Three Rivers is a local government district in southwest Hertfordshire, England. Its council is based in Rickmansworth. The district was formed on 1 April 1974, under the Local Government Act 1972, by the merger of Rickmansworth Urban District, Chorleywood Urban District and part of Watford Rural District. The confluence of the Chess and the Gade with the Colne in Rickmansworth inspired the district's name. The main offices are located in Rickmansworth in Three Rivers House, opened in 1991.