Puget Sound: A uniquely diverse and productive estuary

Puget Sound is the second largest estuary in the contiguous United States. Today, we understand that estuaries — where freshwater and saltwater merge — are among the most productive places for life to exist.

Puget Sound portion of a 1798 chart showing "part of the coast of N.W. America : with the tracks of His Majesty's sloop Discovery and armed tender Chatham / commanded by George Vancouver, Esqr. and prepared under his immediate inspection by Lieut. Joseph Baker." Credit: Library of Congress, Geography and Map Division.
Puget Sound portion of a 1798 chart showing "part of the coast of N.W. America : with the tracks of His Majesty's sloop Discovery and armed tender Chatham / commanded by George Vancouver, Esqr. and prepared under his immediate inspection by Lieut. Joseph Baker." Credit: Library of Congress, Geography and Map Division.

Sailing into Puget Sound in the spring of 1792, Capt. George Vancouver and his crew explored the nooks and crannies of an uncharted inland sea, recording the location of quiet bays, turbulent passages and all manner of rugged shoreline.

Two centuries later, cartographers still marvel at the precision of those first maps of Puget Sound—one of the largest and most productive estuaries in the United States.

Archibald Menzies, assigned to study the plants and animals discovered on the voyage, classified hundreds of “new” species, personally naming many of them. Menzies relished the variety of plants he found, while the ship’s crew feasted on native oysters, crabs, salmon, trout and a new species of flounder.

Long before Vancouver’s voyage, Native American culture embraced the bountiful flora and fauna of the region. Local tribes knew where to hunt, fish and gather plants — and they had their own names for places and things.

It would be nearly a century, however, before early ecologists began to understand that the variety of living things described so carefully by Menzies was a direct consequence of the physical associations among land, water and climate.

​Tidal marsh at the Nisqually National Wildlife Refuge in Puget Sound. Photo courtesy of USFWS.
​Tidal marsh at the Nisqually National Wildlife Refuge in Puget Sound. Photo courtesy of USFWS.

Today, we understand that estuaries—where freshwater and saltwater merge—are among the most productive places for life to exist. Plant and animal communities thrive in these protected areas of brackish water, where freshwater flowing from the land combines with seawater coming from the ocean. In all, an estimated 2,800 streams— from large rivers to small creeks— flow into Puget Sound.

Water circulation

Water circulation—the net result of tides, winds and streamflows—varies from place to place in Puget Sound, playing a direct role in habitat formation and productivity.

Freshwater, being less dense than seawater, tends to float in a surface layer that generally moves toward the ocean. Meanwhile, a deep layer of heavy seawater from the ocean pushes into Puget Sound along the bottom. Both layers ebb and flood with the vigorous tides that drive Puget Sound water movements. Strong winds and underwater formations, including the sills at Admiralty Inlet and Tacoma Narrows, interact with the tides to facilitate mixing between the layers, making nutrients available for phytoplankton. View a working model of Puget Sound circulation patterns.

Because salinity is a continuum from the freshwater rivers to the briny ocean, estuaries are not defined by size. River deltas are considered estuaries, as are the larger bays, inlets and sloughs. More broadly, Puget Sound is itself an estuary.

Complex food webs have evolved from the unique conditions found near the mouths of Puget Sound's rivers. Sediments dislodged from upstream areas and from shoreline bluffs provide the substrate for plants, which flourish in the nutrients and sunlight of the shallow waters.

Geology, water depth, wave action, tides and river currents all influence the unique character of an estuary, including whether the bottom is rocky, sandy or muddy. Conditions hostile to some plants and animals are perfectly suited to others.

Young salmon migrating from rivers to the ocean linger in the estuaries, proceeding slowly as their bodies adjust to the salty water that would kill many freshwater fish. On their return to the 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.

Map of Puget Sound and its main basins. Based on GeoBase and The National Map data. Credit: Pfly (CC BY-SA 3.0)
Map of Puget Sound and its main basins. Based on GeoBase and The National Map data. Credit: Pfly (CC BY-SA 3.0)

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.

Comparative bathymetry for Puget Sound and Chesapeake Bay. Data source: National Geographic Data Center; Image courtesy: UW Center for Environmental Visualization

Another type of estuary is formed by tectonic activity, exemplified by San Francisco Bay, where the ground sank over time as a result of pressure at the junction of the San Andreas and Hayward faults. San Francisco Bay averages 25 feet deep with a maximum depth of 100 feet.

A fourth type of estuary, the bar-built estuary, is characterized by offshore sandbars or barrier islands built up from river deposits. The Outer Banks off the coast of North Carolina helps contain water flowing in from several major rivers to form Albemarle Sound and the adjacent Pamlico Sound, both shallow waterways.

The human factor

Puget Sound’s complex estuarine character is also part of what makes it fragile. Close ties with the land mean that it has had a long and, over the past 100 years, increasingly fraught relationship with humans. Conditions in Puget Sound have changed greatly since Capt. George Vancouver explored the inland waterway, reporting back to England that the area was suitable for settlement. Even the name “Puget Sound” has changed its meaning.

When Vancouver’s ship Discovery stopped at the south end of Bainbridge Island in May 1792, Vancouver sent Lt. Peter Puget and a crew in two small boats to explore every branching inlet to the south.

In 10 days, the work was done and the carefully prepared charts were handed over to Vancouver, who later declared, “by our joint efforts, we had completely explored every turning of this extensive inlet.” He added, “To commemorate Mr. Puget’s exertions, the south extremity of it I named ‘Puget’s Sound.’”

Because of this, the original Puget Sound covered just the waterway south of the Tacoma Narrows to Olympia. Later, after the name came into wider usage, the U.S. Board on Geographical Names placed the boundary of Puget Sound just inside the Strait of Juan de Fuca.

Puget Sound is also recognized as part of the Salish Sea, a vast interconnected estuary that stretches out 6,535 miles and includes the Strait of Georgia in British Columbia, Canada. In 2009, the name “Salish Sea” was officially recognized by the U.S. and Canadian governments.

When creating the Puget Sound Partnership in 2007, the Washington Legislature changed the boundaries of Puget Sound again while declaring, “Puget Sound is in serious decline, and Hood Canal is in a serious crisis.” The law created action areas, defining Puget Sound as all of the inland waterway south of the Canadian border, including the Strait of Juan de Fuca, Hood Canal and the San Juan Islands.

The law creating the Partnership identified many of Puget Sound’s problems, including loss of habitats and native species, increases in nuisance species, contaminated sites, urbanization and stormwater pollution, closures of shellfish beaches and low-oxygen conditions.

“If left unchecked, these conditions will worsen,” the Legislature declared, setting up the governing body that coordinates today’s efforts to restore the health of Puget Sound.

[Funding for this article was provided by the Puget Sound Partnership and the EPA's National Estuary Program.]


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About the Author: 
Christopher Dunagan writes about Puget Sound for the Kitsap Sun and other publications. He is currently a senior writer at the Puget Sound Institute at the University of Washington.