First Reservoir Dam
First Reservoir Dam is an earth dam located in Rockland County, New York, built in 1917. It carries a High hazard potential classification and has a poor condition assessment.
About First Reservoir Dam
The primary purpose of First Reservoir Dam 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 Nysoprhp - Palisades Interstate Park Commission (state). It impounds the Horse Chock Brook near Thiells.
The dam stands 46 feet tall and stretches 650 feet across, creates a reservoir covering 6 acres, has a maximum storage capacity of 116 acre-feet, and collects water from a drainage area of 1 square miles. Completed in 1917, 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 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 05/07/2025.
At 46 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 New York 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 | New York |
| County | Rockland |
| Nearest City | Thiells |
| River/Stream | Horse Chock Brook |
| Coordinates | 41.21623, -74.05820 |
Physical Characteristics
| Dam Type | Earth |
| NID Height | 46 ft |
| Dam Height | 46 ft |
| Dam Length | 650 ft |
| Year Completed | 1917 (109 years old) |
Storage & Hydraulics
| NID Storage | 116 acre-ft |
| Max Storage | 116 acre-ft |
| Normal Storage | 68 acre-ft |
| Surface Area | 6 acres |
| Drainage Area | 1 sq mi |
| Spillway Type | Uncontrolled |
Ownership & Safety
| Purpose | Water Supply |
| Owner | NYSOPRHP - PALISADES INTERSTATE PARK COMMISSION |
| Owner Type | State |
| Hazard Classification | High |
| Condition Assessment | Poor |
| Last Inspection | 05/07/2025 |
| Emergency Action Plan | Yes |
Identifiers
| NID ID | NY01531 |
| Federal ID | NY01531 |
| More Info | State Dam Safety Program |