ARLINGTON — Commuters should expect traffic delays on both I-5 Stillaguamish River bridges this summer, according to the state Department of Transportation.

By mid-July, WSDOT expects contractor Mowat Construction to detour drivers from the southbound Stillaguamish River Bridge onto the west side of the northbound bridge.

WSDOT Assistant Regional Administrator Todd Harrison explained that two lanes of traffic would be maintained for each direction on I-5 over the Stillaguamish River.

Mowat will have a window of 120 days to replace the southbound bridge deck, and some of the steel frame underneath, with an incentive to finish early.

This four-month window is set to start sometime after the Fourth of July, so as not to interfere with its festivities, but it is expected to create delays after that.

During the $8.7 million construction project, Morrison warned that northbound I-5 would be most likely to be congested Thursday and Friday afternoons, and midday on Saturdays and Sundays, while Sunday afternoons are expected to be packed on southbound I-5.