Tillamook County
Tillamook County has two (2) high-hazard dams, Barney Reservoir, located at the headwaters of the North Trask River, and the McGuire Dam, located at the headwaters of the Nestucca River… In the event of a Cascadia Subduction Earthquake, both dams have a very high probability of failure.
Tillamook County Hazard Analysis
Coos County
A recent seismic engineering investigation of the [Lower Pony Creek] Dam completed by a geotechnical consulting engineering firm identified a loose sand layer below the dam. It is likely this material may liquefy in a Cascadia Subduction Zone Earthquake. OWRD is currently doing an all-risks assessment to compare this dam to other HHPD-eligible dams in the state. Based on the preliminary investigation, there is a reasonable likelihood that the dam could fail in a Cascadia Earthquake… Under current conditions, failure appears to be likely in such an event and would result in catastrophic loss of life and also loss of all water supplies for 25,000 people.
Coos County Multi-Jurisdictional Natural Hazards Mitigation Plan
Marion County
Total failure of the Detroit Dam, with a full reservoir, would cause the smaller Big Cliff Dam to fail also and would inundate Gates, Mill City, Stayton, Aumsville, Turner, and Jefferson, as well as cause significant portions of Salem and Keizer to be flooded. Failure of the Detroit Dam is considered very low; however, if it were to fail, 65-70,000 people would be at risk, and damages could well exceed $500 million dollars… The County’s population has 326,110 residents covering 1,182.33 square miles.
Marion County Emergency Operations Plan (EOP)
In 2020, an updated seismic hazard analysis was completed for Detroit Dam to better understand the potential earthquake ground motions at the site. This hazard study has been used to analyze the performance of the spillway gates and found the risk to be higher than previously assessed. The performance of the spillway gates in an earthquake is a function of the potential ground motions due to an earthquake, the height of the gates above the dam foundation, and the water level acting on the gates at the time of an earthquake. Structural analysis of the spillway gates has shown there is a possibility for buckling of the spillway gate’s supporting arms resulting in an uncontrolled release of water from the dam.
US Army Corps of Engineers: Detroit Dam IRRM Pool Restriction
Curry County
The [Ferry Creek] dam is owned by the City of Brookings on the south coast. Based on a geotechnical investigation conducted as part of a feasibility study, the dam will experience severe deformation in a Cascadia subduction earthquake.
Curry County Multi-Jurisdictional Natural Hazards Mitigation Plan
Surviving Cascadia 