Three Mile Reservoir, also known as Three Mile Swamp, is a natural wetland that feeds Beaver Brook, a waterway that flows through Keene. The reservoir is part of a flood control plan for the Connecticut River Basin, and it provides habitats for various birds and mammals. In 2004, the City of Keene initiated a river restoration project on Beaver Brook to improve its water quality and ecological health. The project involved removing invasive plants, such as Japanese knotweed, and restoring native vegetation along the brook.

This area is to the east of NH-10 as it heads north towards Gilsum from NH-9, with .51 acres of marsh and low-lying forest surrounding Beaver Brook at the north edge of Keene. It is an area of marshes, shrubs, and young forests with good habitat for a variety of birds and mammals.