Amid Pierce County floods, Mud Mountain Dam became key. Here’s how
AI-generated summary reviewed by our newsroom.
- Army Corps of Engineers is controlling five dams to reduce downstream flood risks in WA.
- Mud Mountain Dam, the closest to Pierce County, provides protection for Sumner and others.
- The Corps is trying to limit downstream flood risks but also preserve reservoir storage.
The Mud Mountain Dam, located east of Tacoma just outside Pierce County, is one of five dams managed by the U.S. Army Corps of Engineers to battle historic flooding in western Washington that was easing in local areas Friday.
The flooding this week, fueled by atmospheric-river rains, prompted evacuation orders and warnings, school delays and road closures in Pierce County.
Army Corps of Engineers water managers have been regulating water flows around the clock at two dams the agency owns and operates: the Mud Mountain Dam on the White River in Enumclaw and the Howard A. Hanson Dam further northwest along the Green River.
The Corps is also managing flows at three dams it doesn’t own — the Wynoochee, Upper Baker and Ross dams — under established agreements that allow the agency to take over city- and utility-owned dams during floods. The agency said it assumed control of each dam on Monday.
The balancing act is as follows: Reduce the risk of flooding to areas downstream but also preserve reservoir storage through controlled and strategic water releases. At least some cities affected when dams are alleviated, such as Sumner, have been routinely updating residents on social media about when releases are forthcoming.
“We are using every available bit of storage space in these reservoirs to capture floodwater and lessen the impact on downstream communities, but residents need to prepare for sustained high-river levels,” Kyle Comanor, Western Washington senior water manager for the Corps’ Seattle District, said in a statement Wednesday.
Typically, the Mud Mountain Dam’s reservoir is empty, except for the normal flow of the White River, the Corps says. In a sign of how much rain has fallen, the Corps was prepared Thursday for the dam’s peak pool elevation to potentially reach a record 1,200-plus feet on Friday, “depending on actual inflows and operations.”
Mud Mountain Dam, opened in 1948, is important locally as the only flood-control storage on the Puyallup River System, according to the agency. The dam, which provides flood protection to the lower White and Puyallup River valleys, stores water from heavy rain and snowmelt in its reservoir. The Corps says it slowly releases that water back into the river to maintain storage.
“These operations are critical to reducing flood risk for the communities of Pacific, Sumner and Puyallup,” the Corps said in a news release Wednesday.
On Wednesday afternoon, the dam’s outflow reached a peak of 8,480 cubic feet per second (cfs), the highest level seen in recent years, the Corps told The News Tribune.
Determining how much water to release from a dam’s reservoir, and when, is a process that begins much farther north with the Ross and Upper Baker dams in Whatcom County. The Corps’ Seattle District’s Reservoir Control Center starts its work eight hours before an unregulated discharge of 90,000 cfs is forecast for the Skagit River near the town of Concrete.
The Corps “uses forecasts of precipitation and river flows throughout the river basin to determine how best to use reservoir storage space to reduce downstream flow to non-damaging levels in as much as is possible,” the agency said in a statement.
Runoff from unregulated rivers and creeks in the basin is the most significant factor contributing to downstream floods, according to David Doll, chief of the Water Management Section for the Corps’ Seattle District.
“Forecasts for these uncontrolled portions of the basin, such as the Sauk River, are what’s driving the extraordinary flood stages predicted in the lower Skagit Valley,” he said in a statement Thursday.
Beyond building the Mud Mountain Dam, the Corps says it made channel improvements and constructed levees on the lower Puyallup River. The structures collectively have saved an estimated $308 million in flood damages and help to protect the homes and businesses of about 400,000 people, according to the agency.
Several leaks in the levee near Rainier Manor Mobile Home Park in Sumner prompted the city’s Police Department on Thursday to issue a notice for the park’s residents to immediately evacuate, The News Tribune reported. Tenants of the Crossroads at Meridian apartment complex in Puyallup received a similar notice due to flooding from the Puyallup River. By Friday morning, evacuation notices had been lifted in both cities.
The city of Sumner said in a post on X shortly before midnight Friday that the Puyallup River’s levels were declining. A flood watch remained in effect in Pierce County through Friday afternoon, according to the National Weather Service.
