Upper Oso Reservoir Report

Last Updated: December 5, 2025

Upper Oso, located in Mission Viejo, Orange County, California, is a key water resource infrastructure managed by a public utility.


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Summary

This earth dam, completed in 1979, stands tall at 142 feet, with a hydraulic height of 135 feet, serving multiple purposes including flood risk reduction, irrigation, and water supply. With a storage capacity of 3,700 acre-feet and a drainage area of 1.13 square miles, Upper Oso plays a crucial role in managing water resources in the region.

The dam, situated on Oso Creek, is under the regulatory oversight of the California Department of Water Resources for safety and inspection. Despite its satisfactory condition assessment, Upper Oso poses a high hazard potential, necessitating regular inspections and emergency preparedness measures. In the event of any emergencies, the dam operators are required to follow established protocols for ensuring public safety and mitigating risks associated with its operation.

As a vital component of the water supply infrastructure in the area, Upper Oso underscores the importance of sustainable water management practices in the face of changing climate patterns. With a focus on ensuring the dam's integrity and functionality, stakeholders continue to monitor its performance and implement necessary risk management measures to safeguard against potential hazards and support the water needs of the local community.

Year Completed

1979

Dam Length

800

Dam Height

142

River Or Stream

Oso Creek

Primary Dam Type

Earth

Surface Area

115

Hydraulic Height

135

Drainage Area

1.13

Nid Storage

3700

Hazard Potential

High

Nid Height

142
       
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.