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 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, 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.
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 or is a back-formation from Molesey. Domesday Book lists twenty mills on the river in 1086, of which Sidlow Mill was the oldest, dating from Saxon times.

Catchment area

The drainage area of the Mole is and forms 5% of the River Thames's catchment area above Teddington. The catchment area receives of rain each year; the greatest average level of rainfall is around Crawley. The Mole catchment reaches a maximum elevation of 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.

Course

Upper Mole

The Mole rises in Baldhorns Copse to the south of the village of Rusper in West Sussex. It flows initially southwards for 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. The mean flow is 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 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 in the Mole catchment is located on the Gatwick Stream upstream of the confluence with the Mole: Crawley STW discharges 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 is.
The Mole passes Horley to the west, flowing north towards Sidlow and entering a largely rural area. south of Sidlow the mean flow is measured as at Kinnersley Manor gauging station. The Earlswood Brook, a tributary draining the urban area of Reigate and Redhill, joins the Mole at Sidlow. The largest STW in the Mole catchment discharges up to per day into the Earlswood Brook.
From Sidlow, the Mole turns northwest towards Brockham. A number of minor tributaries join the river from the west and are typically second order streams draining the woodland and arable land between Horsham and Dorking. The 18th-century weir at Betchworth was modified in 2004 to facilitate the installation of two 27.5 kW low-head hydro turbines. About 90% of the energy generated is fed into the regional electricity grid, while the remainder is used to supply the Betchworth Park Estate, where the weir is situated. The river leaves the Weald Clay at Brockham, passing Betchworth Castle and flowing briefly across greensand and Gault Clay to Pixham, northeast of Dorking. A mean flow of is measured at a fourth gauging station, located at Castle Mill. At Pixham the Mole meets the Pipp Brook, a tributary draining the northeastern slopes of Leith Hill.

Mole Gap

Between Dorking and Leatherhead the Mole cuts a steep-sided valley though the North Downs, carving a river cliff on the western flank of Box Hill and a smaller cliff at Ham Bank in Norbury Park. The sudden change from impermeable Weald Clay to permeable chalk and the increased gradient of the river, allow the water table to drop below the bed of the river. Water is therefore able to flow out of the river through swallow holes in the bed and banks, decreasing the volume of water carried in the main channel.
The course of the river through Norbury Park was partially straightened when the Epsom to Horsham railway was built in 1837, with the removal of a small meander north of Westhumble. The meander was reinstated in 1997, in an attempt to enhance this area of the Park, although it has since become blocked by silt. The entirety of the Mole Gap lies within the Surrey Hills Area of Outstanding Natural Beauty.

Lower Mole

At Leatherhead, the Mole leaves the chalk and turns northwestwards to flow across impermeable London Clay, winding towards Cobham. The water table rises at this point and much of the water which drained out of the channel through the chalk returns through springs in the riverbed. The aquifer at Fetcham is the only one in the entire catchment area. The next major tributary, The Rye, joins to the north of Leatherhead, before the river is crossed by the M25 motorway. At Cobham the river swings round in a pronounced axehead meander skirting Painshill Landscape Garden, where a diameter waterwheel raises water from the river to feed the ornamental lake in the park.
From Painshill Park the river flows northeastwards to the Thames, passing to the west of Esher Commons and then forming the boundary between Hersham and Esher, where the mean flow of the river is. In response to heavy flooding of East Molesey and Thames Ditton in September 1968, the river was modified downstream of Albany Bridge to the Thames and new flood defences were constructed. Finally the Mole splits into two branches at the Island Barn Reservoir near Molesey: the northern branch continues as the River Mole and the southern branch is known as the River Ember. The two rivers flow either side of the reservoir, before running side by side in a northeasterly direction, merging before the confluence with the River Thames, on the reach above Teddington Lock.
Prior to the last ice age, the River Thames followed a more northerly route to the North Sea, from Reading via Marlow, Chorleywood, St Albans, Hertford and along the present Suffolk–Essex border. During this period, the Mole is thought to have merged with the River Wey near Byfleet and then flowed in a north-easterly direction via Richmond to meet the proto-Thames near Ware in Hertfordshire. Today the Mole and Wey are less than apart at their closest point near Painshill Park. During the Middle Pleistocene period, a large ice sheet built up across much of the East of England, reaching as far south as St Albans and Chelmsford, blocking the path of the proto-Thames. Glacial meltwater from the Anglian ice sheet caused the Thames to divert southwards and flood the valley of the Mole–Wey river, thus adopting its present route through London.

Ecology

Water quality

Standard water quality of Mole and its tributaries has improved markedly since the 1990s. In 1990 the Environment Agency assessed 23% of the watercourses as Grade B or better. In 2002 this figure was 60%. Investment in the Sewage Treatment Works in the catchment area has improved the quality of the discharges into the river, and modifications to the runway and apron drainage systems at Gatwick Airport mean that surface water is diverted to aerated pollution control lagoons and balancing ponds for treatments, including acid/alkali neutralisation, before release into the river.
In 1972 a sub-debate on the future cleanliness of the Mole was had by the House of Lords, involving a member of the South-East Strategic Committee of the Thames Conservancy and four others. In this it was said, "...I can see no future for . The Mole Valley has been polluted. The Mole where I used to tickle trout in my youth is a drain, and it will remain a drain." Rebuttals included that the Mole is a "charming Surrey steam" and "If Lord Lytton went to fish in the River Mole – and there are still angling clubs there – he would find that he would still be able to catch a trout".
Highly polluting discharges have become less common but have taken place since 2000. In 2003, Gatwick Airport Ltd pleaded guilty to charges of allowing chemical pollution to enter the River Mole after a detergent, used to clean rubber and oil from the runway, was washed into Crawters Brook by airport workers. The Environment Agency estimated that up to 5200 fish of 14 different species were killed as the pollution drained downstream. The airport was fined £30,000 by Lewes Crown Court. In May 2003, sewage leaking from a pump operated by Thames Water leaked into the Stanford Brook, killing coarse fish in the Gatwick stream.
Water quality of the River Mole in 2019:

Section	Ecological Status	Chemical Status	Overall Status	Length	Catchment	Channel

						Heavily modified