Rickmansworth is a small town in southwest Hertfordshire, England, approximately 17 miles (27 km) northwest of central London and inside the perimeter of the M25 motorway. The town is mainly to the north of the Grand Union Canal and the River Colne. The nearest large town is Watford, approximately 5 miles (8.0 km) to the east. Rickmansworth is the administrative seat of the Three Rivers District Council. The confluence of the Chess and the Gade with the Colne in Rickmansworth inspired the district's name. The enlarged Colne flows south to form a major tributary of the River Thames. The town is served by the Metropolitan line of the London Underground and Chiltern Railways from London Marylebone to Aylesbury.