Onion Valley Reservoir Report

Last Updated: December 5, 2025

Onion Valley, located in Montrose, Colorado, is a privately owned dam with a primary purpose of irrigation.


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Summary

Completed in 1954, this earth dam stands at a height of 85 feet and has a hydraulic height of 105 feet. With a storage capacity of 10,380 acre-feet, the dam serves to regulate water from Iron Creek for agricultural use in the region.

The dam has a surface area of 340 acres and a drainage area of 22 square miles, with a maximum discharge capacity of 502 cubic feet per second. Despite its fair condition assessment, Onion Valley is classified as having a high hazard potential. The dam's spillway type is listed as 'None', with outlet gates consisting of a slide (sluice gate) and uncontrolled mechanisms.

Although the dam is not under the jurisdiction of the US Army Corps of Engineers, it is subject to state regulation and enforcement. With its strategic location and critical role in providing water for irrigation, Onion Valley is a significant structure that contributes to the water resource management in the region.

Year Completed

1954

Dam Length

407

Dam Height

85

River Or Stream

IRON CREEK

Primary Dam Type

Earth

Surface Area

340

Hydraulic Height

105

Drainage Area

22

Nid Storage

10380

Structural Height

110

Outlet Gates

Slide (sluice gate), Uncontrolled

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

110
       
Seasonal Comparison

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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.