Middlesex

County, northeastern Massachusetts, U.S., west and northwest of Boston and bordered on the north by New Hampshire. The county consists of an upland region drained by the Merrimack, Nashua, Assabet, Concord, Sudbury, and Shawsheen rivers. Other waterways include Whitehall and Cambridge reservoirs, Lake Cochituate, and historic Walden Pond. Parklands include more than