A1 Site, sometimes called Stump Pond, is a 313-acre impoundment of the Assabet River located off Mill Road in Westborough. The average depth of A1 is 3 feet with a maximum of 19 feet. The entire shoreline lies with flood control land owned by the Department of Conservation and Recreation and is undeveloped. The Back two-thirds of the pond is a shallow expansive maze of aquatic vegetation, submerged stumps, and fallen timbers which are the remnants of the trees inundated during the formation of the pond.

The town of Westborough manages a concrete ramp and a small gravel parking area suitable for 5 trailers off Mill Road. Additional parking is provided on the shoulders of Mill Road. The immediate shoreline lies within public land and is accessible.