Major changes ahead for I-5 at Nisqually River delta. County hears plan
People commuting between Thurston and Pierce counties may eventually see major changes to Interstate 5 at the Nisqually River.
The proposed changes would affect a 4.7-mile stretch between the Marvin Road Southeast and Mounts Road Southwest interchanges, according to the Washington State Department of Transportation.
This key passage is a choke point for commuters and commercial trucks. It also runs over the ecologically important Nisqually River delta, home to the Billy Frank Jr. Nisqually National Wildlife Refuge. The already multi-million-dollar project is still in an early planning phase, so construction is likely years away. If completed, WSDOT hopes the project will enhance traffic flow, make the roadway more resilient and restore natural habitat in the delta.
“This project really creates an opportunity to improve transportation and restore the river and estuary habitat at the same time,” Alex Atchison of Parametrix told the Thurston County Board of Commissioners about the project during a May 6 work session.
How will the project improve traffic?
Atchison is the project manager for the consultant team supporting WSDOT. She told the board I-5 needs more capacity in the area due to projected traffic volumes.
“I-5, as you know, is already reaching capacity during peak periods, and congestion is only forecasted to get worse,” Atchison said.
Traffic is forecasted to grow upward of 30% by 2045, Atchison said.
“And there’s currently no safe or direct access for people biking or walking across the Nisqually River delta,” Atchison said.
The proposal includes replacing bridges over the Nisqually River and widening I-5 to add one high-occupancy vehicle (HOV) lane in each direction between exits 111 and 114, according to a WSDOT presentation. There are also plans to construct a new grade-separated crossing over the railway tracks east of the Nisqually River.
The proposal includes a shared-use path for bicyclists and pedestrians that spans from Marvin Road in Lacey to Center Drive in DuPont, Atchison said.
The path will be 14 feet wide, run adjacent to the highway, and have connections at each interchange to the local street network, Kirk Wilcox of Parametrix added.
How will the project benefit the environment?
The agency hopes new bridge designs will make the highway more resilient to environmental challenges.
“This corridor is extremely vulnerable to flooding, to migration of the Nisqually River and aging infrastructure,” Atchison said.
The northbound Nisqually River bridge was built in 1937, Atchison said, and the river channel has been migrating upward of 35 feet per year for the last 20 years. The migration is threatening the embankments and bridge stability of the highway, she said.
“As we all know from the past Amtrak derailment in 2017, there are no viable detours in this area, so short closures have major regional impacts,” Atchison said.
Crews built the current I-5 on a series of berms and embankments in the 1960s, Atchison said. The set-up has blocked the natural flow of the river and tidal movements, she said.
“This has significantly reduced habitat quality in a key estuary for Chinook salmon and steelhead,” Atchison said.
Under this proposal, Atchison said crews would realign McAllister Creek where it crosses below I-5 to “improve tidal exchange, water quality and habitat.”
Additionally, crews would remove two existing fish barrier passages, install facilities to treat stormwater runoff from I-5 and create new habitat in areas where the existing I-5 embankment would be removed.
WSDOT considers options for bridges, alignment
Wilcox, who is the engineering lead for the project, said WSDOT is considering two bridge options.
One is a 6,000-foot-long bridge that would open up the eastern side of the delta, Wilcox said. The other is a 12,000-foot-long bridge that would allow the removal of all the embankment across the valley, he said.
“Either of these options allows us to remove a tremendous amount of fill from the flood plains and wetlands, and really restore the ecological connections across the I-5 corridor,” Wilcox said.
WSDOT is also considering two different bridge alignments near the wildlife refuge. One alignment would go over the existing I-5 right-of-way but require a “massive detour” for I-5 to continue running during construction, Wilcox said.
The other option involves shifting the alignment onto adjacent Nisqually Indian Tribe property.
“That would allow us to build the new southbound bridge offline from existing traffic, and so it would speed up the construction considerably and reduce our temporary detour impacts,” Wilcox said. “We figure it would save about two years off the length of the project.”
How far along is the planning process?
WSDOT completed a federal Planning and Environmental Linkages (PEL) study in 2023 to determine the purpose and need for the project. The final PEL report can be found on the project website.
The project is currently under an environmental review process outlined by the National Environmental Policy Act (NEPA), according to the project website.
As part of that process, WSDOT is conducting an environmental assessment to study potential design options, identify environmental impacts and identify potential strategies to mitigate those impacts.
The NEPA process started in summer 2024, and a draft NEPA report is scheduled to be released in late summer, Atchison said. This will be followed by a public open house and public comment period, she said.
A final environmental assessment decision document is due around late 2027.
No construction has been planned yet, the website states. WSDOT lists the timing for the project as 2023-2029.
Cara Mitchell, WSDOT spokesperson, did not immediately respond to questions about the timeline and cost for construction. Susan Melnyk, Thurston County spokesperson, said the county did not yet have that information either.
No information about a future construction phase was present on the website. Proposed fixes could cost billions of dollars, The News Tribune previously reported.
The state Legislature appropriated $9.6 million between 2021 and 2023 for preliminary engineering and to complete the NEPA review for a proposal, according to the project website. The Legislature also approved $22.4 million for the project that will help pay for preliminary engineering, per the website.
