Project Description
Detroit Dam is located on the North Santiam River 48 miles upstream the confluence with the Willamette River and 13 miles upstream of Mill City, Oregon. Detroit Dam is a 450 ft tall, 1,450 ft long concrete gravity dam, comprised of 32 monoliths, with a gated spillway with six spillway gates, four regulating outlets, and a powerhouse. Detroit Dam is owned, operated, and maintained by the Portland District of the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1949 and was completed in 1953. Detroit Dam is part of a system of 13 multi-purpose dams in the Willamette Valley with the primary purpose of flood risk management and secondary purposes of hydropower, recreation, irrigation, municipal and industrial water supply, fish and wildlife, and water quality. Collectively, this system of dams is referred to as the Willamette Valley Project.

During the winter months, the Willamette Valley Project reservoirs are maintained at their lowest elevations to allow for the temporary storage of rain and snow melt. When managing high flow events, the outflow from the system of dams is coordinated to reduce peak flows and river stages at downstream locations. In spring, USACE begins to fill the reservoirs, increasing the amount stored for conservation purposes and reducing the amount available for flood risk management. During summer, stored water is used for recreation on the reservoirs, and some stored water is released in the river downstream to improve water quality, produce hydropower, support fish and wildlife habitat, and provide water for irrigation and municipal uses. During dry summer months, flows into the reservoirs are generally less than flows needed to meet minimum flow objectives, causing reservoir levels to drop. In fall, stored water remaining in the reservoir is drawn down to minimum levels in preparation for the flood season. Detroit Dam is operated in coordination with Big Cliff Dam to achieve project objectives within the North Santiam sub basin of the Willamette River.

Detroit Dam is located about 95 miles east of the Cascadia Subduction Zone, a megathrust fault along the Oregon Coast. The Cascadia Subduction Zone is capable of producing very large, long duration earthquakes. The last Cascadia Subduction Zone earthquake occurred in the year 1700.

Risk Characterization: Moderate
USACE performs risk assessments as part of an ongoing dam safety program and to assist in the prioritization of investment for aging infrastructure. The risk assessments evaluate the life safety risks associated with the dams to determine if risk reduction actions are needed and, if so, what actions should be taken.  USACE completed a routine risk assessment for the Detroit Dam in May 2018 that characterizes the risk associated with the dam to be Moderate. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The risk associated with Detroit Dam is primarily driven by the potential for a very large earthquake occurring at the same time summer reservoir elevations are the highest. Under these conditions, there is the potential for one or more of the spillway gates to collapse or for a structural failure of the concrete monoliths. In 2020, an updated earthquake hazard study was completed for Detroit Dam to better understand the potential amount of shaking that could occur at the site from a range of all possible earthquake fault sources. USACE has used this hazard study to review the results of the routine risk assessment and found that the amount of shaking from an extreme earthquake could be much greater at the site than originally assessed. Because of the larger predicted amount of shaking, the risks associated with the performance of the spillway gates are higher than originally assessed.  Structural analysis of the spillway gates has shown there is a possibility for damage of the spillway gate’s supporting arms resulting in an uncontrolled release of water from the dam.  Failure of a spillway gate could cause downstream flooding and affect communities downstream of the dam. In the case of an uncontrolled release from multiple spillway gates or one or more concrete monoliths, there is potential for devastating flooding to affect large portions of the narrow river canyon areas and urban areas downstream.

USACE is confident that the Willamette Valley dams are well-built, well-maintained, and will continue to significantly reduce flood risks for the region. However, the dams cannot eliminate potential for flooding. Even with the presence of the Willamette Valley dams, extreme rainfall and snowmelt events may result in flooding in areas downstream of dams.  Flooding can be caused by high flows resulting from unregulated portions of the watershed and/or high flow that must be passed through the dam outlets and spillways when reservoir storage capacities are exceeded

Risk Management Measures
The likelihood is low for an extreme earthquake to occur when the reservoir is at or near its maximum elevation, but the potential impacts of a breach of the spillway gates are very high due to the large downstream population. Therefore, immediate action is warranted to reduce risk to tolerable levels. Targeted measures (called Interim Risk Reduction Measures) were implemented in spring 2021 to reduce life-safety risk while issues are evaluated further in the Study. These measures include reducing the maximum conservation pool (the highest allowable level during summer) of Detroit reservoir by five feet to reduce the likelihood of an earthquake related failure and uncontrolled release. USACE continues to evaluate the seismic performance of the spillway and other components of the dam to determine if long-term modifications or changes to operations will be necessary. USACE regularly conducts routine inspections of its dams and Detroit Dam is equipped with instrumentation to monitor dam performance and seismic activity. Post-earthquake procedures are in place to inspect and evaluate earthquake damages and USACE conducts routine dam safety exercises with local Emergency Managers and first responders. Detroit Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on recent risk assessment findings and with information from updated inundation maps. In addition, USACE will continue and increase its outreach to improve community awareness of flood risks and risks associated with the dam.

View more details about Detroit Dam at the National Inventory of Dams website.