Lake Cynthia
Lake Cynthia is an earth dam located in Napa County, California, built in 1955. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Lake Cynthia
The primary purpose of Lake Cynthia is water supply. The dam stores water for municipal, industrial, or agricultural use, helping ensure a reliable supply for the surrounding area. The dam is owned by Krupp Brothers Vineyard (private). It impounds the Tr Soda Creek near City Of Napa.
The dam stands 20 feet tall and stretches 750 feet across, creates a reservoir covering 12 acres, has a maximum storage capacity of 92 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 1955, the structure is well into its service life at over 50 years old.
This dam carries a high hazard potential classification, which means that a failure or uncontrolled release of water would likely cause loss of human life. This classification reflects the consequences of failure, not the likelihood — it indicates that people live or work in the area downstream. Its condition is rated satisfactory, indicating no existing or potential dam safety deficiencies are recognized. An emergency action plan is in place for this dam. The most recent inspection on record was 06/19/2025.
Living Downstream
If you live or work downstream of this dam, consider taking these steps: learn your local evacuation routes, check whether your county emergency management office has a dam failure inundation map, and sign up for local emergency alerts. The California dam safety program can provide more specific information about emergency planning for this structure.
Search for dams near your address to see all dams in your area with hazard classifications and safety data.
Location
| State | California |
| County | Napa |
| Nearest City | City Of Napa |
| River/Stream | Tr Soda Creek |
| Coordinates | 38.34990, -122.28200 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 20 ft |
| Dam Height | 20 ft |
| Dam Length | 750 ft |
| Year Completed | 1955 (71 years old) |
Storage & Hydraulics
| NID Storage | 92 acre-ft |
| Max Storage | 92 acre-ft |
| Surface Area | 12 acres |
| Drainage Area | 0 sq mi |
Ownership & Safety
| Purpose | Water Supply |
| Owner | Krupp Brothers Vineyard |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 06/19/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA00980 |
| Federal ID | CA00980 |
| More Info | State Dam Safety Program |