Foster Dam is located on the South Santiam River approximately 30 miles upstream of Albany, Oregon. Foster Dam is a 126-foot tall, 2,985-foot long rockfill earthen embankment dam with a 400-foot long concrete spillway, four spillway gates, a concrete non-overflow section, and a powerhouse. Foster Dam is owned, operated, and maintained by the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1964 and was completed in 1968. Foster 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. Foster Dam is downstream of Green Peter Dam.
Foster Dam is 75 miles east of the Cascadia Subduction Zone, a megathrust fault along the Oregon Coast. 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: High
As of May 2021, results of the advanced study for Foster Dam identified the risk associated with the dam to be 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. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. USACE is conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Foster Dam
The risk is primarily driven by the high population downstream of the dam combined with two possible, but very unlikely events: 1) an extreme earthquake occurring at the same time reservoir elevations are the highest or 2) an extreme flood event. According to the study, the shaking from an extreme earthquake could cause the spillway gates and concrete supports on either side to become damaged. If this occurs when the reservoir is at its highest, the cracked and damaged spillway may no longer be able to hold back water, allowing a high volume of water to flow through the spillway and cause flooding of areas downstream. The study also determined that an extreme and very unlikely rainfall event could result in release of higher volumes of water through the dam than the gated spillway was designed to pass. The force and speed of the water flowing out of the dam could damage and erode the concrete lining in the spillway channel, exposing the underlying foundation rock that may be susceptible to erosion if subjected to very high flows. The continued force and speed of the water could erode the rock and destabilize the concrete structure itself. A sudden release of water could result from the destabilized concrete structure’s lost ability to hold back the forces of the reservoir, causing significant downstream flooding. Another different scenario could occur if the extreme rainfall event instead fills the reservoir at a rate faster than the spillway is able to pass flow through the dam. Fallen trees and woody debris floating in the reservoir could partially block the spillway gates during the rainfall event, further limiting the amount of water that can pass through the spillway gates and allowing the reservoir to rise above the top of the embankment dam. As water flows over the top of the dam, the speed and depth of the water could erode the soil and rock that forms the dam. As the soil and rock continue to erode, more water is released over and through the dam at greater speeds and depths, causing significant flooding downstream. Because Foster Dam is located upstream of Sweet Home and Lebanon, there is potential for flooding to affect large downstream populations in the floodplain areas.
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 or an extreme rainfall event to occur, but the potential impacts of a dam failure are high due to the large downstream population. USACE continues to evaluate the performance of the spillway during earthquake and flood loads using advanced computer modeling of the dam’s concrete and gated structures to better understand if and how much the spillway could become damaged. This will help inform whether the potential damages from extreme earthquake or flood loads continue to drive risk at the project, and whether short-term targeted measures (called Interim Risk Reduction Measures) or long-term modifications will be necessary to reduce risk. USACE also continues to study the likelihood of an extreme rainstorm that would be large enough to cause damaging spillway flows. USACE continues to regularly conduct routine inspections of its dams and Foster 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. Foster 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 Big Cliff Dam at the National Inventory of Dams website.