Project Description
Cottage Grove Dam is located on the Coast Fork Willamette River, approximately 5 miles upstream of Cottage Grove, Oregon. Cottage Grove Dam is a 95ft tall, 1,750ft long earthen embankment dam with a 264ft long concrete ungated spillway, and a 96ft wide concrete non-overflow section. Cottage Grove Dam is owned, operated, and maintained by the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1940 and was completed in 1942. Cottage Grove 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 (WVP). Cottage Grove Dam is one of four WVP dams that does not include a powerhouse. 
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, 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.    
Cottage Grove Dam is about 60 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 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.  The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely.  USACE completed a routine risk assessment for Cottage Grove in 2012. The next routine risk assessment is scheduled to occur in 2022.

The 2012 risk assessment identified the risk associated with the dam to be Moderate. According to the 2012 assessment, an extreme earthquake could cause the earthen dam to settle and crack, resulting in water overtopping and flowing through the dam and flooding areas downstream. It is difficult to predict the exact amount of settlement and cracking that could occur to the dam as a result of such an earthquake. The speed and depth of water flowing over and through the damaged dam could erode the soil that forms the dam and cause significant flooding downstream. The study also determined that reservoir water could begin to seep through and under the dam during periods of high pool which could erode the embankment soil. Because Cottage Grove Dam is located upstream of the town of Cottage Grove, 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 to occur, but the potential impacts of a dam failure are high due to the downstream population. In 2018, USACE completed soil sampling and determined that the embankment and foundation soils are not as vulnerable to erosion as initially evaluated during the 2012 routine risk assessment. USACE also installed instrumentation to monitor groundwater levels in the foundation for any changes that could be a sign of potential seepage under and through the dam. These instrument readings will allow USACE to detect seepage and intervene before erosion could occur.

USACE will conduct another routine risk assessment of Cottage Grove Dam in 2022. The assessment will incorporate the updated understanding of the embankment and foundation soils, and their reduced potential for seepage and erosion. Cottage Grove Dam will also be subject to an advanced risk assessment called an Issues Evaluation Study (IES) to evaluate the embankment’s response to earthquake to better understand if and how much the dam could become damaged. The routine and advanced risk assessments will help inform whether the potential damages from extreme earthquake and 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 continues to regularly conduct routine inspections of its dams and Cottage Grove 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. Cottage Grove Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on the results of the advanced risk assessment 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 Cottage Grove Dam at the National Inventory of Dams website.