Petrolia
Petrolia is an earth dam located in Petroleum County, Montana, built in 1951. It carries a High hazard potential classification and has a poor condition assessment.
About Petrolia
The primary purpose of Petrolia is irrigation. The dam stores and diverts water for agricultural irrigation, supporting farming operations in the surrounding area. The dam is owned by Petrolia Irrigation District (local government). It impounds the South Fork Flatwillow Creek near Mosby.
The dam stands 76 feet tall and stretches 3,635 feet across, creates a reservoir covering 515 acres, has a maximum storage capacity of 14,170 acre-feet, and collects water from a drainage area of 612 square miles. Completed in 1951, 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 poor, meaning the dam has recognized safety deficiencies that require remediation to address risks. An emergency action plan is in place for this dam. The most recent inspection on record was 04/26/2023.
At 76 feet, this dam is taller than 97% 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 Montana 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 | Montana |
| County | Petroleum |
| Nearest City | Mosby |
| River/Stream | South Fork Flatwillow Creek |
| Coordinates | 46.94100, -108.23590 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 76 ft |
| Dam Height | 76 ft |
| Dam Length | 3,635 ft |
| Year Completed | 1951 (75 years old) |
Storage & Hydraulics
| NID Storage | 14,170 acre-ft |
| Max Storage | 14,170 acre-ft |
| Normal Storage | 9,192 acre-ft |
| Surface Area | 515 acres |
| Drainage Area | 612 sq mi |
| Max Discharge | 10,200 cfs |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Irrigation |
| Owner | PETROLIA IRRIGATION DISTRICT |
| Owner Type | Local Government |
| Hazard Classification | High |
| Condition Assessment | Poor |
| Last Inspection | 04/26/2023 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | MT00008 |
| Federal ID | MT00008 |
| More Info | State Dam Safety Program |