Rickmansworth is a small town in south-west Hertfordshire, England, situated approximately 20 miles (32 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 (formerly the Grand Junction 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 local authority named from the confluence of three rivers within its borders. The River Gade and the Grand Union Canal join the upper River Colne near Rickmansworth's eastern boundary and are joined by the River Chess near the town centre from where 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 & Chiltern Railways route London Marylebone to Aylesbury.