Erik Gerking, the Port of Everett’s director of environmental programs, walks along the separation dike on the east side of I-5 on Monday. The port, working with a private company, has been stockpiling clean fill dirt for an upcoming habitat project in the Snohomish River estuary. (Andy Bronson / The Herald)

Erik Gerking, the Port of Everett’s director of environmental programs, walks along the separation dike on the east side of I-5 on Monday. The port, working with a private company, has been stockpiling clean fill dirt for an upcoming habitat project in the Snohomish River estuary. (Andy Bronson / The Herald)

Mystery mounds along the freeway explained!

Piles of fill dirt east of I-5 north of Everett are for a dike that will enable new estuary habitat.

EVERETT — It’s hard to miss the mounds.

The huge ones. They rise up from the floodplain east of I-5 between downtown Everett and Marysville.

At least a couple of stories high, they’re about a quarter-mile apart and 150 feet wide. The two most obvious piles of dirt have puzzled more than a few passersby.

With steep slopes that level off to a flat tabletop, they look a little like giant motocross jumps. Or perhaps part of a future freeway off-ramp to nowhere.

They aren’t. We can debunk those theories.

The Port of Everett, working with a private company, has been stockpiling the clean fill dirt for an upcoming habitat project in the Snohomish River estuary. It’s similar to restoration efforts the Tulalip Tribes, Snohomish County and the city of Everett have carried out to either side.

Like those governments, the port project would flood farmland that’s been kept dry by dikes built in the early 20th century.

“Water will pass through this property during the tide cycles,” said Erik Gerking, the port’s director of environmental programs. “Because of the proximity of this property to I-5, those dirt mounds are the beginning of the construction of a protective dike that will prevent the water from affecting the I-5 corridor.”

After building a new 4,000-foot-long dike alongside the freeway, crews would breach the old earthen berm fronting Steamboat and Union sloughs. Four holes would allow brackish water to seep in, altering about 350 acres. The land used to be Biringer Farm, where many locals have memories of picking berries or heading out to the pumpkin patch.

The port calls its project Blue Heron Slough. Similar to its counterparts, the goal is to restore habitat that gives juvenile salmon a more gradual transition from fresh to salt water. Above all, they aim to support runs of endangered Chinook salmon, which is also the main food source for critically endangered Southern Resident orcas.

Combined, the various Snohomish estuary restoration efforts total about 1,300 acres. They’re part of a recovery plan for Puget Sound Chinook developed after the fish was listed as threatened under the Endangered Species Act.

What makes the port’s piece of the estuary different is that it would act as a mitigation bank. The port or other developers could use credits in the bank to offset the environmental impacts of construction projects to wetlands or other aquatic habitat.

Wildlands is the port’s partner for the work. The company, headquartered near Sacramento, California, operates wetland banks up and down the West Coast. It’s taking charge of major components of Blue Heron Slough, such as permitting, construction and running the business side of the mitigation bank. Wildlands also owns a small piece of the land.

The Tulalip Tribes will hold the permanent conservation easement.

The site covers the upper portion of Spencer Island. Snohomish County is preparing for similar work on adjacent property known as Mid-Spencer Island.

The port and Wildlands expect the first phase of their project to unfold in the coming months, as the masses of dirt get turned into the new barrier along I-5.

“The dike will be constructed ideally throughout this summer,” Gerking said. That work could continue into 2020.

Building the new structure will require an estimated 200,000 cubic yards of dirt — equivalent to roughly 20,000 dump-truck loads. Only about a quarter of that amount is there now.

“There’s a lot more dirt coming,” said Lisa Lefeber, a deputy executive director with the port.

After dike construction, the heavy-equipment crews would contour the site’s interior to mimic natural tidal channels. Invasive flora would be taken out and the area replanted with native species, which will be monitored long-term.

Then comes the water.

“The (old) dikes will be opened up to the sloughs in the 2021 to the 2022 time frame,” Gerking said.

The port bought the Blue Heron Slough property from the Biringer family in 1993 for $2.7 million, according to newspaper stories at the time. The berry farm hosted its last major events there about a decade ago, though the family continues to operate a farm in Arlington. Dianna and Mike Biringer remain on Spencer Island through a caretaker agreement with the port that is set to end soon.

Noah Haglund: 425-339-3465; Twitter: @NWhaglund.

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