Port Susan estuary takes a hit from warming, low Stillaguamish River flow

STANWOOD — The early summer, with its unusually hot days and low snowpack, offers a window into the future for local scientists.

Researchers have turned their attention to Port Susan and the mouth of the Stillaguamish River. Important tidal ecosystems there have been dwindling over the past two decades, and a team is studying how tide marshes and estuaries can adapt to a warming climate. Other researches are investigating how changing river flows affect the way sediment is spread, and how that could harm fish and increase flood risks.

A lot of valuable information can be gleaned from Port Susan and the Stilly this year, said Roger Fuller, a plant ecologist with Western Washington University.

“This summer’s really important to get some data because it’s such an outlier,” he said. “It can really help us understand what’s going to happen.”

Looking at low rivers and high tides this year is a good way to see what experts at think tanks and universities, including the University of Washington Climate Impacts Group, predict will be the norm decades from now, said Eric Grossman, a research geologist with the U.S. Geological Survey.

Overall, local rivers are flowing at less than half the rate they normally would be this time of year, Grossman said. Some are down below a third of what would be expected in June. A gauge in the Stillaguamish near Arlington measured 379 cubic feet of water per second Monday afternoon. The average for that day of the year, based on 86 years of records, is 1,650 cubic feet per second. The river’s record low was in September 1938 at 140 cubic feet per second.

“If we’re already at half of normal flows, and then we have a typical summer with more warming in July, August and September, we’re going to see really low lows, possibly the lowest we’ve ever seen,” Grossman said.

Rivers also are warmer than usual. The Stillaguamish hit just shy of 70 degrees by early June, and the water usually doesn’t get that warm until at least mid-July, he said.

It seems extreme now, but the area is likely to see more summers like this, Fuller said. In the winter, scientists expect more rain but less snow, leading to more rain-fueled floods followed by summers with low snowpack and rivers.

Fuller’s focus is tide marshes and estuaries impacted by seasonal extremes. Tidal ecosystems like Port Susan are some of the most productive but least adaptable, he said. Low freshwater flows lead to saltwater intrusion in the marshes, he said. The salinity can hurt plants and animals.

“The dynamics of what’s happening in the marshes are pretty complex, so salinity is just one of the things we’re trying to sort out,” he said.

Marshes also degrade when snow geese flock to the area and feed, and when floods wash through and erode soil.

There were an estimated 1,064 acres of tide marshes around Port Susan in 1990. By 2011, there were 792 acres. A restoration project in 2012 that moved a dike near Hatt Slough added 150 acres, Fuller said.

“But we’re still losing tidal marsh at the north end of the bay at a pretty rapid rate,” he said.

Fuller is looking into options for preserving and restoring the habitat. Grossman is focused on how changing river flows impact the way dirt and debris carried by rivers gets distributed, and how that can increase flood risks.

“The bottom line of all of this is our land use practices,” Grossman said. “Since the 1850s or so we’ve used dikes and levees to protect our land from flooding… but what it’s also done is focus all the rivers like a firehose.”

Sediment has piled on tideflats, extending river deltas farther into the sea. Meanwhile, higher sea levels and lower rivers mean saltwater can push farther inland at high tide, eroding coastal slopes that help floodwaters drain in the winter.

Grossman leads USGS research on coastal habitats and large river deltas. His project studies the Stillaguamish, Snohomish, Skagit, Nooksack, Nisqually and Skokomish rivers.

“We have some information during the last several years suggesting there’s going to be a lot more sediment coming down our rivers with climate change,” he said. “Floodplain managers and public works directors are already struggling with flood hazards.”

Fuller and Grossman hope their studies can help with decisions about development and restoration projects to preserve habitat and protect against flooding. They agree more research is needed.

The rest of the summer is a golden opportunity.

“There’s a need to protect our communities, and we’re trying to figure out how to do that,” Grossman said. “If we want a glimpse into the future of what river flows will be like and what temperatures in the water will be like, this summer is it.”

Kari Bray: 425-339-3439; kbray@heraldnet.com.