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How to mitigate the impact of a new 520 bridge on the Arboretum?

A kayaker paddles through the waters of Union Bay near the west end of the 520 bridge in the Washington Park Arboretum.
A kayaker paddles through the waters of Union Bay near the west end of the 520 bridge in the Washington Park Arboretum.

Unless a lawsuit derails the process, a new 520 bridge will soon be built across Lake Washington. 

A company in Aberdeen is already constructing the huge pontoons that will keep the new, 6-lane structure afloat.

And the state is widening the highway on the east side of the lake. 

But exactly what the project will look like on the Seattle side is still being worked out.

Seattle's Board of Park Commissioners will get a briefing on impacts to the Washington Park Arboretum tomorrow night (Thursday).

Michael Shiusake has been immersing himself in possibilities lately. He's a planner with the Seattle Parks Department and he's just finished writing a new paper about the impact of the state's 520 replacement project on what he calls one of the city's historic jewels. He says removing the massive concrete ramps built at the North end of the Arboretum in the 1960s is a great opportunity for environmental restoration.

"That has the potential to create some significant change in terms of views into and across the Arboretum, also: grading could be changed significantly, the historic alignment of Lake Washington Boulevard could also be changed."


The Olmsted brothers' century-old legacy

That winding road with its decorative bridges and the tree collection it showcases is one of the distinctive features of the park created in the era of the famous Olmstead Brothers. In 1903, the New York landscaping duo came to Seattle and spent more than three decades surveying and designing parks here.  

Paige Miller, with the Arboretum Foundation has been watch-dogging a mitigation plan to protect the park and its ecosystem for four years, as the state forges ahead with a new 520 bridge:

"It is an Olmstead-designed park, but it is also a unique place for growing trees.”

She says the 6 lanes of the new bridge will allow for a wider highway though the park than they had fought for, with more traffic. But it'll be farther north than the existing structure, freeing up open space for kayakers and trail-users, allowing for restoration of wetlands, and maybe even someday allowing salmon to return to the creek that runs along Lake Washington Boulevard. How people will be able to see all of that happening is one of the questions they’re facing now.

"'With the ramps going away, you won’t be able to go up above the arboretum and look down on it, the way people have been doing for the past 30 or 40 years. So, how do we create viewpoints, to look into the wetlands and to look out over the Arboretum. It's that kind of thing that we're working on now."

But she adds there's no funding yet for the Seattle side of the bridge – or the mitigation plan. And she says a lawsuit challenging the final Environmental Impact Statement is likely. 

Still, she hopes the public will come out and give feedback on the preliminary designs for the new open spaces three weeks from now, on June 8th. 

On the web:

University of Washington's Arboretum and Botanical Garden Committee 520 Expansion Guiding Principals

CapitolHill blog on the fight against the 520 plan

Washington State Deparment of Transportation 520 bridge replacement website

The Olmsted Brothers' legacy: Seattle Parks & Recreation Website


Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to