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The South Central Puget Sound Action Area is home to about 3.5 million residents living in three of Washington’s largest cities—Seattle, Bellevue, and Tacoma—and in suburban and rural communities across unincorporated King and Pierce Counties and the southwestern corner of Snohomish County.

The Local Integrating Organization (LIO) area is the most urbanized portion of Puget Sound and includes a variety of industrial, commercial, and residential infrastructure, large areas of pavement, a heavily modified shoreline, and a large network of streets, roads, and highways. Although portions of this area have been intensively developed, approximately 77% of the area is not considered urban, with vast tracts of agricultural lands in rural King and Pierce Counties and wilderness in Mount Rainier National Park and the Mount Baker–Snoqualmie National Forest. There are also large parks and open spaces throughout, including in the cities.

Physical Description

Three major river systems originate in the Cascades near Snoqualmie Pass, Cascade Pass, and Mount Rainier, travel through forests and farms and down through urban landscapes, and empty into Lake Washington and Puget Sound. Glacial melt from Mount Rainier feeds the Puyallup/White River system while the Green/Duwamish and Cedar/Sammamish (Greater Lake Washington) river systems are supplied by snow melt and rainfall.

Lowland areas average 40 inches of rainfall per year. There are a number of small drainages which flow directly into Puget Sound, many of which are channelized and/or lack adequate vegetation. In highly urbanized portions of the region, many streams or stream segments have been routed into drainage pipes and storm sewers that carry runoff from storms and flood events. Both create significant stormwater management challenges as well as opportunities for restoration. There is a high risk of flooding in some parts of these watersheds, potentially causing loss of life and severe impacts to infrastructure, as well as detrimental impacts on salmon habitat. Local jurisdictions are actively managing floodplains to provide multiple benefits and functions, including reduced flood risk and restored habitat.

The two largest bays in the LIO area are Seattle’s Elliott Bay and Tacoma’s Commencement Bay. Vashon-Maury is the largest island south of the Admiralty Inlet. The major currents within the saltwater basin of central Puget Sound generally flow northward along the west side of Vashon Island, and southward through the East Passage. The marine waters of Puget Sound form warm layers at the surface during the summer months due to river input and solar heating. These layers are mixed during winter months by seasonal winds and cool weather. An underwater sill by the Tacoma Narrows also alters the pattern of marine water circulation.

Biological Description

These river and watershed systems are home to four populations of Chinook salmon as well as steelhead and bull trout—all listed as threatened under the Endangered Species Act—and have federally approved watershed-scale recovery plans guiding recovery actions. The 1999 federal listing of Puget Sound Chinook as threatened was the first time a salmon species in such an urban environment had been listed under the Endangered Species Act. Since the listing of Chinook, steelhead and bull trout have also been listed as threatened under the Endangered Species Act in 2007 and 1999, respectively.

Despite the extensive urbanization, Chinook salmon and other salmon species continue to spawn in the South Central Action Area’s major rivers, lakes, and creeks. Unique salmon populations include the spring run of White River Chinook, Issaquah Creek and Cedar River summer and fall Chinook, Lake Sammamish Kokanee, and Lake Washington Sockeye. The Green River is one of the top 10 steelhead rivers in Washington and supports substantial natural and hatchery populations of salmon. Bull, rainbow, and coastal cutthroat trout, as well as coho, chum, and pink salmon, are also present in some of the river systems.

Social Context

The LIO contains a highly diverse population of over 3.5 million people and is one of the fastest growing regions in the nation. There are large immigrant communities from Asia, Latin America and Mexico, Africa, and Eastern Europe. In general, residents of the LIO area are informed and engaged citizens and there is a high level of volunteerism and civic engagement. Many agencies and local nongovernmental organizations benefit from the public's resources and knowledge base through assistance with on-the-ground projects, education, research, and public process for furthering recovery. The LIO area is also home to a large refugee population, and contains several marginalized and impoverished communities that may experience a disproportionate level of environmental challenges.

South Central Puget Sound is the economic driver of the region, and largely of the State of Washington. The region within the Action Area generates over $300 billion in annual economic activity, comprising approximately 71% of the gross state product. Major commercial and industrial enterprises are concentrated here, including in the technology, aerospace, finance, insurance, health care, business and professional services, commercial fishing, recreation, and tourism industries. These industries are served by international seaport facilities in Seattle and Tacoma, along with SeaTac International Airport, Boeing Field, and passenger and freight railroad services. The region has over 16,200 acres of designated manufacturing industrial centers in six locations: Ballard Interbay, Duwamish, North Tukwila, Auburn/Kent, Overlake, and the Port of Tacoma. Water supply for most of the population of the area is provided by the Cities of Seattle and Tacoma through their operations on the Cedar and Green Rivers, respectively.

The varied ports and waterways of this LIO geography have made it an international shipping center for regional and national industries, natural resource extraction (logging, fisheries, mining), and agricultural products. The combined ports of Seattle and Tacoma are the second largest on the west coast. Urban estuaries support many small marine, ship building/repair, and industrial enterprises. A railroad line runs along the nearshore between Tacoma and Everett. In addition, public transportation to Kitsap County and Vashon Island is provided by the Washington State Ferries system, and other vessel traffic consists of passenger ferries, fishing boats, research vessels, small recreational craft, and cruise ships.

Popular locations for recreation include Lake Washington, Lake Sammamish, and Lake Tapps; Puget Sound beaches such as Alki Beach in West Seattle, Seahurst in Burien, Olympic Beach in Edmonds, and Point Defiance in Tacoma; the Mountains to Sound Greenway along Interstate 90; and the middle Green River and the White River above Enumclaw. The headwaters of the major rivers in this area are protected through their status as parklands managed by the National Park Service, wilderness areas managed by the U.S. Forest Service, and the water supply watersheds managed by the cities of Seattle and Tacoma. However, in 2015 and 2016, two of the major rivers in the LIO area, the Green/Duwamish River, and the Puyallup/White River, were named two of the nations’ most endangered.

Source: this Profile is listed in the 2016 South Central LIO Ecosystem Recovery Plan.


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