The busiest corridor from Seattle’s Eastlake neighborhood into South Lake Union will close Monday for 18 months, causing traffic havoc while the city replaces its timber-supported Fairview Avenue North bridge.

The three-lane road crosses a muddy inlet, at such low elevation some drivers may not know it’s a bridge. Timber posts built in 1948 under the west edge are decaying, while concrete columns beneath the inland side show cracks.

The $52 million bridge replacement was scheduled to begin in 2017. The Seattle Department of Transportation (SDOT) blames the two-year delay on environmental and traffic coordination efforts.

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Columns will plunge 125 feet below water level. Tasks were added to remove underwater debris in Lake Union, and remove docks that shade salmon habitat two miles north at Portage Bay Park — to compensate for building the new Fairview bridge wider, said project manager MariLyn Yim.

Seattle consulted with the Muckleshoot Tribe on water and fish effects.

The city took criticism from Eastlake neighborhood advocates for designing a total closure, rather than rebuild only the wood-supported half,  then reinforce the concrete. The state often replaces highways one side at a time, but in this case, the old City Light power plant is only a few feet from the bridge, Yim said, so a two-part rebuild would add cost, bulk and nine months’ time.

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The bridge — the city’s last timber-supported road bridge — serves 8,900 drivers per day, nearly 8,300 riders on the Route 70 bus, some 1,500 bicyclists, and hundreds of walkers and runners, who starting Monday must detour or avoid trips.

Traffic southbound on Eastlake Avenue East to downtown and South Lake Union will be redirected to veer left and stay on Eastlake, parallel to the freeway, instead of dropping toward Fairview parallel to the lake.

If drivers go all the way to Stewart Street, which is already gridlocked at times, that will worsen delays for commuters and deliveries coming from as far as Lynnwood and Everett.

Making things tougher, a biotech building called Alexandria is being built in the muck along Eastlake, where concrete trucks and deliveries will continue to come-and-go among detouring traffic.

To handle buses detouring onto Aloha Street between Eastlake and Fairview, and local traffic to the Seattle Cancer Care Alliance, the city removed some curbside parking along Aloha, while adding a four-way stop to help pedestrians cross and a traffic light at Eastlake and Aloha.

Pass-through motorists might clog Aloha anyway. Yim said she’s asked navigation app companies to avoid directing drivers there. But there are no bus lanes to aid Route 70.

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Business access to lakefront hotels, restaurants and maritime companies will be maintained.

“They’ve been very helpful in making sure it’s not going to affect us,” said Amy Warden, business operations manager at Yachtfish Marine south of the bridge. She said SDOT will maintain boat-worker parking in a city-owned gravel lot, to be fenced and marked.

When the bridge is completed, the 9-foot walk-bike lane on the waterfront side will be widened to a 12-foot bike lane plus an 8-foot sidewalk, to include three pavement bulbs, known as belvederes, where people can pause, lean over the rail and look at the lake. A concrete dock and path at water level, where people rest, jog or launch kayaks, will be returned.

Diahana Vargas, walking Thursday along Lake Union, predicted detour traffic will make life more stressful for neighbors along Eastlake Avenue East, already marked by frustration and traffic jams past the Alexandria construction site.

“I’m kind of teary eyed because it’s going to be my last walk,” she said, after ascending from the dock at water level. “It’s so beautiful and tranquil.”

In other changes this weekend:

Highway 18 will close from Snoqualmie to Issaquah-Hobart Road from 10 p.m. Friday until noon Sunday, for asphalt and guardrail repair.

Interstate 5 at Tukwila will lose three lanes northbound over the Duwamish River, while the Washington State Department of Transportation (WSDOT) replaces old expansion joints, from 10 p.m. Friday until midnight Sunday, followed by a similar closure next weekend.

WSDOT left 1960s-era steel components in place during a repaving job in June 2017, mistakenly thinking parts were still sound. They’ve popped up since then. Highway 509 at Burien provides a detour option, or travelers could board light rail in SeaTac.

West Seattle and Burien buses return Sunday to northbound Highway 99 and First Avenue South. The right lane will be re-striped as a bus lane, weather permitting. That forces general traffic into two lanes, similar to the 2011-18 tunnel-construction era.

Some commuters fear a chokepoint will return, where drivers stop and merge immediately after reaching Highway 99 from the West Seattle Bridge. Earlier this month, viaduct demolition diverted northbound buses to the Sodo busway and Fourth Avenue South. Southbound trips leaving downtown will stay on Second Avenue Extension and Fourth Avenue South until early 2020, Metro says.

King County Metro will add service beginning Saturday to 35 routes, totaling 200 more trips daily. These include routes C, D, E in Seattle, plus late-night service on Route 40 to Ballard, 101 from Seattle to Renton, and 150 to Southcenter and Kent Station.

Seahawks football returns Sunday for a 1:25 p.m. game against the New Orleans Saints. Sound Transit will operate three gameday Sounder trains from Pierce County, and two from Snohomish County. Schedules are at soundtransit.org/seahawks. For drivers, stadium exits from the Highway 99 tunnel returned to normal this week, following completion of viaduct demolition nearby.

Everett’s West Marine View Drive is closed at the waterfront until Thursday, while a new steel-truss Grand Avenue Park pedestrian bridge is lifted, rotated and fastened 36 feet above the street.