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2015 Puget Sound Factbook Book | v3.0
river, spawning adult salmon reverse that acclimation process. In this way, salmon and
steelhead take advantage of the most beneficial conditions in both streams and ocean.
Estuary formation
Puget Sound, as we know it today, owes much of its size and shape to massive ice sheets that
periodically advanced from the north, gouging out deep grooves in the landscape. The most
recent glacier advance, about 15,000 years ago, reached its fingers beyond Olympia. The ice
sheet, known as the Vashon glacier, was more than a half-mile thick in Central Puget Sound and
nearly a mile thick at the Canadian border.
As the glacier melted, freshwater filled in the holes, creating many lakes, including Lake
Washington and portions of Puget Sound that later became inundated with seawater.
Puget Sound is actually four deep basins, three of which are separated by prominent “sills,” or
rises in the seabed. These sills play a major role in the circulation of water in Puget Sound,
impeding the waterway’s ability to flush out pollution and restore healthy oxygen levels. One sill
at Admiralty Inlet reduces the flow of seawater from the Strait of Juan de Fuca into the Main
Basin of Puget Sound. Other major sills provide partial barriers between the Main Basin and the
basins of northern Hood Canal and the southern Sound at the Tacoma Narrows. (The Whidbey
Basin has no sill at its entrance.)
Estuaries carved by glaciers, such as Puget Sound, are known as fjord estuaries. They are
prominent in areas where the glaciers once loomed, including Alaska and Scandanavia in the
Northern Hemisphere and Chile and New Zealand in the Southern hemisphere.
More common types of estuaries, called coastal plain estuaries, were formed when a rising sea
level flooded a major river valley. Coastal plain estuaries, including Chesapeake Bay on the East
Coast and Coos Estuary in Oregon, tend to be shallower with less physical diversity than fjord
estuaries.
Chesapeake Bay, which filled the immense valley of an ancient Susquehanna River, covers about
4,480 square miles—more than four times the area of Puget Sound (not including waters north
of Whidbey Island). But Chesapeake Bay is shallow—averaging just 21 feet deep. In comparison,
Puget Sound averages 205 feet deep, with the deepest spot near Point Jefferson in Kitsap County
at more than 900 feet.
Consequently, Puget Sound can hold a more massive volume of water—some 40 cubic miles,
well beyond Chesapeake Bay’s volume of 18 cubic miles.
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