Circle S is a privately owned dam located in Napa, California, along the Tr Milliken Cr river. Built in 1979, this earth dam stands at a height of 28 feet with a hydraulic height of 24.7 feet, serving primarily for water supply purposes. With a storage capacity of 131 acre-feet, Circle S has a significant hazard potential but is currently assessed to be in satisfactory condition as of September 2017.

Managed by the Department of Water Resources (DWR) and regulated by the Safety of Dams agency, Circle S undergoes regular inspections, with the last one conducted in December 2019. Its location in a high-risk earthquake zone raises concerns, but the dam meets guidelines for emergency action plans, although specific details on preparedness and risk management measures are not outlined in the data. Despite its age, Circle S continues to play a vital role in irrigation, recreation, and water supply within the region.

For water resource and climate enthusiasts, Circle S presents an intriguing case study of a crucial infrastructure in California's water supply network. Its design and construction details, along with regulatory oversight and hazard assessment, offer valuable insights into the challenges and responsibilities associated with maintaining dams in earthquake-prone regions. As the debate around water resource management and climate change intensifies, understanding the complexities of structures like Circle S becomes increasingly important for ensuring the safety and sustainability of our water systems.