The River Mole is a tributary of the River Thames in southern England, rising near Gatwick Airport in West Sussex and flowing north-west through Surrey for approximately 80 kilometres (50 miles) before joining the Thames at Hampton Court Palace. The river lends its name to the Surrey district of Mole Valley and is widely regarded as having the most diverse fish population of any river in England. The Mole cuts through a variety of landscapes, most notably passing through the chalk North Downs between Dorking and Leatherhead, where it carves a steep-sided valley known as the Mole Gap. Much of the catchment lies on impermeable rock — including Weald Clay and London Clay — meaning river levels respond rapidly to heavy rainfall, which significantly influences fishing conditions throughout the year.