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Mole Rat

Nestled between the Cascade and Olympic mountains in northwest Washington, the Puget Sound basin covers more than 16,000 square miles of land and water. The basin's surface area is roughly 80 percent land and 20 percent water. Puget Sound offers a breadth of landscapes unique in the world -- the rocky shores of the San Juan Islands, the forested slopes of the Olympic Mountains, Skagit Valley's fertile floodplain, and rich, tidal mudflats in the southern inlets. The health of the Sound depends on these rich and diverse environments.

The Puget Sound System extends approximately 100 statute miles from Deception Pass south to Olympia. A shoreline of 1,33? statute miles encloses a water area of 1,020 square miles at Mean High Water. Fresh water inflow from rivers amounts to a yearly mean of 41,000 cfs (cubic feet per second), ranging between a peak of about 367,ooo cfs to a minimum of about 14,000 cfs.

Puget Sound is an estuary -- a semi-enclosed, glacial fjord where saltwater from the ocean is mixed with fresh water draining from the surrounding watershed. Made up of a series of underwater valleys and ridges called basins and sills, Puget Sound is deep, with an average depth of 450 feet. The maximum depth (930 feet) occurs just north of Seattle. A relatively shallow sill at Admiralty Inlet separates the waters of the Strait of Juan de Fuca from the waters of Puget Sound proper. South of Admiralty Inlet, Puget Sound proper consists of four interconnected basins. The largest and deepest of these, the Main Basin, consists of two sub-basins and extends some 60 miles from Admiralty Inlet to the Tacoma Narrows. Around the Tacoma Narrows, a shallow sill separates the Main Basin from the Southern Basin. To the north and east of the Main Basin (but not separated by a sill) is the Whidbey Basin. This basin lies to the east of Whidbey Island and includes the waters of Possession Sound, Port Susan, Saratoga Passage, and Skagit Bay. The smallest of the four basins, in terms of area, is the Hood Canal Basin on the western side of the Sound. This long, narrow channel branches from the Main Basin south of Admiralty Inlet and extends about 80 miles south between the Olympic Mountains and the Kitsap Peninsula.

Vancouver, George (1757-1798), British naval officer and explorer. He was born in King's Lynn, England. He served with the British explorer Captain James Cook on his second (1772-1775) and third (1776-1780) voyages. In 1791 Vancouver began an expedition to survey the Pacific coast of North America. On this expedition he became the first European to circumnavigate the island now named Vancouver.

The two-layer circulation system is disturbed by shallow sills, a series of underwater valleys and ridges, which recirculate water from the surface back into the depths of the basin. In particular, sills at the Tacoma Narrows and Admiralty Inlet greatly influence water movement through the basin. Mixing at the Admiralty Inlet sill draws seaward-moving surface water down into the inward-moving salty water from the Strait of Juan de Fuca. Puget Sound's circulation pattern acts as a pump to raise deep water toward the surface at the south end of the main basin. Water flow is also complicated by the islands, narrow passages, and changes in water depth that characterize Puget Sound. In some of the shallow, semi-enclosed bays of the southern basin, water moves sluggishly. Water is funneled at high speeds through passages connecting with the main system. These estuarine circulation patterns also affect the millions of tons of sediment and other materials transported to or resuspended in the Sound. However, unlike the waters that eventually move seaward, most particles are permanently trapped in the basin. In the main basin, only a small fraction of the particles initially present in the surface water are carried past Admiralty Inlet. The terrain [back to top] Puget Sound's striking terrain is largely the result of extensive glacial and tectonic activity. Other geologic processes, including weathering, erosion, and sedimentation, have further defined the region's landforms and physical characteristics. The region's soils are relatively immature with shallow accumulations of organic material. Fertile soils are only found along the southern and western margins of the basin and in the lower reaches of the river valleys. Dense coniferous forests dominate, interspersed with a variety of deciduous woodlands, wetlands, and grass and shrub prairies. The soil and plant cover provide important and diverse habitat for wildlife and protection against precipitation and runoff, naturally slowing, storing, and cleansing water as it drains to the Sound. The shoreline environment is a complex mixture of beaches, bluffs, deltas, mudflats, and wetlands. Forming a bridge between land and ocean, they nurture some of the most dynamic and productive habitats in the world.



Movie: Glacial Carving of Puget Sound




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Copyright Pacific Science Center 2000
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Updated 24, May 2000