East Mountain stands within the watershed of the Connecticut River, which drains into Long Island Sound in Connecticut.

The north side of East Mountain drains into Madison Brook, thence into Paul Stream, and the Connecticut River.

East Mountain is the site of the former USAF North Concord Air Force Station, a ground-based radar facility of the US military.

Built in 1955, its name was changed to Lyndonville Air Force Station in 1962; it was closed in 1963.

[2] The North Concord AFS reported an unidentified flying object (UFO) sighting in 1961 — a couple hours before the reported abduction of Barney and Betty Hill.