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. 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.