USACE uses strategic safe reservoir drawdowns to protect Green, White River valleys amid winter storm cycles
As Western Washington moves through the peak of flood season, the U.S. Army Corps of Engineers (USACE), Seattle District, is providing critical information on the process of drawdowns at Howard A. Hanson Dam and Mud Mountain Dam.
Recent back-to-back atmospheric rivers have highlighted the necessity of these controlled water releases. Responsible drawdown operations can make the difference between significant flooding and catastrophe for the residents of Auburn, Kent, Puyallup, and Sumner.
Think of Howard A. Hanson and Mud Mountain dams as massive batteries. During heavy rain, they "charge" by capturing thousands of cubic feet per second (cfs) of rushing water, preventing it from overwhelming downstream river systems.
A drawdown is the process of "discharging" that battery. Once a storm passes, USACE water managers must release the stored water as quickly—and safely—as possible to empty the reservoir. This creates the "empty space" (storage capacity) needed to catch the next incoming storm.
“As our water management team directs outflows from Mud Mountain and Howard A. Hanson Dams to maximize flood risk reduction downstream of the dams, we also manage the reservoir elevation behind the dam as it rises with the increased rain,” said Kyle Comanor, the district’s senior western Washington water manager.
“We determine outflow using several factors, including how much rain and runoff are in the forecast, how high and fast the reservoir is filling and how much tributary flow is coming into the river downstream of the dam. We're using the most current and accurate information we have, to make sound flood risk management plans and decisions in both flood and non-flood operations.”
Seattle District manages drawdowns differently at each project to meet specific community needs:
Howard A. Hanson Dam (Green River): The reservoir reached a record 74% capacity Dec. 12. USACE water managers began a strategic drawdown to return the pool to its "seasonal target." By drawing down the reservoir by 26 feet in just a few days, the Army Corps regained the space necessary to absorb the next forecast atmospheric river, protecting the $4.1 billion in economic activity within the Green River Valley.
Mud Mountain Dam (White River): Unlike Howard A. Hanson Dam, Mud Mountain is a "dry bed" dam with a primary goal of remaining as empty as possible. During a drawdown at this location, USACE water managers must also manage sediment and debris. Using specialized tunnels, USACE "flushes" glacial flour (fine-grained, silt-sized rock particles) and wood through the dam to maintain the reservoir’s capacity and ensure the White River’s natural ecosystem remains healthy.
Seattle District water managers carefully time drawdowns so that the total flow at the Auburn Gage (Green River) or Puyallup Gage (White River) does not exceed the capacity of local levee systems. However, sustained high flows in these systems do make for a dynamic environment and can lead to unexpected changes in the river channel. Water managers are routinely evaluating these changing conditions to provide effective flood risk management.
While winter is about emptying the pool, the final drawdown of the season at Howard A. Hanson Dam is timed to transition into "refill" mode by March, ensuring the City of Tacoma has drinking water and the Green River has enough flow for salmon migration in the summer.
During drawdown operations, river levels can rise rapidly even after the rain has stopped.
The public is urged to stay safe and remain vigilant as flood conditions can change rapidly. Monitor real-time reservoir levels at the USACE Northwest Division website (or http://www.nwd-wc.usace.army.mil/nws/hh/www/index.html).
Residents seeking resources or assistance should contact their local government offices.
If forecast or observed conditions change, these operations may also change. USACE advises residents to check https://www.nwrfc.noaa.gov/rfc/ for the status of current operations.
NEWS INFO
Date Taken:
12.19.2025
Date Posted:
12.31.2025 17:13
Story ID:
555475
Location:
WASHINGTON, US
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