Construction of a $15 million road-connector project in Skagit County is being delayed by returning salmon, whose spawning would suffer if the project put too much extra dirt into Maddox Creek.

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MOUNT VERNON — Maddox Creek once took a rollicking path from the Mount Vernon foothills to Skagit Bay. Decades of residential growth and agriculture have tamed it — but not the coho salmon that call it home.

Driven by instinct, the red salmonids have tried for more than 60 years to surpass a mighty barrier: a culvert under Blodgett Road that prevented their upstream migration. Generation after generation, the salmon deposited their eggs in less-than-ideal conditions.

In October, that restrictive culvert was replaced with a fish-friendly box culvert, thanks to a $15 million road-connector project. When done — likely next summer — the road could lift up to 25 percent of peak-hour traffic from College Way and give residents another path to Interstate 5.

What no one expected was how quickly salmon would return to lay claim to their former home.

The salmon have caused at least a month’s delay in the bridge’s construction. To install the bridge over Maddox Creek, the county has to drive 24 pilings into the ground deep enough to support the 80-foot-long bridge and the traffic it will carry. Last month, the contractor drove in a test piling.

Once the piling reached a certain depth, each impact of the pile driver shook the stream bed and loosened enough dust particles for the state Department of Fish and Wildlife to shut down the operation, said Ron Smith, Exeltech bridge-inspection engineer. Extra dirt in the stream could smother sensitive salmon eggs, and salmon had already spawned in the job site.

This sent engineers back to the drawing board about how to drive in the piles without harming habitat, said Skagit County Engineer Paul Randall-Grutter. Next, the crew will use a lighter weight to drive the piles in hopes that the vibrations will be less disruptive for the fish.

“If the lighter hammer doesn’t work, we can start other methods,” Randall-Grutter said.

In a third method, the county could cut the solid end off of the piling and drive it into the ground, which would require even less force to drive it to the required depth.

Workers would then auger the dirt out of its hollow center.

If that doesn’t work, construction might have to wait until mid-June, when salmon are not in the creek, said state Fish and Wildlife biologist Jeff Kamps.