Johnston is a private gravity dam located in Half Moon Bay, California, built in 1919 for water supply purposes on the Arroyo Leon. With a height of 31 feet and a storage capacity of 30 acre-feet, Johnston serves as a vital water resource in the region. The dam has a low hazard potential and was last inspected in September 2020, with a satisfactory condition assessment reported in September 2017.

Managed by the California Department of Water Resources, Johnston is regulated, permitted, inspected, and enforced by state agencies to ensure its safety and compliance with regulations. The dam's primary purpose is water supply, with no associated structures and no federal involvement in its ownership, funding, design, construction, or operations. Despite its age, Johnston continues to play a crucial role in water management in San Mateo County, highlighting the importance of maintaining and monitoring aging infrastructure for climate resilience and sustainable water resource management.

With its historical significance and continued functionality, Johnston stands as a testament to the engineering and management of water resources in California. As climate change impacts water availability and quality, dams like Johnston play a key role in ensuring a reliable water supply for communities while also serving as potential tools for flood control and ecosystem management. As stakeholders continue to assess and address the risks associated with aging infrastructure, Johnston remains a valuable asset in the region's water infrastructure network.