The Puget Lowland ecoregion

Ecoregions provide a useful framework and background for the discussion of marine, freshwater, and terrestrial environs of the county. The discussion of ecoregions is based on U.S. Environmental Protection Agency (EPA) ecoregion conventions, which result in units similar to European biogeographical regions because they denote areas of general similarity in ecosystems.

Aerial view of the Vashon Island shoreline.
Aerial view of the Vashon Island shoreline. Photo copyright King County.

EPA’s ecoregions have been mapped across the North American continent and are classified hierarchically into four levels of increasing complexity and detail. There are only 15 Level I ecoregions across the entire continent, and King County falls within 2 of these: Marine West Coast Forests and Northwestern Forested Mountains. There are 52 Level II ecoregions, and the two that occur across King County correspond identically with the Level I ecoregions: Marine West Coast Forest and Western Cordillera. Level III regions describe smaller ecological areas nested within level II regions. There are three Level III ecoregions in King County: Puget Lowlands, Cascades, and North Cascades. Level IV ecoregions describe an even finer scale of nested ecological areas and allow locally defining characteristics to be identified and more specific management strategies to be formulated for such local conditions. In King County, there are nine Level IV Ecoregions (see Table 2 and Landscape Diversity Map).

Table 2. Level III and Level IV Ecoregions that lie within King County’s geographic boundaries..

Level III

Level IV

Puget Lowland


Eastern Puget Riverine Lowlands

Eastern Puget Uplands

Central Puget Lowland

North Cascades

North Cascades Lowland Forests

North Cascades Highland Forests

North Cascades Subalpine/Alpine



Western Cascades Lowlands and Valleys

Western Cascades Montane Highlands

Cascade Subalpine/Alpine


The Puget Lowland Ecoregion

The Puget Lowland Ecoregion of King County, including its component Central Puget Lowland ecoregion, Eastern Puget Riverine Lowland ecoregion, and Eastern Puget Upland ecoregion, have undergone perhaps the greatest change since settlement, and this landscape is where biodiversity has declined the most. These ecoregions were the first to be logged, the first to be turned to agriculture, and have borne the brunt of encroaching settlement and urbanization. In the lowlands of King County, from the shores of Puget Sound to the uplands and foothills of the Cascades, the once-continuous forests of Western hemlock, Western Redcedar, and Douglas-fir have largely been replaced with forest plantations, farms and fields, cities, towns, and their suburbs. The remains of this ancient and great forest, where trees grew to 70 meters (230 feet) tall, 2 meters (6.6 feet) in diameter, and over 800 years old, are now found only in small, scattered reserves or in remnant groves, mostly in the foothills, and mostly in state or federal ownership. The scale of this loss is illustrated by the presence of two small “pioneer” groves within the City of Seattle, the only remaining lowland examples of this formerly dominant landscape in King County.

Central Puget Lowland

Based on 2002 landcover data, approximately 710 square kilometres (274 square miles) of urban area lie in the county, and nearly all of it is in the Puget Lowlands. This entire Central Puget (CP) Lowland ecoregion, approximately 933 square kilometers (333 square miles), is now dominated by urban and suburban uses, criss-crossed by roads, and fragmented into patches of residential, commercial, and industrial uses. Little area has been spared the pressure of development. Few areas remain undeveloped along the mainland shoreline of King County; Elliott Bay, for example, is the main harbour and industrial area for Seattle. Virtually all the remaining undeveloped marine shoreline in King County occurs on Vashon Island, approximately 4.5 kilometers (2.5 miles) offshore from the mainland. Yet even this area has no completely undisturbed shoreline remaining. Lake Washington and Lake Sammamish, King County’s two largest lakes, lie within the CP lowlands; each has an extensively developed shoreline, and little native habitat remains intact amid retaining walls, bulkheads, docks, and lawns to the water’s edge. Even the lake levels have been adjusted downward as a result of canal construction and river diversions in 1906 and more recent adjudication of water levels to prevent residential flooding.

Protection of biodiversity in King County occurs mainly at the habitat level through the use of restrictive regulations and covenants within recent developments. Wetland and stream protection regulations specify buffers and setbacks for these “critical areas” from development activities and finished infrastructure; nesting trees and breeding sites of certain species of animals are also protected through regulation. Even as these buffers have increased in width and breeding areas are protected, the greater landscape continues to fragment. The County has attempted to conserve animal populations against this fragmentation by connecting existing habitats using a codified Wildlife Habitat Network and by recognizing habitat “complexes” (currently only for wetland systems). However, until very recently, the use of landscape planning tools such as “SmartGrowth” (the densifying of existing population centers in an attempt to reduce urban sprawl), the transfer of development rights from outlying lands to urban areas, and the outright purchase of land or development rights in forest areas has not been directed expressly at preserving biodiversity. As a result, large-scale, landscape ecosystem biodiversity, as represented in the ecoregion discussion, has declined more dramatically than either habitat or species biodiversity, and many habitats, especially wetland, stream, and forest habitats, have been compromised by the effects of land use in the surrounding landscape. Few, if any, complete native landscapes remain in this lowland ecoregion.

