Charting the flood: WWU ocean researchers investigate the extent and impact of Skagit River flooding

In the days leading up to the Skagit River cresting on Dec. 12, 2025, Western Washington University Professor of Environmental Sciences and physical oceanographer Erika McPhee-Shaw looked for changes in the river.

When she saw trees floating down the river, she began to wonder how the flooding was affecting the ocean. Physical oceanographers like McPhee-Shaw study the movement of the ocean — waves, tides and currents.

“I was like, this is a big flood. This is the kind of opportunity that is so rare that I think we should jump on it because we may never see this again,” said McPhee-Shaw.

McPhee-Shaw and Assistant Professor of Marine and Coastal Science (MACS) and Environmental Sciences Sam Kastner, also a physical oceanographer, began messaging each other with thoughts and questions.

What was the extremely high volume of fresh water from the river doing to the intensity of water mixing as it entered Skagit Bay, flowed through the Whidbey Basin and, eventually, out into Puget Sound?

“It was obvious that we were going to see extremely low salt levels in Whidbey Basin, but what was not obvious was how exactly that would manifest,” Kastner said.  

Normally, fresh and salt water mix quickly, but would the flooding cause distinct layers? And if the fresh water formed a layer like a blanket on top, how much and where?

Ocean water enters Puget Sound from the Strait of Juan de Fuca through Admiralty Inlet along the west edge of Whidbey Island. This is where the oceanic waters (roughly 35 parts per thousand salinity) of the Strait of Juan de Fuca meet the mostly oceanic waters of Puget Sound (32 parts per thousand salinity).

McPhee-Shaw calls Admiralty Inlet “the eggbeater of Puget Sound” because of how aggressively and rapidly fresh and salt water are blended there.  

This same mixing phenomenon also occurs at the west end of Deception Pass, but not on this particular day.

“The freshwater layering was so strong that the outflow of Deception Pass, which I have always observed to be completely mixed, top to bottom, was layered,” said Kastner.

McPhee-Shaw explained that under typical conditions, strong tides combine ocean and river water to create estuarine exchange, making the freshwater signature of rivers indistinguishable from that of salty marine waters.  

“The tidal mixing manages and keeps Puget Sound breathing and happy,” McPhee-Shaw said. “Without it, we would just have relatively stagnant water.”

On Wednesday evening, Dec. 10, McPhee-Shaw and Kastner quickly assembled a team to go out on the water, observe firsthand, and collect measurements. Fall quarter was ending, so they had a little free time and flex in their schedules.

The next morning, they were joined by Amelia Bourne, a research technician in the WWU Hydrodynamics Lab who received a bachelor’s degree in Marine and Coastal Science from WWU in 2025, and Marisa Viola, a graduate student in the Marine and Estuarine Science program, who is advised by Kastner. The group boarded the R/V Zoea navigated by Shannon Point Marine Center’s Research Vessel and Diving Safety Officer Nate Schwarck.  

The research vessel rounded Fidalgo Island counterclockwise from Shannon Point, traveled through Deception Pass and then into the Whidbey Basin.

At waypoints along the route, the team deployed a CTD (Conductivity, Temperature, and Depth) instrument at the surface and at various depths to measure salinity.

“If the salinity varied gradually from top to bottom, then that would show that you had a lot of exchange going on,” McPhee-Shaw said. “Whereas if the density profile showed that it was all fresh water on top, like a skinny little lake layered on top of the ocean water, that would mean there’s a weaker exchange.”  

Just before entering Deception Pass, they started to note small changes.

“We’re out there in the open Salish Sea enough to know what these waters look like under normal conditions,” said McPhee-Shaw. “That day, the waters looked slightly browner than usual, and we saw logs and trees float by.”

The color comes from the volume of soil and debris being transported down the river to the sea along with the floodwaters. As they continued east through Deception Pass, the water grew increasingly brown and cloudy.  

“Coming out of Deception Pass, we hit this front, and we were like, oh my gosh, it looks just like muddy Mississippi River water. I’d never seen that before. It was fresh water,” McPhee-Shaw said. “Usually, river water almost immediately interacts with the salt water, becoming mostly ocean-like.”

Kastner said the front itself was remarkable.

“In addition to the completely fresh surface water, we observed obvious changes in water properties by eye from the vessel — estuarine fronts, visible as churning lines of foam and debris that separate pulses of riverine water sucked out of the Skagit River mouth by the tide,” he said. “I am a total ‘front nerd,’ and I have never seen fronts that strong in Whidbey Basin.”

They are still evaluating the data, but their observations showed that river water formed a distinct layer. Fresh water is less dense than salt water, so while this was unusual for the area, it wasn’t unexpected.

The historic flooding in Northwest Washington last December created a unique and ephemeral scenario that they could have easily missed.

“We saw an opportunity and just decided to see if we could pull it off. In science, this is called a ‘rapid response,’” said McPhee-Shaw. “A question arises and you jump on the chance to research something that’s going on right now.” 

Learn more about the College of the Environment’s Department of Environmental Sciences, the Marine and Coastal Sciences (MACS) program, Kastner’s Environmental Hydrodynamics Lab, and Shannon Point Marine Center.

Jennifer Nerad covers Western’s College of the Environment and College of Business and Economics for the Office of University Communications. Have a great story idea? Reach out to her at neradj@wwu.edu.