Keywords: Physical environment, Water quality, Water quantity, Algae, Marine habitat, Estuarine habitat, Freshwater habitat, Modeling, Nutrient pollution, Circulation, Tidal energy

In a new series we are calling “Ask a scientist” we interview local researchers to get their thoughts on some of the important but lesser-known scientific facts about the Puget Sound ecosystem. Today, we speak with University of Washington oceanographer Parker MacCready about Puget Sound’s “underwater Amazon” and why it has profound implications for Puget Sound science and policy. It all begins, he says, with the mixing of fresh and salt water and something called the “estuarine exchange flow.” 

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 2,800 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 ocean currents and fresh 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.

View from space of the Salish Sea and major river valleys that flow into Puget Sound.

This view of the Salish Sea from space captures some of the major rivers that flow into Puget Sound. Photo: Stuart Rankin using ISS040 images (CC BY-NC 2.0)

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 (thus our use of the term “estuarine”), 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 2,800 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 University of Washington Puget Sound Institute, which publishes the Encyclopedia of Puget Sound, is exploring many factors related to water circulation in a series of workshops focusing on the science of Puget Sound water quality. The workshops and this article are jointly sponsored by King County and the Puget Sound Institute.

The mechanism behind this system is well understood, at least from the perspective of 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.

Satellite image of Washington state showing bright Hood Canal as a bright teal color.

A huge phytoplankton bloom (teal blue area) in Hood Canal was visible from space on July 31, 2017. Image: Joshua Stevens, NASA Earth Observatory, using MODIS data from LANCE/EOSDIS Rapid Response and Landsat data from the U.S. Geological Survey.

That problem has led to controversy over the degree to which human sources such as wastewater and agriculture also contribute nutrients to the water. 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.”

This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality.

Did you know that the Columbia River can sometimes flow into Puget Sound?

“When the wind is from the south, there’s a lot of water from the Columbia River that comes into the Salish Sea,” University of Washington oceanographer Parker MacCready told us. Those southern winds create a fast-moving and narrow current along the coast that brings water from the Columbia River all the way up to the Strait of Juan de Fuca. It happens in just a few days. “Then it turns the corner — it takes a right into the Strait of Juan de Fuca — and comes in as an inflowing surface layer. Sometimes you can trace that Columbia River plume water all the way to Admiralty Inlet.”

View from space of the Oregon and Washington coastline showing sediment plume of the Columbia River

A view from space shows where the sediment plume from the Columbia River empties into the Pacific Ocean. Under certain wind conditions the plume may extend more than 125 miles north to the Strait of Juan de Fuca. Graphic: Sylvia Kantor/UW based on SeaWiFS image from NASA/Goddard Space Flight Center

Citation: Hickey, B. & McGabe, R. & Geier, S. & Dever, E.P. & Kachel, N.. (2009). Three interacting freshwater plumes in the northern California Current System. J. Geophys. Res.. 114. 10.1029/2008JC004907.

Related: Measuring Puget Sound’s 'headwaters'

The knowledge that huge amounts of water flow into Puget Sound from the ocean is nothing new, but the source of this underwater river — its "headwaters" in Juan de Fuca Canyon — was first measured in detail by University of Washington oceanographers Matthew Alford and Parker MacCready in 2014. The findings shed new light on why Puget Sound is especially rich in nutrients that flow in from the deep waters of the Pacific Ocean. The related paper in the journal Geophysical Research Letters is described in a report from UW News.

Four people stand on the deck of a boat as an instrument is lowered into the water.

Oceanographers lower an instrument called the Shallow Water Integrated Mapping System (SWIMS) into the Juan de Fuca Canyon. The instrument measures detailed water properties over a depth of water as it zig-zags up and down while being towed behind a UW research vessel. Photo: Parker MacCready/UW

About the author: Jeff Rice is managing editor of the Encyclopedia of Puget Sound.

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