Detroit Dam IRRM Final EA FONSI

A Finding of No Significant Impact: Detroit Dam IRRM pool reduction - Environmental Assessment (April 29, 2021)


Detroit Dam Interim Risk Reduction Measure Pool Restriction

The U.S. Army Corps of Engineers is conducting an environmental review on a project to provide Interim Risk Reduction Measure at Detroit Dam. The Detroit Dam and Reservoir spans the Linn County–Marion County border in the Oregon Cascades on the North Santiam River near the city of Detroit.


The U.S. Army Corps of Engineers (Corps) operates 13 dams in the Willamette River Basin, referred to as the Willamette Valley Project (WVP). The authorized purposes of the dams include flood risk management, water quality, hydropower, navigation, irrigation, recreation, fish and wildlife, and municipal and industrial water supply. The dams are operated as a system to manage flood risk during major flood season (December- January). In spring, the reservoir pools are allowed to fill, capturing a portion of rainfall and snowmelt for the conservation release season (May-November). During the conservation release season, stored water is released for conservation benefits, such as minimum flows, water supply, irrigation, and power production. The dams are operated to meet minimum releases at the individual dams and as a system to meet flow targets at downstream river control points.

As part of its comprehensive dam safety program, the Corps is continuously assessing its dams to better understand dam safety risks and inform future actions. Seismic hazard and seismic performance of the dams in the Willamette Valley is a focus area for the program. Detroit Dam is located about 150 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 every 300-500 years. In 2020, as a part of the comprehensive dam safety program, an updated seismic hazard analysis was completed for Detroit Dam to better understand the potential earthquake ground motions at the site. This hazard study has been used to analyze the performance of the spillway gates and found the risk to be higher than previously assessed. The performance of the spillway gates in an earthquake is a function of the potential ground motions, the height of the gates above the dam foundation, and the water level acting on the gates at the time of an earthquake. 

Structural analysis of the spillway gates has shown there is a possibility for buckling of the spillway gate’s supporting arms resulting in an uncontrolled release of water from the dam. With this possibility, there is potential for devastating flooding to affect large portions of the narrow North Santiam river canyon and urban areas because Detroit Dam is located upstream of several communities including the state capital, Salem Oregon.

Although the likelihood of an extreme earthquake is low, the potential impacts of a breach of the spillway gates are 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) are being analyzed for implementation in spring 2021 to reduce life-safety risk while issues are evaluated further. These measures include reducing the maximum conservation pool (the highest allowable level during summer) of Detroit reservoir by 5 feet. The Corps 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, including the potential for major rehabilitation or reconstruction of the spillway structures.

The pool restriction elevations from 1563.5 feet to 1558.5 feet at Detroit Damn (5-foot reduction) was assessed as an alternative based on the recommendation of the risk assessment teams and additional analysis performed by the Corps to reduce life-safety risk to tolerable levels.

The project requires review under applicable laws and regulations, including the National Environmental Policy Act (NEPA). The proposed pool restriction (Action Alternative) would begin 29 April 2021, after the Corps has received all required environmental clearances and completed the NEPA process and continue until 7 September 2021 and recur annually until a permanent solution is developed. To evaluate the impacts of implementing this interim measure, the Corps team used a Hydrologic Engineering Center (HEC) reservoir simulation model (HEC-ResSim) to determine the potential project and system-wide impacts of a pool restriction at Detroit Dam. The Corps has developed this Draft Environmental Assessment (EA) to evaluate the environmental impacts of this summer conservation pool restriction at Detroit Dam. The interim pool restriction is presented as the Action Alternative in the EA. The Corps is the lead federal agency for this EA.

This draft EA is being made available for a 15-day public review period. All comments, whether by conventional mail or email, must be received no later than the expiration date of this public notice to ensure consideration.

Submit Your Comment

Comments will be accepted until March 30, 2021
By email: 
By mail: Jess Jones, U.S. Army Corps of Engineers
Attn: PM-E, PO Box 2946 Portland, OR

Public Meeting


A virtual meeting was conducted Monday, March 22, 2021 and can be viewed here:

The purpose of the meeting was to provide the public with an overview of the proposed action. The draft EA is available for public comment through April 1, 2021 at 5 p.m.


NEPA - The Environmental Policy Act Process, Scoping Phase and Your Role

The National Environmental Policy Act of 1969 (NEPA) is one of the nation’s oldest environmental laws that encourages federal agencies to make environmentally responsible decisions. NEPA requires all federal agencies to consider and disclose the environmental effects of their proposed actions in an Environmental Impact Statement or EA. The Citizen's Guide to NEPA explains this law and how to effectively submit your input.

Per NEPA, an EA assessing impacts of the proposed project on the human environment will be provided to the public for review and comment. The public will be notified when opportunities for comment arise. Collecting the right level of information at the right time is important to developing a project plan, requiring early and frequent engagement of all affected federal, state and local agencies, Native American Tribes, and interested groups and individuals. 

Areas for evaluation in the EA include:
• Aesthetics
• Air quality
• Aquatic resources/wetlands 
• Invasive species
• Fish and wildlife habitat 
• Threatened/Endangered species/critical habitat 
• Historic properties 
• Other cultural resources 
• Floodplains 
• Hazardous, toxic & radioactive waste 
• Hydrology 
• Land use
• Navigation 
• Noise levels 
• Public infrastructure 
• Socio-economics
• Environmental justice 
• Soils
• Tribal trust resources 
• Water quality 
• Climate change

To provide opportunities for the public to learn about the project, the Corps will continue to provide project updates on the project website. If you would like to be included in the project distribution list to receive updates on the project via email, please send an email with the subject line “Detroit IRRM Distribution List” to the project email:

Interested parties can also send questions to the project team via the project email or by calling the Corps’ Public Affairs Office at 503-808-4510. The public is being provided an opportunity to review and provide comments on the Draft EA. A public meeting will be held to present the preferred alternative, answer questions, and provide an opportunity for members of the public to submit written comments. The Corps will consider these comments when finalizing the EA. The public will then be provided an opportunity to review the final EA.

An informed public is an empowered public

We understand the importance of Detroit Dam and Reservoir to the communities of the North Santiam River Basin. An informed public that understands risk is an empowered public that can take responsibility for its safety. The Corps wants the community to be informed of these dam safety risks and seeks your input on the Environmental Assessment. Public comments on the EA will be considered, responded to, and part of the public record, but may not impact the decision. The Corps will share its continued findings and recommendations with stakeholders, agencies, and officials as they become available.