Green Peter Dam is located on the South Santiam River seven miles upstream of Foster Dam and ten miles upstream of Sweet Home, Ore. Green Peter Dam is a 378-foot tall, 1,500-foot long concrete gravity dam with a concrete spillway, two spillway gates, two regulating outlets, and a powerhouse. Green Peter 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 1963 and was completed in 1966. Green Peter 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.
Green Peter Dam is located about 80 miles east of the Cascadia Subduction Zone, a megathrust fault along the Oregon Coast. The Cascadia Subduction Zone can produce very large, long duration earthquakes. The last Cascadia Subduction Zone earthquake occurred in the year 1700.
Risk Characterization: Low
USACE has been conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Green Peter Dam which was completed in 2020. As of result of the advanced study for Green Peter Dam, USACE identified the risk associated with the dam to be low.
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. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The assessment considered five scenarios that could be damaging to the structure driven by an earthquake and two damaging scenarios driven by extreme flood events. For all the scenarios considered, the associated risks were found to be low and not to require any short-term measures or long-term modification.
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. Floods could occur in the future that may cause some inundation and flooding below Green Peter and Foster Dams in communities along the South Santiam River including areas of Sweet Home and Lebanon.
Risk Management Measures
USACE continues to evaluate the condition and risks associated with its dams and will continue to review the risk associated with Green Peter Dam in future routine studies. USACE regularly conducts routine inspections of its dams and Green Peter 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. Green Peter 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 Green Peter Dam at the National Inventory of Dams website.