Tualatin Basin Water Supply

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Design Concepts

Doing nothing is not an option.

Scoggins Dam has been classified as a seismically at-risk dam. This means that the dam needs to be modified in order to reduce the hazards downstream in the event of a large earthquake. The Tualatin Basin Joint Project is currently exploring three options: two at the location of the current dam and one downstream. Additional alternatives may be explored as part of the National Environmental Policy Act (NEPA) process:

Options One and Two: Existing location

  • Option 1, Modifies the existing dam at its current location without raising the dam height. While this option does not provide increased water storage, it provides needed safety improvements to the existing dam so that it could withstand a major earthquake.

  • Option 2, modifies and raises the dam at its existing location, which would allow for additional water storage. Modifications to the existing dam would allow an additional 17.5 feet of water depth to Hagg Lake. This would provide an additional 21,000 acre-feet of water storage for the region. An acre-foot is the volume of one acre of surface area to a depth of one foot and equals 326,000 gallons. 

Option Three: Proposed new downstream dam

  • Option 3, The third engineering option currently under consideration includes a new dam downstream of the Stimson Mill where the valley is narrower. The dam in this location would stretch approximately 900 feet across the valley compared to 2,700 feet at the location of the existing dam. Because it is downstream, this dam would be able to store more water while not increasing the depth of Hagg Lake. This new concrete dam would provide up to an additional 50,000 acre-feet of storage.

Alternatives will be designed to withstand a major earthquake. Options 2 and 3 will also provide additional water. CWS is working closely with Reclamation's Dam Safety Office to coordinate the engineering and environmental reviews necessary to select a preferred alternative for design and construction.  A preferred alternative is scheduled to be selected in 2019.

Design Options 1-3: Existing Location and Proposed New Downstream Dam