SEATTLE -- U.S. Army Corps of Engineers (USACE) officials are seeking public comments on the draft supplemental environmental assessment for the proposed update to Mud Mountain Dam’s Water Control Manual. USACE is poised to finalize the water control manual in October 2026, ahead of the 2026-2027 winter flood season.
The proposed water control manual update has been studied for over five years and is result of a deliberate process to balance the dam’s finite reservoir capacity with changing river conditions to protect downstream communities in the Puyallup River Basin. A water control manual, and the plan within it, serves as the engineer’s handbook, establishing the rules and guidelines for storing and releasing water to meet its congressionally authorized purpose for flood risk management.
USACE Seattle District will host an open house Sept. 1 in Auburn, Washington to inform the public about the district’s efforts to update Mud Mountain Dam’s Water Control Manual, learn how USACE ensures its dams operate safely, and explain how help members of the public can submit comments for the draft supplemental environmental assessment.
A supplemental environmental assessment analyzes a project’s potential impacts on social, biological, and economic resources and guides USACE’s decision-making process.
Under National Environmental Policy Act requirements, the public can submit comments during a 30-day period open from Aug. 18 to Sept. 17, 2026. Comments must be submitted before Sept. 18 to be considered in the final supplemental environmental assessment.
The public can review the draft supplemental environmental assessment online at: www.nws.usace.army.mil/Missions/Environmental/Environmental-Documents/
Additional information about the Water Control Manual update is available on the USACE Project Assistance Library.
Submit comments via email to CENWS-MMDWCP26@usace.army.mil or postage mail to:
U.S. Army Corps of Engineers, Seattle District
Attention: PECRB
4735 E. Marginal Way S., Bldg. 1202
Seattle, WA 98134-2329
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