The River Mole is a tributary of the River Thames in southern England. It rises in West Sussex near Horsham and flows northwest through Surrey for 80 km to the Thames at East Molesey, opposite 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 , meaning that the river level responds rapidly to heavy rainfall.
During the second half of the 20th century pollution levels in the river were high, however since 1995 the water quality has improved dramatically and the Mole now boasts the greatest diversity of fish species of any river in England. Twelve Sites of Special Scientific Interest that include wetland habitats are located within the Mole catchment area and the stretch of river through Leatherhead has been designated a Local Nature Reserve. The Mole Gap forms part of a Special Area of Conservation and is an SSSI of European importance.
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