River Mole

River Mole
The River Mole at the foot of Box Hill in Surrey
Map showing the River Mole (dark blue) and part of the River Thames (light blue)
Location
Country	England
Counties	West Sussex, Surrey
Districts / Boroughs	Horsham, Crawley, Mole Valley, Reigate and Banstead, Elmbridge
Towns	Crawley, Horley, Dorking, Leatherhead, Cobham, Surrey, Esher, Walton-on-Thames, Hersham
Physical characteristics
Source	Baldhorns Copse
• location	Rusper, Horsham, West Sussex
• coordinates	51°7′30″N 0°16′26″W / 51.12500°N 0.27389°W / 51.12500; -0.27389
• elevation	105 m (344 ft)
Mouth	River Thames
• location	East Molesey, Elmbridge, Surrey
• coordinates	51°24′4″N 0°20′21″W / 51.40111°N 0.33917°W / 51.40111; -0.33917
• elevation	6 m (20 ft)
Length	80 km (50 mi)
Basin size	512 km2 (198 sq mi)
Discharge
• location	Esher
• average	5.43 m3/s (192 cu ft/s)
• minimum	1.00 m3/s (35 cu ft/s)(9 August 1993)
• maximum	99.9 m3/s (3,530 cu ft/s)(9 December 1994)
Discharge
• location	Castle Mill, Dorking
• average	3.74 m3/s (132 cu ft/s)
Discharge
• location	Kinnersley Manor, Sidlow
• average	2.21 m3/s (78 cu ft/s)
Discharge
• location	Horley
• average	1.40 m3/s (49 cu ft/s)
Discharge
• location	Gatwick Airport
• average	0.33 m3/s (12 cu ft/s)

The River Mole is a tributary of the River Thames in southern England. It rises in West Sussex near Gatwick Airport and flows northwest through Surrey for 80 km (50 miles) to the Thames at Hampton Court Palace.^[1] 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.^[2] Much of the catchment area lies on impermeable rock (including Weald Clay and London Clay), meaning that the river level responds rapidly to heavy rainfall.^[2]

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.^[1][3] Twelve Sites of Special Scientific Interest (SSSIs) 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.^[4][5] The Mole Gap forms part of a Special Area of Conservation and is an SSSI of European importance.^[6]

The river has captured the imagination of several authors and poets,^[7] particularly since in very hot summers the river channel can become dry between Dorking and Leatherhead, most recently in 2022.^[8] In John Speed's 1611 map of Surrey, this stretch of the river is denoted by a series of hills accompanied by the legend "The river runneth under". However the river's name is unlikely to have derived from this behaviour: The Oxford Dictionary of English Place Names suggests that Mole either comes from the Latin mola (a mill) or is a back-formation from Molesey (Mul's island).^[9] Domesday Book lists twenty mills on the river in 1086, of which Sidlow Mill was the oldest, dating from Saxon times.^[10][11]