Big Cliff Dam is located on the North Santiam River approximately 47 miles east of Salem, Ore. and approximately 11 miles northeast of Mill City, Ore. Big Cliff Dam is a 182-foot tall, 295-foot-long concrete gravity dam with a concrete spillway, three spillway gates, a non-overflow section, and a powerhouse. Big Cliff 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 1951 and was completed in 1954. Big Cliff 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.
Big Cliff Dam is 2.8 miles downstream of Detroit Dam. Big Cliff is a re-regulating dam for Detroit and is operated to provide consistent flows into the river downstream while allowing for fluctuations in outflow from Detroit Dam. The re-regulation of flow allows for Detroit Dam's powerhouse to meet peak electricity demands daily for the region.
Big Cliff Dam is located about 95 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 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 Big Cliff Dam in April 2012 that characterizes the risk associated with the dam to be Low.
The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The risk is driven by two factors including: downstream population combined with the potential for an extreme earthquake occurring during routine dam operations. According to the study, an extreme earthquake could damage the spillway gates or spillway gate piers resulting in an uncontrolled release through the spillway causing flooding downstream. Because Big Cliff Dam is located upstream of several rural communities along the North Santiam River there is potential for flooding to affect downstream communities in the floodplain and along the 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
USACE regularly conducts routine inspections of its dams and Big Cliff Dam is equipped with instrumentation to monitor dam performance. 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. Big Cliff 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 to increase its outreach to improve community awareness of flood risks and risks associated with the dam.
View more details about Big Cliff Dam at the National Inventory of Dams website.