Ish #3 (main Dam)
Ish #3 (main Dam) is an earth dam located in Larimer County, Colorado, built in 1905. It carries a High hazard potential classification and has a satisfactory condition assessment.
About Ish #3 (main Dam)
The primary purpose of Ish #3 (main Dam) is irrigation. The dam stores and diverts water for agricultural irrigation, supporting farming operations in the surrounding area. The dam is owned by The Ish Reservoir Company (private). It impounds the Big Thompson River-os near Berthoud.
The dam stands 48 feet tall and stretches 6,700 feet across, creates a reservoir covering 314 acres, has a maximum storage capacity of 9,065 acre-feet, and collects water from a drainage area of 1 square miles. Completed in 1905, the structure is over a century 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. An emergency action plan is in place for this dam. The most recent inspection on record was 07/20/2022.
At 48 feet, this dam is taller than 92% 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 Colorado 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 | Colorado |
| County | Larimer |
| Nearest City | Berthoud |
| River/Stream | Big Thompson River-os |
| Coordinates | 40.26309, -105.09555 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 48 ft |
| Dam Height | 42 ft |
| Dam Length | 6,700 ft |
| Year Completed | 1905 (121 years old) |
Storage & Hydraulics
| NID Storage | 9,065 acre-ft |
| Max Storage | 9,065 acre-ft |
| Normal Storage | 7,128 acre-ft |
| Surface Area | 314 acres |
| Drainage Area | 1 sq mi |
| Max Discharge | 3,066 cfs |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Irrigation |
| Owner | THE ISH RESERVOIR COMPANY |
| Owner Type | Private |
| Hazard Classification | High |
| Condition Assessment | Satisfactory |
| Last Inspection | 07/20/2022 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | CO01771 |
| Federal ID | CO01771 |
| More Info | State Dam Safety Program |