Harrison County Roadgrade Dam 18-79-43 is an earth dam located in Harrison County, Iowa, built in 1959. It carries a Significant hazard potential classification and has a fair condition assessment.
Dam profile
NID height · 66 ft66 ft tall and 395 ft long, taller than 96% of U.S. dams with a recorded height. Height is drawn to scale against a ten-story building, about 100 ft; the profile is a generic embankment section and the water level is illustrative.
About Harrison County Roadgrade Dam 18-79-43
Harrison County Roadgrade Dam 18-79-43 is a dam in Harrison, Iowa. The dam is owned by Harrison County Board of Supervisors (local government). It impounds the Tr-willow Creek near Missouri River.
The dam stands 66 feet tall and stretches 395 feet across, creates a reservoir covering 14 acres, has a maximum storage capacity of 486 acre-feet, and collects water from a drainage area of 1 square miles. Completed in 1959, the structure is well into its service life at over 50 years old.
This dam has a significant hazard potential classification, meaning that a failure could cause significant economic damage — flooding roads, bridges, or property downstream — but is not expected to result in loss of life. Its condition is rated fair — the dam has minor deficiencies that are being addressed or do not pose an immediate safety threat. The most recent inspection on record was 10/11/2023.
At 66 feet, this dam is taller than 96% of all dams in the United States.
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Location
| State | Iowa |
| County | Harrison |
| Nearest City | Missouri River |
| River/Stream | Tr-willow Creek |
| Coordinates | 41.65890, -95.90080 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 66 ft |
| Dam Height | 66 ft |
| Dam Length | 395 ft |
| Year Completed | 1959 (67 years old) |
Storage & Hydraulics
| NID Storage | 486 acre-ft |
| Max Storage | 486 acre-ft |
| Normal Storage | 246 acre-ft |
| Surface Area | 14 acres |
| Drainage Area | 1 sq mi |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Other |
| Owner | Harrison County Board of Supervisors |
| Owner Type | Local Government |
| Hazard Classification | Significant |
| Condition Assessment | Fair |
| Last Inspection | 10/11/2023 |
| Emergency Action Plan | Not Required |
Identifiers
| NID ID | IA00475 |
| Federal ID | IA00475 |
| More Info | State Dam Safety Program |