If you ask people to name some of the rivers that flow into Puget Sound, they might list the Skagit or the Nisqually or any one of thousands of creeks and streams that arrive from the surrounding mountains. They are less likely to mention that Puget Sound’s largest river is one you can’t see from the land.

It’s a river that has no name but starts deep off the Washington coast and pushes trillions of gallons of ocean water through the Strait of Juan de Fuca in a raging torrent the size of the Amazon. This underwater Amazon flows through Juan de Fuca Canyon stirring up turbulent waves and eddies that churn the waters of Puget Sound deep below the surface.

“Yeah, it’s pretty close to the size of the Amazon except that it’s flowing in, toward the land, and then getting mixed and flowing out again,” University of Washington oceanographer Parker MacCready told us recently. This oceanic river, MacCready said, is a big reason Puget Sound is Puget Sound. For one thing, it’s part of why the water is salty — imagine a giant spigot that fills the basin with ocean water — but there’s more to the story.

The interaction between fresh and salt water stokes an engine that drives water circulation throughout the entire basin, something intensely important to the understanding and management of Puget Sound.

This giant current is part of Puget Sound’s estuarine exchange flow, a phenomenon little-known outside of scientific circles that means everything to Puget Sound’s vitality and health. This flow brings in life-giving nutrients from the ocean that feed Puget Sound’s creatures, from salmon to orcas. It also drives water circulation throughout the entire basin, something intensely important to the understanding and management of Puget Sound. It can help explain the impacts of wastewater, the movement of toxic chemicals, where our fish and orcas roam, and even whether there is enough oxygen in the water to sustain life.

Ask a scientist: The estuarine exchange flow

We asked MacCready to help us understand Puget Sound’s estuarine exchange flow and to explain why oceanographers care so much about it. The first thing you need to grasp, he said, is that Puget Sound is not the ocean. It is an estuary, a fact that has all kinds of ramifications.

Loosely defined, an estuary is a place where salt water and fresh water intermingle, and this simple mixture influences most aspects of the region’s ecology, including the types of plants and animals that live here. But for oceanographers, this mixing also tells a story about how Puget Sound behaves as a system. To begin with, Puget Sound's huge influx of ocean water — the underwater Amazon — wouldn't happen without it. 

Consider those creeks and rivers that flow into Puget Sound, the ones fed by snowmelt and rainwater and ancient glaciers. Their volume is relatively tiny in comparison to what comes in from the Pacific — all the fresh water flowing into Puget Sound makes up only about 1/20th of the incoming salt water — but their impact is transformative.

Those many rivers do more than just lower Puget Sound's salinity (Puget Sound is about 83% seawater). When the fresh water they provide meets the salt water, it stratifies and churns and moves. This mixing action creates a liquid force that helps to pull in water from the ocean. It stokes an engine that drives Puget Sound's massive underwater flow — and subsequently water circulation — throughout the entire basin. "Estuaries exchange water with the ocean very efficiently," MacCready said. 

The mechanism behind this system can be explained by basic physics. As lighter fresh water floats near the surface, the tides and currents mix the fresh water downward creating potential energy. As the water moves back up and becomes stratified again, energy is released that draws more water into Puget Sound (a full description of this process can be found on MacCready’s website LiveOcean). This push-and-pull is what scientists call the estuarine exchange flow. 

What if there were no rivers?

Without the incoming fresh water, MacCready told us, Puget Sound would look very different. Aside from the lack of salmon, which need rivers to spawn, there would be much less circulation of the water, and perhaps less life in Puget Sound overall. 

“You’d still have tides and winds, but the big difference would be that there would be much less exchange of water between the ocean and Puget Sound,” MacCready said. “It would all be driven by tides and tidal stirring, which is pretty ineffective. You’d see the water inside Puget Sound having much longer residence times, like probably years instead of months. [The rivers] give you a lever to work with that otherwise would not be there.”

The loss of the exchange flow would have another noticeable effect. The influx of ocean water brings in nutrients that feed much of the life in Puget Sound. That includes huge amounts of nitrogen and other elements such as phosphorous that flow in from the California Current. This feeds Puget Sound’s plankton, making Puget Sound among the most biologically productive estuaries in the world.

“This ocean water helps feed the ecosystem,” MacCready said. “In the case of Puget Sound or the Salish Sea, most of the nutrients that are used by phytoplankton come from the ocean. There are lots of nutrients— essentially nitrate — out in the Pacific at the depths that the water flows. The nutrients feed phytoplankton and zooplankton, then little fish, then big fish, then orca whales and people and sushi restaurants and everything. It all relies on that.”

Not everything about these nutrients is good for life. Large amounts can also make Puget Sound naturally low in oxygen, especially in certain areas like Hood Canal and in some small embayments where water circulation is less defined. During certain times of the year, such as the warmer summer months, high levels of nutrients can cause blooms of phytoplankton that can create toxins or eventually die and draw oxygen from the water.

That problem has led to controversy over the degree to which human sources such as wastewater and agriculture also contribute nutrients. Some argue that the input from humans is tipping the balance while others maintain that the effect is less severe.

While that may be a matter of dispute — the issue is now being argued in the courts — MacCready said there is little question that we owe much of what makes Puget Sound special to the meeting of fresh water and salt water. Without that bit of alchemy, he said, there would be no estuarine exchange flow, no subsurface Amazon, and Puget Sound as we know it would not exist. “Yes, it’s huge. It’s extraordinarily important. It’s the whole deal.”

"Ask a Scientist" will appear occasionally in the Encyclopedia of Puget Sound. This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality.