Pool level at Howard Hanson Dam set to reach 1,167-foot benchmark

The reservoir at Howard Hanson Dam will reach yet another benchmark elevation — 1,167 feet above sea level, likely this past Sunday.

Since April 21, when the reservoir reached the benchmark of 1,147 feet, the U.S. Army Corps of Engineers has conducted intensive monitoring, including scientists on the ground around the clock. Since early March, the Seattle District of the Corps has been slowly filling Howard Hanson Dam’s reservoir for its annual conservation pool.

During the conservation pool, the Corps has been testing and conducting in-depth investigations of the dam’s current effectiveness. Depending on what engineers, geologists and other scientific experts on the ground witness during the reservoir fill, the Corps may elect to fill it slightly higher to more fully test the merit of the interim seepage barrier, also known as a grout curtain. During the fill, if anything is deemed as unsafe, the reservoir level can be reduced appropriately.

Initial testing results indicate that water levels at some locations between the grouted seepage barrier and the drainage tunnel are lower than last year, suggesting that the grouted seepage barrier is reducing flow through the north abutment. However, this data is preliminary and should be considered provisional, as it has not been fully analyzed.

The Corps operates the Howard Hanson Dam project, which provides flood damage reduction to the Green River valley, as well as low-flow augmentation to the Green River for environmental benefits. More details and history about Howard Hanson Dam and the right abutment issue can be found at

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