district, administrative and historic county of Hertfordshire, southeast England, in the southwestern corner of the county. Three Rivers district is essentially a dormitory suburban area on the northwest border of Greater London. The district's gently rolling countryside is traversed by the wooded valley of the River Colne. Rickmansworth is the district seat. Sands and gravels in the district have been extensively worked, and the resulting series of flooded pits are used now for recreation or for water storage. Rickmansworth and Croxley Green have paper mills; other industries include printing works and asbestos sheet and truck manufacturing. Area 34 square miles (88 square km). Pop. (2001) 82,843.