The Cascades 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.

King County map, showing incorporated land and major water bodies. Copyright King County.
King County map, showing incorporated land and major water bodies. 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 Cascades Ecoregion

To the south of Interstate 90, the Cascade Mountains take on some characteristics quite distinct from the northern portion of the range. These southern mountains are bedded mainly on volcanic rocks rather than the granitics that typify much of the northern North Cascades Region. Peaks along the crest are not so high, only reaching into the truly alpine at Blowout Mountain (1,732 meters; 5,680 feet) at the very eastern extreme of the upper Green River watershed. In King County, the Cascade Ecoregion includes the Western Cascade Lowlands and Valleys, the Western Cascades Montane Highlands, and the very limited Western Cascades Subalpine/Alpine.

Western Cascade Lowlands and Valleys

In King County, the Western Cascade Lowlands and Valleys ecoregion encompasses 666 square kilometers (238 square miles) and is dominated by three river systems: the Cedar River, which penetrates along the northern edge of the Cascade region; the Green River in the central portion; and the White River, which marks the boundary between King County and Pierce County to the south. The ecoregion also extends to the northeast for approximately 25 kilometers (15 miles), along a broad, glacial meltwater-formed valley that penetrates the Puget Uplands. This valley links the Puget Lowlands to the Cascade Highlands. According to certain historical accounts, this “thumb” was a major corridor for both human and animal travelers between the two regions. Because of their proximity to Seattle and other settled areas of Central Puget Sound, these valleys were among the first to be logged and the first to be converted to agriculture in their lower reaches; logging and agriculture continue here today. In the Duwamish-Green River Valley, for example, the first Euro-American settlers arrived in 1851; by 1853 a steam-powered sawmill was operating at the mouth of the Duwamish-Green river at Elliott Bay and by 1880, logging had extended into the upper watershed, some 97 kilometers (60 miles) from the bay. Between about 1910 and the end of WW I, the lower elevations of this ecoregion were cut almost completely and converted to agriculture. Flood control levees and revetments along the three major rivers aided this conversion. In 1948, the Mud Mountain Dam was completed on the White River. This was soon followed by Howard Hanson Dam on the Green in 1963. With the damaging floods controlled on these rivers, industry began to push aside agriculture in the lower valleys. Today, the ecoregion remains primarily in forestry production compatible with the use of the major rivers as water supplies for the metropolitan area. In fact, virtually all of the ecoregion within the Cedar River Watershed is managed to assure the quality of Seattle’s water supply. A Habitat Conservation Plan developed for the 36,600 hectare (90,500 acres) watershed in 2002 has an objective to “Eliminate timber harvest for commercial purposes to effectively create a watershed ecological reserve” and management is now focused on regaining the old forest structure.  (the City of Seattle has committed to stop commercial logging in their 36,600-hectare (90,500-acre) municipal watershed). Even so, little of the native forest landscape remains, even in this protected area. The only other large old-growth stands occur in Federation Forest State Park in the upper White River Watershed. Despite these changes, cougar, black bear, elk and deer are all common residents of this ecoregion, even at its lowest elevations.

Clearcut in King County. Photo by Jennifer Vanderhoof.

Western Cascades Montane Highlands

The Western Cascade Montane Highlands (612 square kilometers; 219 square miles) are also dominated by timber harvest. Most of the lands in this ecoregion are in private ownership, except for the Cedar River Watershed and the Tacoma Watershed lands. A patchwork of clearcuts and reforested areas characterizes this landscape, and all but the steepest and most inaccessible areas are traversed by forest roads. This fragmentation has produced a forest cover that is predominantly in early to mid seral stages (less than 75 years old); less than 10 percent of the ecoregion is in a late seral stage (old growth). Many of the existing late seral forest stands tend to be located in riparian areas of headwater streams or areas on very steep slopes. Much of the riparian corridor was harvested during the original timber harvest in the 1880s or burned in fires at the turn of the century. Currently, riparian vegetation along the mainstem rivers (with the exception of the Cedar, where the riparian zone is being managed for large mature conifers) is predominantly small to medium-sized deciduous or mixed deciduous and coniferous stands. These vegetation communities are in sharp contrast to the size of the pre-harvest trees that once lined the streambanks. This patchwork of forest harvest has almost certainly altered the distribution and abundance of many forest dwelling birds and mammals. Although few historic records exist, the use of pre-logging forests by a variety of species, some now rare, has been suggested by a number of researchers. Among the species using these mid-elevation forests is the spotted owl, now listed under the U.S. Endangered Species Act.

Western Cascades Subalpine/Alpine

The Western Cascades Subalpine/Alpine ecoregion occupies only about 2.5 square kilometers (1 square mile) in King County, mostly on the slopes of Blowout Mountain (at 1,732 meters, 5,680 feet ASL). This area differs little from the subalpine areas of the North Cascades in vegetation and animal species. This small area of King County lies on the Pacific Crest Trail, the main north-south recreational trail along the crest of the Cascade Range.

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.