A FlatironDragados-Aecon Joint Venture, which includes Aecon Group Inc. (40%) and FlatironDragados (60%), has finalized a US$691 million contract with the U.S. Army Corps of Engineers (USACE) to deliver the Additional Water Storage Fish Passage Facility project at the Howard A. Hanson Dam in Washington State.
Located in the Green-Duwamish River basin in King County, near Ravensdale southeast of Seattle, the dam plays a central role in regional water management, providing flood risk reduction, municipal water supply and seasonal flow regulation for aquatic ecosystems. It serves as a key line of defence against flooding in the Green River Valley by capturing and regulating runoff from the upper basin, and has historically prevented significant flood damages in the region. Flood damage prevented during the January 2009 event has been estimated at approximately US$4.6 billion.
The project will be delivered under an integrated design and construction model, bringing designers and contractors together from early stages to enhance coordination, mitigate risks and improve cost and schedule certainty. Construction is expected to begin in the second quarter of 2026, following completion of the development phase, with delivery scheduled for the third quarter of 2031. Aecon stated that its share of the contract will be included in its Construction segment backlog in the first quarter of 2026.
The initiative combines infrastructure upgrades with environmental restoration objectives. Planned works include a downstream facility with vertically stacked intake ports, as well as a multiport collector, a steep-slope bypass, a deceleration tunnel and energy dissipation structures. These elements will enable downstream-migrating salmon to bypass the dam and reconnect with spawning habitats along the Green River, while maintaining flood control functions.
“We are excited to start construction on the Additional Water Storage Project at Howard A. Hanson Dam,” said Kathryn Sanborn, Seattle District Commander at the U.S. Army Corps of Engineers. “Howard A. Hanson Dam serves as our primary line of defense against catastrophic flooding in the Green River Valley, holding a record pool and decreasing peak flood by over 5 feet during the record-breaking flows in December 2025. This project significantly expands our water storage, ensuring a reliable supply for thousands of homes and supporting the future economic growth of the region, while also providing salmon access to nearly half of the Green River habitat above the dam.”
