Hempstead Lake Dam

About Hempstead Lake Dam

The primary purpose of Hempstead Lake Dam is recreation. The dam creates a reservoir used for recreational activities such as fishing, boating, and swimming. The dam is owned by Nys Office Of Parks, Recreation & Historic Preservation- Long Island Region (state). It impounds the Mill River near Rockville Centre.

The dam stands 24 feet tall and stretches 1,200 feet across, creates a reservoir covering 149 acres, has a maximum storage capacity of 1,220 acre-feet, and collects water from a drainage area of 14 square miles. Completed in 1873, the structure is one of the older dams in the region, with over a century and a half of service.

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 11/22/2024.

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.

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