Big Johnson Reservoir Report
Last Updated: December 6, 2025
Big Johnson is a privately-owned dam located in Clear Lake, Modoc County, California, with a primary purpose of water supply for irrigation and other uses.
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Summary
Completed in 1959, this earth dam stands at 18 feet tall and spans 1900 feet in length, with a storage capacity of 410 acre-feet. It is situated on the Tr Lost River and is regulated by the California Department of Water Resources for safety and maintenance.
Despite being categorized as having a low hazard potential, Big Johnson is regularly inspected and deemed to be in satisfactory condition as of the last assessment in September 2017. The dam has a hydraulic height of 12.5 feet and covers a surface area of 65 acres, serving a drainage area of 3.8 square miles. Its location within the San Francisco District underlines its importance in the local water resource management and its role in supporting agricultural activities in the region.
With its strategic position and crucial role in water supply, Big Johnson serves as a vital infrastructure for the community, ensuring a reliable source of water for irrigation and other needs. Its history, design, and regulatory oversight highlight the importance of responsible dam management in the face of climate change and evolving water resource challenges. Enthusiasts of water resources and climate conservation can appreciate the significance of Big Johnson in sustaining a balance between human needs and environmental preservation.
Year Completed |
1959 |
Dam Length |
1900 |
Dam Height |
18 |
River Or Stream |
Tr Lost River |
Primary Dam Type |
Earth |
Surface Area |
65 |
Hydraulic Height |
12.5 |
Drainage Area |
3.8 |
Nid Storage |
410 |
Hazard Potential |
Low |
Nid Height |
18 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
Klamath Straits Drain Near Worden
|
1cfs |
Ady Canal At Highway 97
|
39cfs |
North Canal At Highway 97
|
104cfs |
Link River At Klamath Falls
|
928cfs |
Klamath River At Keno
|
698cfs |
Pit R Nr Canby Ca
|
56cfs |
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.
Big Johnson