Big Rock Detention Dam
Big Rock Detention Dam is a gravity dam located in Valley County, Montana, built in 1961. It carries a Low hazard potential classification and has a unsatisfactory condition assessment.
About Big Rock Detention Dam
The primary purpose of Big Rock Detention Dam is flood risk reduction. The dam is designed to control downstream flooding by temporarily storing excess water during storms and releasing it at a controlled rate. The dam is owned by Doi Blm (federal). It impounds the Willow Ck./milk River near Nashua.
The dam stands 13 feet tall and stretches 1,793 feet across, creates a reservoir covering 62 acres, has a maximum storage capacity of 265 acre-feet, and collects water from a drainage area of 0 square miles. Completed in 1961, the structure is well into its service life at over 50 years old.
This dam has a low hazard potential classification, meaning that failure would cause minimal damage, limited primarily to the dam owner's property, with no expected loss of life. Its condition is rated unsatisfactory, indicating serious safety deficiencies exist that require immediate attention or remedial action. The most recent inspection on record was 11/02/2019.
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Location
| State | Montana |
| County | Valley |
| Nearest City | Nashua |
| River/Stream | Willow Ck./milk River |
| Coordinates | 48.08686, -106.95780 |
Physical Characteristics
| Dam Type | Gravity |
| NID Height | 13 ft |
| Dam Length | 1,793 ft |
| Year Completed | 1961 (65 years old) |
Storage & Hydraulics
| NID Storage | 265 acre-ft |
| Max Storage | 265 acre-ft |
| Surface Area | 62 acres |
| Drainage Area | 0 sq mi |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Flood Risk Reduction |
| Owner | DOI BLM |
| Owner Type | Federal |
| Hazard Classification | Low |
| Condition Assessment | Unsatisfactory |
| Last Inspection | 11/02/2019 |
| Emergency Action Plan | Not Required |
Identifiers
| NID ID | MT00711 |
| Federal ID | MT00711 |
| More Info | State Dam Safety Program |