Chehalis Basin Strategy in review: Replacement dam on Mill Creek still bringing flood protection for 200-plus homes and businesses


Editor’s note: This article is part of an ongoing series highlighting projects that have been completed as part of the Chehalis Basin Strategy. These projects are implemented in cooperation with partners such as the Chehalis River Basin Flood Authority, the Confederated Tribes of the Chehalis Reservation, the Quinault Indian Nation, state agencies, local governments, nonprofit organizations, landowners and the Aquatic Species Restoration Plan team. The Chronicle has partnered with the Office of Chehalis Basin for this series. Read previous installments at

Mill Creek flows through the City of Cosmopolis in Grays Harbor County, meeting with the Chehalis River north of the city. Mill Creek Park, within walking distance of the city, is a 39-acre popular green space for the community, and used to include a large pond held by a dam on the creek.

The area was already prone to flooding. Then, in 2008, a major flood wiped out the 1930s-era dam on the creek. The pond inside the park spilled over, causing significant flood damage to the park and the city. Now, an updated, modern dam completed in 2018 is helping communities be better protected from flooding today and in the future.


The problem

In November 2008, torrential rains wiped out an earth and concrete dam on Mill Creek in the City of Cosmopolis in Grays Harbor County, an area already prone to flooding. After the dam collapsed, the pond inside the city’s Mill Creek Park emptied, causing significant flooding in the popular park and throughout Cosmopolis.

The Mill Creek Dam was originally built around 1913 and updated in 1970. The mostly earthen dam blocked fish passage, and afforded some flood relief for the homes that line the creek through Cosmopolis. After the dam collapsed in 2008, Mill Creek Park had to close, and the city sought funding for a new dam to help control flooding, enhance the picturesque park, and also support habitat for salmon and other aquatic life.


The project

In 2015, the City of Cosmopolis received nearly $3.1 million from the Chehalis River Basin Flood Authority and the state Recreation and Conservation Office to replace the dam and foot bridge over the structure.

“We knew we couldn’t just replicate the old dam. We needed a dam that would stand the test of time and help protect our community,” said former Cosmopolis City Administrator Darrin Raines. “We also wanted to make sure it fit in aesthetically and made environmental sense.”

Before the new concrete, steel and earthen dam was constructed, Raines said the city worked with the Quinault Indian Nation and Confederated Tribes of the Chehalis Reservation regarding the structure’s design and purpose.

The new dam was constructed during summer 2017 by Quigg Bros, Inc., using authorization from the Washington state Department of Fish and Wildlife to work in state waters. Construction crews secured steel base supports by drilling 30 feet into bedrock every 18 inches. The structure’s 3-foot thick wall is made up of a combination of concrete blocks and earth. Interlocking steel plates of sheet pile are anchored 40 to 60 feet down, with any seepage channeled through sand to a drain.

Cosmopolis also worked with the Federal Emergency Management Agency (FEMA), the U.S. Army Corps of Engineers, and the Washington State Department of Ecology, the agency responsible for regulating the dam.


The impact

Finished in September 2018, the new Mill Creek Dam now protects 130 homes and businesses in Cosmopolis and another 90 homes in south Aberdeen from flooding. One of these businesses is the Grays Harbor County Road Maintenance Shop, which stores critical equipment for the maintenance of roads in the central part of Grays Harbor County.

While the old earth and concrete dam blocked anadromous fish from reaching spawning habitat in Mill Creek for more than 100 years, the new dam has flood control gates, a fish ladder, and several weirs to accommodate migrating fish. The new dam also has a viewing platform for watching returning salmon.

The design of the new dam and its fish ladder opened up an additional two miles of accessible fish habitat. Chum salmon, coho salmon, and resident trout are among the species who utilize the creek. After the new dam was complete, construction crews first spotted returning coho salmon, then chum salmon. Today, the city is working on a grant to correct undersized culverts further downstream to support ongoing fish passage improvement for Mill Creek, and there is still ample opportunity for habitat improvements.


The Chehalis Basin Strategy is a network of partners and projects dedicated to protecting communities from flood damage, restoring critical habitat for aquatic life, and ensuring the Chehalis Basin is safe and prosperous for people, fish and wildlife for generations to come.

The strategy, through the Department of Ecology’s Office of Chehalis Basin, has invested nearly $152 million in the basin and completed more than 140 projects, with dozens more underway.

The strategy is led by the Chehalis Basin Board, a group of representatives with diverse interests and perspectives from across the Basin and is administered and funded by the Office of Chehalis Basin. It depends on its many partners across the Chehalis Basin and the state to inform the path forward and bring projects from ideas to reality.