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.

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.

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.

Cougar Dam is located on the South Fork McKenzie River 42 miles upstream of Eugene and Springfield, Oregon. Cougar Dam is a 519 ft tall, 1,600 ft long rockfill earthen embankment dam with a concrete spillway, two spillway gates, an intake structure with regulating outlet, and a powerhouse. Cougar 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 1959 and was completed in 1963. Cougar 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 low 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 refill 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.
Cougar Dam is located about 100 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.   

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.  USACE has been conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Cougar Dam.  
The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely.  As of June 2020, results of the advanced study for Cougar Dam identified the risk associated with the dam to be High. The risk is driven by the large population downstream combined with the potential for an extreme earthquake occurring at the same time reservoir elevations are the highest.  According to the study, a large earthquake could cause the rockfill earthen dam to settle, resulting in water overtopping the dam. It is difficult to predict the exact amount of settlement that could occur to the dam as a result of such an earthquake. The speed and depth of water overtopping a damaged dam could erode the soil and rock that forms the dam and cause significant flooding downstream.  The shaking from an earthquake could also open up cracks through the rockfill earthen dam, allowing water to flow through the dam. The force and speed of the flowing water through the cracks 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.  Because Cougar Dam is located upstream of Eugene and Springfield, Oregon, there is potential for devastating flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities in the floodplain and 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. 

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.

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.

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.

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.

Detroit Dam is located on the North Santiam River 48 miles upstream the confluence with the Willamette River and 13 miles upstream of Mill City, Oregon. Detroit Dam is a 450 ft tall, 1,450 ft long concrete gravity dam, comprised of 32 monoliths, with a gated spillway with six spillway gates, four regulating outlets, and a powerhouse. Detroit 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 1949 and was completed in 1953. Detroit 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. Detroit Dam is operated in coordination with Big Cliff Dam to achieve project objectives within the North Santiam sub basin of the Willamette River.

Detroit Dam is located about 95 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.

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.  USACE completed a routine risk assessment for the Detroit Dam in May 2018 that characterizes the risk associated with the dam to be Moderate. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The risk associated with Detroit Dam is primarily driven by the potential for a very large earthquake occurring at the same time summer reservoir elevations are the highest. Under these conditions, there is the potential for one or more of the spillway gates to collapse or for a structural failure of the concrete monoliths. In 2020, an updated earthquake hazard study was completed for Detroit Dam to better understand the potential amount of shaking that could occur at the site from a range of all possible earthquake fault sources. USACE has used this hazard study to review the results of the routine risk assessment and found that the amount of shaking from an extreme earthquake could be much greater at the site than originally assessed. Because of the larger predicted amount of shaking, the risks associated with the performance of the spillway gates are higher than originally assessed.  Structural analysis of the spillway gates has shown there is a possibility for damage of the spillway gate’s supporting arms resulting in an uncontrolled release of water from the dam.  Failure of a spillway gate could cause downstream flooding and affect communities downstream of the dam. In the case of an uncontrolled release from multiple spillway gates or one or more concrete monoliths, there is potential for devastating flooding to affect large portions of the narrow river canyon areas and urban areas downstream.

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

The likelihood is low for an extreme earthquake to occur when the reservoir is at or near its maximum elevation, but the potential impacts of a breach of the spillway gates are very high due to the large downstream population. Therefore, immediate action is warranted to reduce risk to tolerable levels. Targeted measures (called Interim Risk Reduction Measures) were implemented in spring 2021 to reduce life-safety risk while issues are evaluated further in the Study. These measures include reducing the maximum conservation pool (the highest allowable level during summer) of Detroit reservoir by five feet to reduce the likelihood of an earthquake related failure and uncontrolled release. USACE continues to evaluate the seismic performance of the spillway and other components of the dam to determine if long-term modifications or changes to operations will be necessary. USACE regularly conducts routine inspections of its dams and Detroit 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. Detroit Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on recent risk assessment findings and with 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.

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.

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.

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. 

