Rickmansworth is a town in the Three Rivers district of Hertfordshire, England, 4¼ miles (7km) west of Watford. The town has a population of around 15,000 people and lies on the Grand Union Canal and the River Colne, at the northern end of the Colne Valley Park. It began to grow in the 1920s and 1930s as part of the Metro-land area, due to its station on the Metropolitan line of the London Underground. As such, it is largely a commuter town. It is contained within the M25 with good transport links to Luton and Heathrow Airports.