Cottonwood Creek Middle Reservoir Report

Last Updated: December 13, 2025

Cottonwood Creek Middle, also known as Cottonwood Detention Dam #2, is a crucial water resource infrastructure located in Boise, Idaho.


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Summary

Built in 1961, the dam serves the primary purpose of flood risk reduction along the Cottonwood Creek and Boise River. With a dam height of 20.4 feet and a hydraulic height of 15.4 feet, it plays a significant role in managing water flow and protecting the surrounding area from potential flood events.

Managed by the local government and regulated by the Idaho Department of Water Resources, Cottonwood Creek Middle has a fair condition assessment and a significant hazard potential. Despite its age, the dam continues to operate effectively, with regular inspections conducted to ensure its structural integrity and functionality. The dam's storage capacity of 40 acre-feet and drainage area of 16.6 square miles make it a critical component of the region's water management infrastructure, with a moderate risk assessment rating of 3.

Overall, Cottonwood Creek Middle stands as a vital piece of infrastructure in the region, providing essential flood risk reduction measures and safeguarding communities along the Cottonwood Creek and Boise River. Its strategic location and design exemplify the importance of water resource management in mitigating the impact of extreme weather events and climate change on local ecosystems and populations.

Year Completed

1961

Dam Length

1210

Dam Height

20.4

River Or Stream

COTTONWOOD CREEK BOISE RIVER

Surface Area

4

Hydraulic Height

15.4

Drainage Area

16.6

Nid Storage

40

Hazard Potential

Significant

Foundations

Rock, Soil

Nid Height

20
       
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.