Cottonwood Creek Lower Reservoir Report
Last Updated: December 13, 2025
Cottonwood Creek Lower, also known as Military Reserve Flood Basin #3, is a vital flood risk reduction structure located in Boise, Idaho.
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Summary
Constructed in 1961 by Morrison Knudsen Corp, this earth dam with a stone core stands at a height of 15.4 feet and has a hydraulic height of 10.4 feet. With a storage capacity of 88 acre-feet, this structure serves to protect the surrounding area from potential flooding events along Cottonwood Creek and the Boise River.
Managed by the Idaho Department of Water Resources, Cottonwood Creek Lower is subject to regular state inspections and enforcement to ensure its operational readiness. The dam has a spillway width of 280 feet and can handle a maximum discharge of 3540 cubic feet per second. While the structure's hazard potential is deemed significant, its overall condition assessment is fair as of the most recent inspection in September 2018. The surrounding community benefits from this flood control measure, which plays a crucial role in mitigating flood risks in the region.
Given its strategic location and importance in flood risk reduction, Cottonwood Creek Lower serves as a key asset in the water resource management infrastructure of Ada County, Idaho. With its proximity to the Boise River and its effective storage capacity, this dam plays a significant role in protecting the local community from potential inundation events. As climate change continues to impact precipitation patterns and increase the frequency of extreme weather events, structures like Cottonwood Creek Lower are essential in safeguarding both lives and property from the dangers of flooding.
Year Completed |
1961 |
Dam Length |
1710 |
Dam Height |
15.4 |
River Or Stream |
COTTONWOOD CREEK BOISE RIVER |
Surface Area |
9 |
Hydraulic Height |
10.4 |
Drainage Area |
16.6 |
Nid Storage |
88 |
Hazard Potential |
Significant |
Foundations |
Rock, Soil |
Nid Height |
15 |
Seasonal Comparison
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Dam Data Reference
Condition Assessment
SatisfactoryNo 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
HighDams 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.
Cottonwood Creek Lower
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