the following text is based on an article in Wikipedia
Three Rivers is a local government district in Hertfordshire in the east of England. The local authority is "Three Rivers District Council", which is based in Rickmansworth. The three rivers of the name are the Chess and the Gade which meet the Colne in the area around Rickmansworth. The population of the district, according to a 2014 estimate, was 90,500.
Three Rivers District was formed on 1 April 1974 by the merger of