Lake Cynthia in Napa, California, is a privately owned water supply reservoir on Tr Soda Creek. Built in 1955, this earth dam stands at 20 feet high and has a storage capacity of 92 acre-feet, serving primarily for irrigation and water supply purposes. With a drainage area of 0.65 square miles and a surface area of 12 acres, Lake Cynthia is a vital resource in the region, providing crucial water storage for agricultural and domestic use.

Despite its relatively small size, Lake Cynthia plays a significant role in water management in the area. With a high hazard potential due to its location and structural characteristics, the dam is regularly inspected and maintained to ensure its safety and integrity. The most recent inspection in September 2017 deemed the dam to be in satisfactory condition, with a comprehensive emergency action plan in place. The dam's proximity to the city of Napa and its crucial role in water supply make it a key asset in the region's water resource infrastructure, highlighting the importance of proper management and maintenance for sustainable water usage and climate resilience.