Dorena Dam is located on the Row River six miles upstream of Cottage Grove, Oregon. Dorena Dam is a 145 ft tall, 2,600 ft long earthen embankment dam with an ungated 200 ft long concrete spillway and gated regulating outlets. Dorena Dam is owned, operated, and maintained by the Portland District of the U.S. Army Corps of Engineers (USACE).  Construction of the dam was completed in 1949. A privately owned hydropower facility was added to the dam and began operation in 2014. The hydropower facility is regulated by Federal Energy Regulatory Commission (FERC). Dorena 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, 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.

Dorena Dam is located about 65 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.

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 Dorena Dam in September 2012 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. 

USACE continues to evaluate the condition and risks associated with its dams and will continue to review the risk associated with Dorena Dam in future routine studies. The next routine risk assessment for Dorena Dam is scheduled for the spring of 2022. USACE regularly conducts routine inspections of its dams and Dorena 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. Dorena 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. 

Fall Creek Dam is located on Fall Creek, a major tributary to the Middle Fork Willamette River, one mile upstream of Unity, Oregon and 25 miles upstream of Eugene and Springfield, Oregon. Fall Creek Dam is a 205-foot tall, 5,050-foot long rockfill earthen embankment dam with a concrete spillway, two spillway gates, and a regulating outlet. Fall Creek 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 1964 and was completed in 1965.  Fall Creek 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, conservation, water quality, and hydropower. Collectively, this system of dams is referred to as the Willamette Valley Project (WVP). Fall Creek 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 of storage 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. Fall Creek 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.    

HIGH

USACE completed a routine risk assessment for the Fall Creek Dam in October 2014 that characterizes 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. The 2014 assessment concluded that the risk at Fall Creek Dam is driven by the high population downstream 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 that fills the reservoir at a rate faster than the dam can pass water through the spillway.  

An extreme earthquake could cause the rockfill earthen dam to settle and crack, resulting in water overtopping and flowing through the dam. 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 and rock that forms the dam and cause significant flooding downstream. The strong shaking from an earthquake could also damage the spillway’s concrete structure and gates. The assessment also determined that fallen trees and woody debris floating in the reservoir could partially block the spillway gates during an extreme and unlikely rainfall event. The partial blockage of the spillway gates from the debris could reduce or prevent passage of flow through the dam, which would cause the reservoir to rise above and flow over 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. Because Fall Creek Dam is located upstream of Eugene and Springfield, Oregon, there is potential for devastating flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities in the floodplain.  

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.  

The likelihood is very low for an extreme earthquake or extreme flood resulting in a breach of the dam to occur, but the potential impacts of a dam failure are very high due to the large downstream population. Therefore, Fall Creek Dam will enter an advanced risk assessment called an Issues Evaluation Study (IES) starting in 2021 to further evaluate the dam’s performance during extreme seismic and flood events. The study will also determine whether short-term targeted measures (called Interim Risk Reduction Measures) or long-term modifications are necessary to reduce the risk.  USACE regularly conducts routine inspections of its dams and Fall Creek 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. Fall Creek Dam’s Emergency Action Plan (EAP) outlines actions to be taken during an emergency. USACE will update the EAP based on advanced 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.  

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. 

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.  

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.

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. 

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.  

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. 

Green Peter Dam is located on the South Santiam River seven miles upstream of Foster Dam and ten miles upstream of Sweet Home, Ore. Green Peter Dam is a 378-foot tall, 1,500-foot long concrete gravity dam with a concrete spillway, two spillway gates, two regulating outlets, and a powerhouse. Green Peter 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 1963 and was completed in 1966. Green Peter 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.  

Green Peter Dam is located about 80 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. 

LOW

USACE has been conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Green Peter Dam which was completed in 2020. As of result of the advanced study for Green Peter Dam, USACE identified the risk associated with the dam to be 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. The assessment considers a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The assessment considered five scenarios that could be damaging to the structure driven by an earthquake and two damaging scenarios driven 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. Floods could occur in the future that may cause some inundation and flooding below Green Peter and Foster Dams in communities along the South Santiam River including areas of Sweet Home.

USACE continues to evaluate the condition and risks associated with its dams and will continue to review the risk associated with Green Peter Dam in future routine studies. USACE regularly conducts routine inspections of its dams and Green Peter 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. Green Peter 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.

