Dexter Dam is located on the Middle Fork Willamette River in Lowell, Oregon and 20 miles upstream of Eugene and Springfield, Oregon. Dexter Dam is a 90 ft tall, 2,319ft long rockfill earthen embankment dam with a 359 ft long concrete spillway equipped with seven spillway gates, regulating outlet, and a powerhouse. Dexter 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 1947 and was completed in 1955. Dexter 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.
Dexter Dam is 1 mile downstream of Lookout Point Dam. Dexter Dam is a re-regulating dam for Lookout Point and is operated to provide consistent flows into the river downstream while allowing for fluctuations in outflow from Lookout Point Dam. The re-regulation of flow allows for Lookout Point Dam's powerhouse to meet peak electricity demands on a daily basis for the region.
Dexter Dam is located about 70 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: 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 Dexter Dam in June 2015 that characterizes the risk associated with the dam to be Low. The assessment considered a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The assessment considered several scenarios that could be damaging to the structure and the embankment driven by an earthquake and or 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.
Risk Management Measures
USACE continues to evaluate the condition and risks associated with its dams and will continue to review the risk associated with Dexter Dam in future routine studies. USACE regularly conducts routine inspections of its dams and Dexter 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. Dexter 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 Dexter Dam at the National Inventory of Dams website.