OSO — Winter weather is expected to be the highway’s first big test.

For more than six months, crews have worked to rebuild and landscape the stretch of Highway 530 between Arlington and Darrington that was buried in the Oso mudslide.

Planners knew flooding would be a threat to the new road. The March 22 slide, which killed 43 people and destroyed a neighborhood, also altered the course of the North Fork Stillaguamish River in ways that remain to be seen.

In an effort to prepare for the river’s unpredictable behavior, crews elevated the road and added new culverts and drainage. Nearby slopes were tapered down, rock barriers installed between the road and the river, and plants put in the ground with roots meant to clutch the soil and soak up water.

“We’ve been making sure as we opened the road that there’s no more movement on the slopes,” said Travis Phelps, a spokesman for the state Department of Transportation. “Those plants are really going to help pull things together and hold that soil.”

Planting thousands of trees and shrubs near the highway is the last piece of the project. Contractors expect to be done in December, weather permitting, Phelps said.

Construction on the road itself wrapped up in September. The new highway is about five feet higher than the previous road on its east end, and up to 20 feet higher on the west end. Six culverts were installed to help reroute water during a flood.

The state is working with the county to monitor the Stillaguamish River’s depth in the slide zone and farther afield. Sensors have been placed along the river to keep track of water levels after drizzles and downpours.

“We’re seeing the Stillaguamish River rise and change, and we’re watching how that affects the land around the road,” Phelps said. “It’s one of our hot zones still. We’re keeping a close eye on it.”

Planners also wanted to lessen slide risks along the new roadway. Crews hauled about 7,500 dump trucks of earth away from “the knoll,” a sloped outcropping on the south side of the road about halfway through the slide zone, Phelps said. This allowed workers to taper the slope so it wasn’t as steep. They installed 25 drains that stretch 150 feet into the hillside to release water so it doesn’t build up in the soils and cause the slope to sag or slide.

A flood warning has already been issued this fall along the Stillaguamish River. Historically, Snohomish County’s flood season peaks between November and February.

Rainstorms during the last couple of weeks shifted some of the top soils along Highway 530 that had been loosened for planting, but the weather so far hasn’t significantly affected the land or river near the highway, Phelps said.

“It’s so far, so good,” he said.

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