Eastern Puget Riverine Lowland

The major river valleys of King County within the Eastern Puget Riverine Lowland ecoregion have undergone similar changes to the Central Puget Lowland ecoregion. This ecoregion comprises only about 291 square kilometres (112 square miles) in King County but contains a wide variety of aquatic and terrestrial habitats and ecotones, and a rich complex of animal and plant communities. The rivers and streams of this ecoregion were, and still remain, the major spawning and rearing areas for the seven native species of Pacific Salmon and trout, and two species of char that occur in King County. Of this group, three species (Chinook salmon, steelhead and bull trout, a char) have been listed recently under the U.S. Endangered Species Act and are the subject of region-wide recovery efforts. Although these three species have been the first listed, virtually all the salmon, trout, and char species (with the possible exception of Pink salmon) of King County have declined in abundance and distribution over the last 100 years and the potential exists for further listings. More factors are at work in the salmon declines than landscape change and habitat degradation, however. Over-harvest and the overuse of hatchery programs have eroded genetic and life history diversity and migration barriers have reduced the distribution of salmon. The recovery programs for Chinook salmon address all these factors and progress is steady in changing past management regimes.

Among the first area to be logged and turned to agriculture, this ecoregion would also be unrecognizable to the first explorers or to the 18th Century Native American inhabitants of King County. These valleys were once heavily forested by flood tolerant species such as western redcedar, black cottonwood and Sitka spruce; the largest trees in the county occurred in these fertile lowland valleys.  By about 1890, the valleys of the Green River and the lower Snoqualmie River had been extensively logged and converted to agriculture, which remains the dominant land use for the Snoqualmie. The lower Green has seen considerable urbanization with residential and commercial land uses replacing the agricultural areas over the past 25 years. With human land use eventually came a desire for protection from the regular floods that swept these rivers. A major flood control dam was completed on the Green River in 1963, and the lower river has an extensive levee system. In the Snoqualmie, agricultural use required only the construction of revetments and a few levees but no major dam. Nevertheless, the floodplains in both systems have been grossly modified for crops and pastures, and the sloughs and oxbows, side channels and backwaters, along with the extensive riparian forests were mostly lost by 1950.

Eastern Puget Uplands

The Eastern Puget Uplands, the area of moraine and rolling foothills up to about 823 meters (2,700 feet) above sea level (ASL), encompasses approximately 1,307 square kilometers (467 square miles) and is considered an ecological transition zone from the Puget lowlands to the forests on the western slope of the Cascade Mountains. This ecoregion should also be considered a transition zone for land use as well because the intensity of settlement declines from west to east across the ecoregion. From its western edge, newly created cities and their suburbs gradually give way to farming areas, woodlots and forests, a few small towns and, finally, to the current forest production zone on the highlands and in the rising foothills. This area remains prominent in the production of forest products and includes extensive private forest lands, two state forests, and the western edge of federal forest lands. Working in concert with local conservation groups, it is in this area that much of the County’s land acquisition and protection is directed. This attention is both timely and warranted as expanding local cities begin to include farmland within their growth boundaries and forest companies dispose of extensive forest holdings along the foothills. This area is probably experiencing the pressure of an expanding King County population more than any other ecoregion.

In the eastern reaches of this ecoregion many habitats and species can be found that were once common and even abundant throughout the lowlands to the west. Large wetland systems, nestled in morainal troughs or in the depressions of ancient kettle lakes, dot the landscape and small lakes and ponds can be found throughout. Plant and animal species have either retreated to these areas as the landscape has been developed or are survivors of the past alterations of the landscape.  Herds of elk, once common in the lower river valleys and across the highlands have been pushed into the foothills and forests of the west slope; bobcats are encountered less and less in the interspersed forests and woodlands that remain. Black bear and cougar, now sharing their foothill habitats with more people, are seen in backyards and along trails (see Mammals discussion below). 

Like the Puget Lowland, the Eastern Puget Uplands have been altered considerably from their past condition. Although little irreversible change has occurred here, the landscape bears little resemblance to that first encountered by settlers. Mostly unbroken forest covered the land; these forests are similar to the lowlands, with the addition of a few tree species that were at the lower limit of their elevation range and a rare few that were holdovers from the glacial past. Douglas-fir, western redcedar, and some white pine were present in the lowlands, and these species transitioned to a mix with some silver fir and noble fir upslope. Truly a transition zone, this area was probably rich in species and in complex forest habitats that differed because of slope exposure, soils, and fire history. A few remnants of the original forest are scattered throughout the eastern edge of this ecoregion. However, the forests that are present today are much less diverse in species, age, and size, which are characteristics of the vertical and horizontal complexity of mature stands.

About the Author: 
Robert Fuerstenberg, Jennifer Vanderhoof, Jonathan Frodge, Kathryn Gellenbeck, Kollin Higgins, Klaus Richter, and Kim Stark collaborated to write the King County Biodiversity Report, which was published by the King County Department of Natural Resources and Parks as part of the Local Action for Biodiversity (LAB) project, a global biodiversity initiative. The report serves as the foundation of a plan for long-term protection of biodiversity in King County. Citation: King County. 2007. King County biodiversity report. King County Water and Land Resources Division, Department of Natural Resources and Parks. Prepared for International Council for Local Environmental Initiatives (ICLEI), Biodiversity Initiative. 102 pp.