A new channel south of Northgate Mall in Seattle will filter sediment and clean the water of Thornton Creek. A parking lot used to cover this 2.7-acre creek channel.

Share story

Dragonflies alight and swoop delicately from plant to plant. For biologists and project managers alike, the presence of these insects, which are sensitive to pollution, indicates a healthy ecosystem.

That wasn’t always so across the stark, paved grounds of Northgate Mall. Polluted runoff from the parking lots and neighborhood streets had nowhere to go except directly into Thornton Creek, a stream that weaves through Shoreline and North Seattle and empties into Lake Washington at Matthews Beach Park.

But with enough community pressure and compromise, change can happen.

A large, paved lot once devoted to overflow mall traffic and RV parking has been replaced with a landscaped, open space that allows the beginnings of Thornton Creek to flow above ground for the first time in decades. Before, a large underground pipe diverted the water to an outfall several blocks away.

This project now lets water in the creek’s south fork flow as it should above ground and nourish its new stream bed before exiting under Fifth Avenue Northeast into the existing creek.

“If you look at it 10 years from now, it will be the defining development that helped transform the whole neighborhood into a different place,” said John Lombard, a Northgate resident with Thornton Creek Alliance.

Seattle Public Utilities recently completed the stream-restoration channel as part of a new development that brings more than 100 condos, 278 apartments, senior housing, a 14-screen movie theater and more retail space to the North Seattle neighborhood.

Lorig and Stellar Holdings say they’ve rented about 50 of the apartments, which exceeds their goal to date. The market has been slow for the condos, however, with only one unit sold, said Stephen Holt, partner at Lorig in charge of the project.

Still, Holt said he’s hopeful that as more people discover the area, the condos will find buyers. He said the open space and creek will become a place to gather within the new development.

“I think it is going to put a mark on that as a place, as a neighborhood, that’s not just identified by a shopping mall and traffic problems.”

The new creek channel is seen by many as the aesthetic glue holding together Northgate’s new development.

It’s also the key compromise that allowed the building project to move forward after residents strongly opposed an option that didn’t include stream restoration, said Janet Way, a Shoreline City Council member and president of the Thornton Creek Legal Defense Fund, an advocacy group.

“It took incredible courage for everybody to get on board,” Way said.

The new channel spanning nearly 3 acres helps to filter and clean almost all runoff from more than 600 acres around the project, mostly from neighborhood streets.

It also stores and slowly releases water to Thornton Creek, providing a holding place during heavy rains.

Seattle Public Utilities designed the channel to naturally filter pollutants in stormwater runoff from streets and parking lots.

Runoff enters the channel on one end, then flows through four chambers. Small rocks and sediment settle and can be removed by the city using trucks with large vacuums. Aquatic plants help to capture and remove pollutants, similar to how a wetland functions.

The water exits the channel into free-flowing Thornton Creek at Fifth Avenue Northeast and Northeast 103rd Street.

Before, all the runoff carrying pesticides, oils and other pollutants was flushed through a pipe and discharged into the creek at the same intersection, said Nancy Ahern, Seattle Public Utilities deputy director for utility-systems management.

“Ultimately, it was the best design solution for what we were trying to do,” Ahern said. “The city needed a project that provided benefit to the creek. A do-nothing solution wasn’t acceptable.”

The city started construction on the $14.8 million creek-channel project about a year and a half ago. Young native plants dot the landscape, and project managers expect vegetation to fill more of the channel and surrounding walls within a year.

The final outcome is acceptable for most supporters of Thornton Creek’s restoration. Years of development have encroached and narrowed the stream, causing faster water flow and more stream-bank erosion.

As Northgate and Shoreline have developed, more paved surfaces give stormwater fewer places to naturally sink into the ground, said Dan Mahler, president of Thornton Creek Alliance.

The nearly 12-square-mile Thornton Creek watershed is the largest in Seattle and Shoreline, and about 67,000 people live in homes that send pollutants into the watershed. Coho salmon have been seen in the creek and Mahler recently saw a cutthroat trout in the stream behind his Northgate home.

“Before this (channel), you wouldn’t know there was a creek there at all,” he said. “What we’ve got here is a great start. It’s serving a very vital function in delivering clean water to the south fork.”

Michelle Ma: 206-464-2303 or mma@seattletimes.com