Keywords: Physical environment, Water quantity, Estuarine habitat, Circulation, Salish Sea Currents magazine, Floodplains, History

Puget Sound is often referred to as the second largest estuary in the United States behind only Chesapeake Bay, but its overall size may be less important than its complexity. The place is defined by the mixing of saltwater from the ocean and freshwater from inflowing creeks and rivers that create an almost alchemical transformation of habitat. In this article, we look at the geologic forces that formed Puget Sound and made it the dynamic system that we understand today.   


In 1849, Captain Charles Wilkes, who had visited Puget Sound eight years earlier as head of the U.S. Exploring Expedition, described the waterway as “one of the most noble estuaries in the world.” While Wilkes focused much of his admiration on the Sound’s calm, easy to navigate waters and “a climate unsurpassed in salubrity,” he could easily have described the richness of the water itself. What he had seen was one of most productive, complex, diverse, and dynamic ecosystems on earth, with more than 200 fish species, almost 200 types of birds, more than 35 species of mammals, and several thousand invertebrate species.

Bringing in the water

Wilkes used the word “estuary” to describe Puget Sound but what did he mean? Like all estuaries, Puget Sound is a mix of freshwater and saltwater. Much of this mixing is driven by the great inflow of tidal water at Admiralty Inlet. That underwater torrent combines with the many rivers and creeks flowing in from the land, but it creates more than just a single estuary. Puget Sound is 1,016-square-miles of bays, basins, islands, and inlets, as well as many, smaller estuaries formed at the mouths of the rivers that enter the Sound.

Map showing five Puget Sound marine basins include: Hood Canal, Main Basin (Admiralty Inlet and the Central Basin), South Basin, and Whidbey Basin. Map: Kris Symer. Data source: WDFW.

Map showing five Puget Sound marine basins: Hood Canal, Main Basin (Admiralty Inlet and the Central Basin), South Basin, and Whidbey Basin. Map: Kris Symer. Data source: WDFW.

All of those features owe their existence to the region’s geological past. Beginning about 35 million years ago, plate tectonic action led to the mountains that rise on either side of the waterway and which supply the water for 2,800 rivers and streams via glaciers, snowfall, and rain. Much more recently, during the last ice age, a finger of the northern continent-covering Cordilleran Ice Sheet, extended down between the mountains and resulted in the deep depression now filled with that mix of salt and freshwater. Some also refer to the Sound as a fjord estuary, because of its glacial origin. 

(Coincidentally, the Cordilleran Ice Sheet, and a second finger of ice carved the Strait of Georgia and Strait of Juan de Fuca as well. Although they share a glacial origin, these two bodies of water, which along with Puget Sound comprise the Salish Sea, function differently ecologically because of other aspects of geology, which will not be covered in this article.)

Carving the channels

Imagine draining the water out of Puget Sound. No longer would a viewer be confronted with shimmering sheet of blue, rippled by wind-blown waves, and the occasional breaching orca. Instead, one would see a chasm that bottomed out 938 feet deep at Point Jefferson, due west of Seattle’s northern boundary. If the city’s tallest building, the 76-story Columbia Center, rose from the depths at this location, a person standing on top would be about even with the shoreline. Because of its great depth, Puget Sound has a volume of about 40 cubic miles — the equivalent of more than a trillion standard-sized (42 gallon) bathtubs. In contrast, Chesapeake Bay has a bit less than half that volume with four times the surface area.

Puget Sound began to form when a sheet of ice crossed the 49th parallel around 18,800 years ago, moving about 125 yards a year. Known as the Puget lobe, the great glacier pushed between the foothills of the Olympics and Cascades and was at least 4,500 feet thick at Bellingham at its maximum extent. In Seattle, it was 3,000 feet thick, thinning to about a thousand feet near its southern most point, a bit south of Olympia. The icy origin means that Puget Sound is a fjord, or a deep, glacially carved, coastal inlet.

Map showing extent of the Puget lobe of the Corillerian ice sheet

At its maximum extent about 16,000 years ago, the Puget lobe of the Cordilleran ice sheet was about 5000 feet thick at Bellingham, 3500 feet thick at Seattle, and 2000 feet thick at Tacoma. By about 15,500 years ago the ice retreated northward creating Puget Sound. Map: Courtesy Washington Geological Survey, modified from Patrick Pringle, Roadside Geology of Mount Rainier Park and Vicinity, Washington Division of Geology and Earth Sciences Information Circular 107, (Olympia, Wa.: Washington State Department of Natural Resources, 2008).

