Cottage Grove Dam Reservoir Report

Last Updated: December 7, 2025

Cottage Grove Dam, also known as Cottage Grove Lake, is a federally owned structure located on the Coast Fork Willamette River in Oregon.


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Summary

Built in 1942, this earth dam stands at 81 feet tall and serves multiple purposes including flood risk reduction, irrigation, navigation, and recreation. With a storage capacity of 33,000 acre-feet, the dam covers a surface area of 1,158 acres and has a maximum discharge of 40,800 cubic feet per second.

A routine risk assessment conducted in 2012 identified Cottage Grove Dam's risk level as moderate, particularly in the event of an extreme earthquake causing settlement and cracking that could lead to flooding downstream. To address potential risks, the US Army Corps of Engineers (USACE) has implemented risk management measures such as soil sampling, groundwater monitoring, and ongoing inspections. Plans for an advanced risk assessment in 2022, along with the development of an updated Emergency Action Plan, aim to enhance the dam's resilience and reduce risks associated with extreme events.

USACE's commitment to conducting regular risk assessments, implementing interim risk reduction measures, and enhancing community awareness of flood risks underscores its dedication to ensuring the safety and functionality of Cottage Grove Dam. By staying proactive in risk management strategies and engaging with local stakeholders, USACE aims to mitigate potential threats and maintain the dam's critical role in flood risk reduction for the region.

Year Completed

1942

Dam Length

2110

Dam Height

81

River Or Stream

COAST FORK WILLAMETTE RIVER

Primary Dam Type

Earth

Surface Area

1158

Hydraulic Height

76

Drainage Area

104

Nid Storage

50000

Structural Height

103

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

103
       
Seasonal Comparison

Weather Forecast

Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.