Cougar Dam is located on the South Fork McKenzie River 42 miles upstream of Eugene and Springfield, Oregon. Cougar Dam is a 519 ft tall, 1,600 ft long rockfill earthen embankment dam with a concrete spillway, two spillway gates, an intake structure with regulating outlet, and a powerhouse. Cougar 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 1959 and was completed in 1963. Cougar 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 low 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 refill 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.
Cougar Dam is located about 100 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: High
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 has been conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Cougar Dam.
The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. As of June 2020, results of the advanced study for Cougar Dam identified the risk associated with the dam to be High. The risk is driven by the large population downstream combined with the potential for an extreme earthquake occurring at the same time reservoir elevations are the highest. According to the study, a large earthquake could cause the rockfill earthen dam to settle, resulting in water overtopping the dam. It is difficult to predict the exact amount of settlement that could occur to the dam as a result of such an earthquake. The speed and depth of water overtopping a damaged dam could erode the soil and rock that forms the dam and cause significant flooding downstream. The shaking from an earthquake could also open up cracks through the rockfill earthen dam, allowing water to flow through the dam. The force and speed of the flowing water through the cracks could cause the dam to be further damaged by eroding away the embankment material, resulting in uncontrolled release of water from the reservoir, and flooding areas downstream. Because Cougar Dam is located upstream of Eugene and Springfield, Oregon, there is potential for devastating flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities in the floodplain and narrow canyons.
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, but the potential impacts of a dam failure are very high due to the large downstream population. USACE continues to evaluate the seismic performance of the embankment to determine if short-term targeted measures (called Interim Risk Reduction Measures), or long-term modifications will be necessary to reduce risk. USACE regularly conducts routine inspections of its dams and Cougar 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. Cougar Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on recent risk assessment results and 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 Cougar Dam at the National Inventory of Dams website.