High Quality Content by WIKIPEDIA articles! The River Mole is a tributary of the River Thames in southern England. It rises in West Sussex near Gatwick Airport and flows north west through Surrey for 80 km (50 mi) to the Thames near Hampton Court Palace. The river gives its name to the Surrey district of Mole Valley. The Mole crosses the North Downs between Dorking and Leatherhead, where it cuts a steep-sided valley, known as the Mole Gap, through the chalk. Much of the catchment area lies on impermeable rock (including Wealden Clay and London Clay), meaning that the river level responds rapidly to heavy rainfall.