Lake William
Lake William is an earth dam located in Napa County, California, built in 1960. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Lake William
The primary purpose of Lake William 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 William E. Jarvis (private). It impounds the Tr Milliken Cr.
The dam stands 66 feet tall and stretches 575 feet across, creates a reservoir covering 16 acres, has a maximum storage capacity of 340 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 1960, 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. Notably, no emergency action plan is currently on file for this high-hazard dam. The most recent inspection on record was 09/17/2025.
At 66 feet, this dam is taller than 96% of all dams in the United States.
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 |
| River/Stream | Tr Milliken Cr |
| Coordinates | 38.36170, -122.22310 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 66 ft |
| Dam Height | 66 ft |
| Dam Length | 575 ft |
| Year Completed | 1960 (66 years old) |
Storage & Hydraulics
| NID Storage | 340 acre-ft |
| Max Storage | 340 acre-ft |
| Surface Area | 16 acres |
| Drainage Area | 0 sq mi |
Ownership & Safety
| Purpose | Water Supply |
| Owner | William E. Jarvis |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 09/17/2025 |
| Emergency Action Plan | No |
Identifiers
| NID ID | CA00586 |
| Federal ID | CA00586 |
| More Info | State Dam Safety Program |