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River Mole
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 miles) to the Thames at 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 Weald Clay and London Clay), 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 (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. The Mole Gap forms part of a Special Area of Conservation and is an SSSI of European importance.
The river has captured the imagination of several authors and poets, particularly since in very hot summers the river channel can become dry between Dorking and Leatherhead, most recently in 2022. 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). Domesday Book lists twenty mills on the river in 1086, of which Sidlow Mill was the oldest, dating from Saxon times.
The drainage area of the Mole is 477 km2 (184 sq mi) and forms 5% of the River Thames's catchment area above Teddington. The catchment area receives 761 mm (30.0 in) of rain each year; the greatest average level of rainfall is 800 mm (31.5 in) around Crawley. The Mole catchment reaches a maximum elevation of 265 m (869 ft) above Ordnance Datum at Leith Hill to the southwest of Dorking. There is only one aquifer in the drainage basin, at Fetcham, which means that the majority of the water in the river is from surface drainage, particularly from Gatwick Airport and the urban areas of Horley and Crawley, and that the flow rate responds rapidly to rainfall.
The Mole rises in Baldhorns Copse 700 m (0.4 mi) to the south of the village of Rusper in West Sussex. It flows initially southwards for 1 km (0.6 mi) to a small lake at Baldhorns Park, before running eastwards through a largely rural area towards Crawley. The first tributaries to join the young river drain the northernmost part of St Leonard's Forest, between Horsham and Crawley, although much of the forest is in the catchment area of the River Arun. The Mole skirts the northern suburbs of Crawley where it is joined by its first major tributary, Ifield Brook, which drains Ifield Mill Pond.
The first gauging station on the Mole is south of Gatwick Airport (57 m [187 ft] above OD). The mean flow is 0.33 m3/s (12 cu ft/s) and the river ran dry at this point for the first time in the summer of 1995. The Mole runs under the airport runway in a culvert completed in 1985. The course of the Mole within the airport perimeter has been altered several times since commercial flights began in 1945; however the meanders visible on the 1839 tithe map in the 1.5 km (0.93 mi) stretch immediately north of the runway were reinstated in 1999, in a £1.2 million project to facilitate airport expansion.
The Mole enters Surrey to the south of Horley, where it meets the Gatwick Stream, a tributary draining Worth Forest to the southeast of Crawley. The second-largest Sewage Treatment Works (STW) in the Mole catchment is located on the Gatwick Stream 3 km (1.9 mi) upstream of the confluence with the Mole: Crawley STW discharges 15,000 m3 (530,000 cu ft) of water per day, and in prolonged dry periods it accounts for up to 75% of the flow of the Mole downstream of the confluence. The mean flow measured at Horley gauging station (52 m [171 ft] above OD) is 1.40 m3/s (49 cu ft/s).
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River Mole
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 miles) to the Thames at 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 Weald Clay and London Clay), 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 (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. The Mole Gap forms part of a Special Area of Conservation and is an SSSI of European importance.
The river has captured the imagination of several authors and poets, particularly since in very hot summers the river channel can become dry between Dorking and Leatherhead, most recently in 2022. 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). Domesday Book lists twenty mills on the river in 1086, of which Sidlow Mill was the oldest, dating from Saxon times.
The drainage area of the Mole is 477 km2 (184 sq mi) and forms 5% of the River Thames's catchment area above Teddington. The catchment area receives 761 mm (30.0 in) of rain each year; the greatest average level of rainfall is 800 mm (31.5 in) around Crawley. The Mole catchment reaches a maximum elevation of 265 m (869 ft) above Ordnance Datum at Leith Hill to the southwest of Dorking. There is only one aquifer in the drainage basin, at Fetcham, which means that the majority of the water in the river is from surface drainage, particularly from Gatwick Airport and the urban areas of Horley and Crawley, and that the flow rate responds rapidly to rainfall.
The Mole rises in Baldhorns Copse 700 m (0.4 mi) to the south of the village of Rusper in West Sussex. It flows initially southwards for 1 km (0.6 mi) to a small lake at Baldhorns Park, before running eastwards through a largely rural area towards Crawley. The first tributaries to join the young river drain the northernmost part of St Leonard's Forest, between Horsham and Crawley, although much of the forest is in the catchment area of the River Arun. The Mole skirts the northern suburbs of Crawley where it is joined by its first major tributary, Ifield Brook, which drains Ifield Mill Pond.
The first gauging station on the Mole is south of Gatwick Airport (57 m [187 ft] above OD). The mean flow is 0.33 m3/s (12 cu ft/s) and the river ran dry at this point for the first time in the summer of 1995. The Mole runs under the airport runway in a culvert completed in 1985. The course of the Mole within the airport perimeter has been altered several times since commercial flights began in 1945; however the meanders visible on the 1839 tithe map in the 1.5 km (0.93 mi) stretch immediately north of the runway were reinstated in 1999, in a £1.2 million project to facilitate airport expansion.
The Mole enters Surrey to the south of Horley, where it meets the Gatwick Stream, a tributary draining Worth Forest to the southeast of Crawley. The second-largest Sewage Treatment Works (STW) in the Mole catchment is located on the Gatwick Stream 3 km (1.9 mi) upstream of the confluence with the Mole: Crawley STW discharges 15,000 m3 (530,000 cu ft) of water per day, and in prolonged dry periods it accounts for up to 75% of the flow of the Mole downstream of the confluence. The mean flow measured at Horley gauging station (52 m [171 ft] above OD) is 1.40 m3/s (49 cu ft/s).