Hills Creek Dam is located on the Middle Fork of the Willamette River three miles upstream of Oakridge and 42 miles upstream of Eugene and Springfield, Oregon. Hills Creek Dam is a 304-foot tall, 1,920-foot long rockfill earthen embankment dam with a concrete spillway, three spillway gates, an intake structure with regulating outlet, and a powerhouse. Hills Creek dam is owned, operated, and maintained by the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1956 and was completed in 1961.  Hills Creek 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 reservoirs are maintained at their lowest elevations to allow for the temporary storage of rain and snowmelt. 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. Hills Creek is upstream of Lookout Point Dam and Dexter Dam.    

Hills Creek Dam is located about 90 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. 

HIGH

USACE has been conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Hills Creek Dam. As of April 2019, results of the IES for Hills Creek Dam identified the risk associated with the dam to be High. 

USACE preforms 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 assessments consider a wide range of hazard scenarios from the most likely to the most extreme and unlikely. The risk is driven by two factors: the high population downstream combined with the potential for an extreme earthquake occurring at the same time reservoir elevations are the highest.  According to the IES, an extreme earthquake could cause the earthen dam to settle, resulting in water overtopping the dam. It is difficult to predict the exact amount of settlement that could occur to the dam as a result of such an earthquake. The speed and depth of water overtopping a damaged dam could erode the soil and rock that forms the dam and cause significant flooding downstream. Because Hills Creek Dam is located upstream of Oakridge, Lowell, Jasper, Eugene, and Springfield Oregon, there is potential for devastating flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities in the floodplain and 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. 

The likelihood is very low for an extreme earthquake to occur, but the potential impacts of a dam failure are very high due to the large downstream population. Therefore, immediate action is warranted to reduce risk to acceptable levels.  Targeted measures (called Interim Risk Reduction Measures) were implemented in spring 2020 to reduce life-safety risk while issues identified in the Study are evaluated further. These measures include reducing the maximum conservation pool (the highest allowable level during summer) of Hills Creek reservoir by 10 feet.  USACE continues to evaluate the seismic performance of the embankment to determine if long-term modifications will be necessary.  USACE regularly conducts routine inspections of its dams and Hills Creek 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. Hills Creek 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. 

Lookout Point Dam is located on the Middle Fork of the Willamette River 22 miles upstream of Eugene and Springfield, Oregon. Lookout Point Dam is a 246-foot tall, 1,875-foot long rockfill earthen embankment dam with a concrete spillway, five spillway gates, a concrete non-overflow section, and a powerhouse with regulating outlets. Lookout Point dam is owned, operated, and maintained by the U.S. Army Corps of Engineers (USACE). Construction of the dam began in 1948 and was completed in 1954. Lookout Point 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. Lookout Point Dam is downstream of Hills Creek Dam and upstream of Dexter Dam.   

Lookout Point Dam is located about 75 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. 

MODERATE

USACE is conducting advanced risk assessments, called Issue Evaluation Studies (IES), at several Willamette Valley Project dams including Lookout Point Dam.  As of May 2019, results of the advanced study for Lookout Point Dam identified 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 is 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 that fills the reservoir at a rate faster than the dam can pass water through the spillway.  According to the study, an extreme earthquake could cause the spillway gates and the concrete supports on either side to become damaged. If this occurs when the reservoir is at its highest, the damaged gates may no longer be able to hold back the 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 fill the reservoir at a rate faster than the spillway is able to pass flow through the dam, 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 allowed to pass over and through the dam at greater speeds and depths, causing significant flooding downstream. Because Lookout Point Dam is located upstream of Eugene and Springfield, Oregon, there is potential for flooding to affect large downstream populations in urban areas and surrounding suburbs, as well as rural communities 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.  

The likelihood is very low for an extreme earthquake to occur, and very low for an extreme rainfall event that could overtop the dam to occur, but the potential impacts of a dam failure are very high due to the large downstream population.  Immediate action is therefore warranted to reduce risk to acceptable levels.  Interim risk reduction measures (IRRMs) were implemented in spring 2020 to reduce life-safety risk while issues are studied further. These measures include reducing the maximum conservation pool (summer refill target) of Lookout Point reservoir by 5 feet.  A modification study is underway that will address replacement of the spillway gates to reduce the likelihood that they become damaged during an earthquake.  USACE regularly conducts routine inspections of its dams and Lookout Point 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. Lookout Point 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.