Fern Ridge Dam is located on the Long Tom River 13 miles upstream of Monroe, Ore., and 24 miles upstream of the confluence of the Long Tom and Willamette Rivers. Fern Ridge Dam is a 50-foot tall, 6,330-foot long earthen embankment dam built on a soil foundation with a concrete spillway, six spillway gates, a gated regulating outlet, and two earthen auxiliary dikes. Fern Ridge 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 1940 and was completed in 1942. In 2005, the downstream portions of the earthen embankment were completely reconstructed to address concerns with seepage through the embankment’s soil foundation. The reconstruction included installation of a drainage system to collect seepage through the foundation and instrumentation to monitor the drainage system's performance during normal reservoir operations and during flood events. Fern Ridge 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 recreation, irrigation, municipal and industrial water supply, fish and wildlife, water quality and hydropower. Collectively, this system of dams is referred to as the Willamette Valley Project (WVP.) Fern Ridge is one of four WVP dams that do 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.
Fern Ridge Dam is located about 50 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: Moderate
USACE performed a routine risk assessment for Fern Ridge Dam in 2020 that characterizes the risk associated with the dam to be 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. The risk at Fern Ridge Dam is driven primarily by the potential for an earthquake occurring at the same time reservoir elevations are the highest. The Cascadia Subduction Zone fault can produce very large earthquakes that can cause up to three to five minutes of ground shaking. The last subduction zone earthquake occurred in 1700. Fern Ridge Dam has never experienced a large earthquake, but because of its proximity to the Cascadia Subduction Zone and its soil foundation, the earthen embankment dam at Fern Ridge Dam is expected to experience stronger shaking from a Cascadia Subduction Zone earthquake than the other Willamette Valley Projects. It is difficult to predict the exact amount of damage or settlement that could occur to a dam as a result such an earthquake. A large earthquake could cause the earthen dam to settle, resulting in water overtopping the dam. The speed and depth of water overtopping a damaged dam could erode the soil that forms the dam and cause flooding downstream. The shaking from an earthquake could also open up cracks through the earthen embankment, allowing water to flow through the dam. The force and speed of the flowing water through a crack in the earthen embankment 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. The flooding would be most severe in the agricultural lands between the dam and the confluence with the Willamette River, between Monroe and Corvallis, Oregon.
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 is further addressing the risk at Fern Ridge Dam by pursuing an advanced risk assessment of the earthen embankment's response to earthquake shaking so that potential damages are better understood. This will help inform the likelihood of an uncontrolled release of water following an earthquake and whether there is a need to pursue long-term risk reduction measures. USACE regularly conducts routine inspections of its dams and Fern Ridge 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. Fern Ridge 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.