.

Although altered and under stress, both the terrestrial and marine environments of the Puget Trough ecoregion are still extremely productive.

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