After reaching its terminus, the ice began to retreat, or melt, back to the north, about twice as fast as it advanced. By 15,500 years ago, it had shrunk to the point that salt water from the Strait of Juan de Fuca could flood the depression via Admiralty Inlet and Puget Sound was born.

Like all glaciers, the Puget lobe generated great volumes of water, which flowed under the ice in what geologists call subglacial rivers. As the glacier advanced, the subglacial water cut deep into the sediments under the ice, excavating Puget Sound, as well as Lake Washington, Lake Sammamish, and the Duwamish River valley, which was subsequently filled by eruption-generated mudslides, or lahars, from Mount Rainier. (Two lines of evidence point away from the Puget lobe carving the depressions. They are too sinuous, and geologists have evidence that lakes formed in front of the glacier and the ice would have floated when encountering the water.)

Gouging out these depressions was not a straightforward process. In areas comprised of softer underlying material, the terrain-carving water cut well below the average depth of about 230 feet, creating the Sound’s steep-sided walls. In harder areas, the rock resisted and led to the formation of four large sills, or ridges, which in turn resulted in three of the Sound’s four basins: Main, South, and Hood Canal; Whidbey basin lacks a sill. The biggest sill rises in Admiralty Inlet and runs 18 miles north to south with two high points. Half as long, the Tacoma Narrows sill separates what oceanographers call the Main, or Central, Basin and South Sound. A third prominent sill rises at the entrance to Hood Canal.

Mixing it up

What make the sills important to the ecology of Puget Sound is that the submarine structures act as plugs that impede the uninterrupted flow of tidal water moving into, out of, and through the Sound. In the Strait of Juan de Fuca, which has less bathymetric variation, cold, dense ocean water enters and travels along the bottom. Flowing in the opposite direction on the surface is a layer of river-supplied, fresher water, generally warmer than the ocean in summer and colder in winter. These two currents have limited mixing, creating two distinct layers of stratified water.

The same basic pattern of incoming ocean water and outflowing freshwater occurs in the Sound except that the sills add a level of complexity by mixing the two currents. (If you have seen rising boils, or large, smooth, round bubbles of water, over the sills at Admiralty or the Tacoma Narrows, you have seen the effects of turbulence causing the water to mix.) Averaging over the tidal cycle, a river of cold, ocean water, twenty to thirty times the volume of all fresh water entering Puget Sound, travels in from the deep of the Strait of Juan de Fuca and over the sill at Admiralty Inlet. Because the top of the sill rise unevenly to within 213 feet of the surface, water flows at different rates across the impediment, which generates turbulence that mixes the incoming saltwater and outgoing freshwater.

The sills and the blending of currents has both positive and negative ecological effects. On the negative side, the sills lead to pollutants remaining longer in the Main Basin. Studies have shown that about two-thirds of the pollutants in the Main Basin get returned during tidal exchange. The same effect, known as reflux, also occurs in the South Sound, where the blending of salt and freshwater recirculates pollutants and restricts their removal.

Graphic showing water circulation pattern and sills of Puget Sound

Illustration showing how sills influence circulation patterns and create reflux in Puget Sound. The illustration is not to scale and arrows showing current exchange do not indicate relative volumes of saltwater or freshwater. For example, the surface layer labeled "freshwater" is a mixture of seawater (90%) and freshwater (10%). Graphic: Courtesy of Su Kim, Northwest Fisheries Science Center/NOAA Fisheries

On the plus side, oxygen from the freshwater mixes with the saltwater, which prevents anoxic conditions that can cause havoc for many marine organisms. In addition, outgoing surface water doesn’t flow freely out of the Main Basin into the Strait. Instead, about two-thirds of the fresh water gets mixed back in with the inflowing saltwater and remains in the basin. This type of reflux limits the movement of fish and invertebrate larvae and translates to more nutrients and greater productivity.

Reflux may also influence evolution. The sills in the Sound have narrowed the interchange of several species of rockfish larvae and adults between the inland waters and the outer coast. Over time this led to species such as brown, yelloweye, and quillback to evolve and become genetically isolated from their coastal relatives.

Hood Canal and Whidbey Basin, in contrast to the other basins, have different flow patterns. Because of the large volume and length of the Hood Canal basin, water has a longer residence time—the amount of time to flush water and replace it—and tendency to mix less. This results in stagnant, deep water in the basin, one of the main reasons why Hood Canal experiences low oxygen levels, or hypoxia, at depth most years in autumn. Whidbey Basin experiences yet another pattern because it is fed by a high volume of freshwater from the Skagit River, which creates more stratified water.

