Coordinates: 42°39?32?N 71°19?06?W? / ?42.6589°N 71.3184°W? / 42.6589; -71.3184 Beaver Brook is a 30.7-mile (49.4 km) river located in New Hampshire and Massachusetts in the United States. It is a tributary of the Merrimack River, part of the Gulf of Maine watershed.