Big Rock Ranch
Big Rock Ranch is an earth dam located in Marin County, California, built in 2002. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Big Rock Ranch
The primary purpose of Big Rock Ranch 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 Lucasfilm, Ltd (private). It impounds the Nicasio Creek Trib To Lagunitas Creek near Nicasio.
The dam stands 45 feet tall and stretches 490 feet across, creates a reservoir covering 7 acres, has a maximum storage capacity of 91 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 2002, the structure is relatively new.
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 12/17/2025.
At 45 feet, this dam is taller than 91% of dams nationwide.
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 | Marin |
| Nearest City | Nicasio |
| River/Stream | Nicasio Creek Trib To Lagunitas Creek |
| Coordinates | 38.05060, -122.62770 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 45 ft |
| Dam Height | 45 ft |
| Dam Length | 490 ft |
| Year Completed | 2002 (24 years old) |
Storage & Hydraulics
| NID Storage | 91 acre-ft |
| Max Storage | 91 acre-ft |
| Surface Area | 7 acres |
| Drainage Area | 0 sq mi |
Ownership & Safety
| Purpose | Water Supply |
| Owner | Lucasfilm, LTD |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 12/17/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CA01457 |
| Federal ID | CA01457 |
| More Info | State Dam Safety Program |