Building the beaches

In addition to eroding the landscape, glaciation also resulted in deposition of silt, sand, cobbles, and boulders, which now makes up the miles of bluffs that border Puget Sound. The bluffs range up to 300 feet high and develop from relentless wave action pounding and gnawing the glacial sediments. Erosion typically occurs too slowly for us to notice, perhaps one foot per decade, but bluffs can fail catastrophically when the wave-undercut base collapses and tons of sediment and vegetation crash to the beach below. (It might seem intuitive that larger tidal ranges cause more erosion, the opposite is true; smaller tidal ranges concentrate wave energy, making the waves more effective.) The other major natural triggers are big rainstorms, especially when combined with the removal of bluff vegetation and altered drainage caused by human activities.

Beach with gravel and drift wood in foreground; bluff in background.

Bluff erosion is a primary source of beach sediment around Puget Sound. The accumulation of drift logs and beach wrack, shown here at Ebey's Landing on Whidbey Island, provides habitat for insects, worms, and amphipods. Photo: Sylvia Kantor

Researchers have found that bluff erosion around the Sound is the primary source of beach sediment and the associated plant and animal communities. As bluffs change shape, they create feeding, roosting, and nesting habitat for marine birds and shorebirds. On the upper beach, the accumulation of drift logs and beach wrack, such as algae, seagrass, leaf litter, and other tide-deposited debris, provides habitat for insects, worms, and amphipods. Closer to the water, in the intertidal zone, beach-spawning forage fish such as surf smelt and sand lance, deposit their eggs. Sand lance also bury themselves in the substrate to conserve energy and avoid predation. In addition, the intertidal zone supports numerous shellfish species, including oyster, geoduck, and razor, horse, and butter clams. Out in the water, juvenile Chinook, coho, chum, and pink exploit the nearshore during outmigrattion and rearing.

Building the rivers

Large scale, continental glaciation was not the lone type of glaciation to influence ecological processes in Puget Sound. During the last ice age, and continuing to the present, alpine glaciers helped carve the great valleys now filled with the many rivers that reach the Sound. By far the biggest river is the Skagit River, which contributes from one third to one half of all the freshwater that enters Puget Sound, followed by the Snohomish, Nooksack, Puyallup, Stillaguamish, Skokomish, and Nisqually Rivers.

Aerial image of the Skagit River

The Skagit River channel. Image: Washington Geological Survey, Washington State DNR (CC BY-SA 2.0)

Together, the streams and rivers would take about five years to fill the Sound. Because of what is a relatively small amount of freshwater input, the average salinity of Puget Sound is about 83 percent of Washington’s coastal waters, dropping to about 66 percent in Budd Inlet, at the south end. In contrast, Chesapeake Bay is about half seawater because of the greater percentage of freshwater that enters it.

Puget Sound’s rivers are essential to the health of the saltwater ecosystem, particularly for salmon. The glacial fed rivers provide sediment, necessary for salmon redds. Trees shade riparian areas, creating a cooler, salmon-friendly environment. When the trees die, they end up as logjams that offer additional habitat and influence seasonal flow. Sedimentation also moves down river to form deltas comprised of wetlands, emergent marshes, mudflats, and freshwater swamps that provide shelter and food for fish such as salmon, filtered runoff, and protected shorelines from storms and waves.

These pocket, estuarine habitats are rich with plants and animals adapted to the protected areas of brackish water. They are also critical ecosystems, particularly for salmon, which exploit the mix of salt and fresh to undergo the physiological transformation (called smoltification) necessary for their migration between the sea and their riparian birthplace. And, like the other Puget Sound ecosystems, the delta estuaries result from a combination of tectonic and glacial events, which has led to the entire waterway’s unique and valuable productivity.

This article is adapted from David B. Williams’ book Homewaters: A Human and Natural History of Puget Sound, published in 2021 by the University of Washington Press.

Some of the information for this story is drawn from the Puget Sound Fact Book, published by the Encyclopedia of Puget Sound. 


About the author: David B. Williams is an author, naturalist, and tour guide whose book Homewaters: A Human and Natural History of Puget Sound is a deep exploration of the stories of this beautiful waterway. He is also the author of the award-winning book Too High and Too Steep: Reshaping Seattle’s Topography, as well as Seattle Walks: Discovering History and Nature in the City. Williams is a Curatorial Associate at the Burke Museum and writes a free weekly newsletter on Substack, the Street Smart Naturalist.

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