Editing River Mole (section)

==Course==
[[File:River Mole at Baldhorns Park - geograph.org.uk - 1811633.jpg|thumb|River Mole at Baldhorns Park, approximately {{cvt|1|km|1}} downstream of the source at Rusper]]

===Upper Mole===
The Mole rises in Baldhorns Copse {{convert|700|m|mi|1|abbr=on}} to the south of the village of [[Rusper]] in [[West Sussex]]. It flows initially southwards for {{convert|1|km|mi|1|abbr=on}} to a small lake at Baldhorns Park, before running eastwards through a largely rural area towards Crawley.<ref name=OS_Rusper>{{cite map |publisher=Ordnance Survey |title=Crawley & Horsham |year=2009 |scale=1:25000 |series=Explorer |isbn=978-0-319-24085-4}}</ref> 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 Water Mill|Ifield Mill Pond]].

The first [[stream gauge|gauging station]] on the Mole is south of Gatwick Airport ({{cvt|57|m|disp=sqbr}} above [[Ordnance Datum|OD]]).<ref group=note>The gauging station south of Gatwick Airport was installed in 2005, replacing an earlier station located {{cvt|200|m|yd}} downstream.</ref> The mean flow is {{convert|0.33|m3/s|cuft/s|abbr=on}} and the river ran dry at this point for the first time in the summer of 1995.<ref name=CEH/> The Mole runs under the airport runway in a culvert completed in 1985.<ref>{{cite web |url= https://www.environmental-expert.com/news/forty-years-on-from-the-floods-of-1968-37047 |title= Forty years on from the floods of 1968 |date= 11 September 2008 |publisher= Environmental Expert |access-date= 11 February 2023 |archive-date= 11 February 2023 |archive-url= https://web.archive.org/web/20230211210036/https://www.environmental-expert.com/news/forty-years-on-from-the-floods-of-1968-37047 |url-status= live }}</ref> 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 {{cvt|1.5|km}} stretch immediately north of the runway were reinstated in 1999, in a £1.2 million project to facilitate airport expansion.<ref>{{cite web |url= http://www.baa.com/assets//B2CPortal/Static%20Files/glsstnbltyrprt2002.pdf |title= Sustainability Report 2001/02 |author= Earl G |year= 2001 |publisher= BAA London Gatwick |access-date= 9 February 2010 |archive-url= https://web.archive.org/web/20110522191841/http://www.baa.com/assets//B2CPortal/Static%20Files/glsstnbltyrprt2002.pdf |archive-date= 22 May 2011 |url-status= live }}</ref><ref name="autogenerated1">{{cite web |url= http://www.tpsconsult.co.uk/tps/assets/pdf/profiles/gatwick_river_mole.pdf |title= River Mole Diversion, Gatwick |year= 2001 |publisher= TPS |access-date= 9 February 2010 |archive-url= https://web.archive.org/web/20080627171133/http://www.tpsconsult.co.uk/tps/assets/pdf/profiles/gatwick_river_mole.pdf |archive-date= 27 June 2008 |url-status= live }}</ref>

The Mole enters [[Surrey]] to the south of Horley, where it meets the [[Gatwick Stream]], a tributary draining [[Worth, West Sussex|Worth Forest]] to the southeast of Crawley. The second-largest Sewage Treatment Works (STW) in the Mole catchment is located on the Gatwick Stream {{convert|3|km|mi|abbr=on}} upstream of the confluence with the Mole: Crawley STW discharges {{cvt|15000|m3}} 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.<ref name=Hazelton>{{cite journal |author= Hazelton C|year= 1998 |title=Variations between continuous and spot-sampling techniques in monitoring a change in river-water quality |journal= Water and Environment Journal |volume=12 |issue=2 |pages=124–129 |doi=10.1111/j.1747-6593.1998.tb00161.x|s2cid= 108508464 }}</ref> The mean flow measured at Horley gauging station ({{cvt|52|m|disp=sqbr}} above OD) is {{convert|1.40|m3/s|cuft/s|abbr=on}}.<ref name=CEH/>

The Mole passes Horley to the west, flowing north towards [[Sidlow]] and entering a largely rural area. {{convert|0.7|km|mi|1|abbr=on}} south of Sidlow the mean flow is measured as {{convert|2.21|m3/s|cuft/s|abbr=on}} at Kinnersley Manor gauging station ({{cvt|48|m|disp=sqbr}} above OD).<ref name=CEH/> The Earlswood Brook, a tributary draining the urban area of [[Reigate]] and [[Redhill, Surrey|Redhill]], joins the Mole at Sidlow. The largest STW in the Mole catchment (Reigate STW) discharges up to {{cvt|118,500|m3}} per day into the Earlswood Brook.<ref group=note>A principal tributary of the Earlswood Brook is the Redhill Brook, which includes The Moors wetland nature reserve within its catchment area.</ref>

From Sidlow, the Mole turns northwest towards [[Brockham]]. A number of minor tributaries join the river from the west and are typically [[Strahler number|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&nbsp;kW [[low head hydro power|low-head]] [[water turbine|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.<ref>{{cite web |url= http://www.tvenergy.org/pdfs/betchworth-estate.pdf |title= Small hydroelectric power: Betchworth Park |publisher= TV Energy |access-date= 8 February 2010 |archive-url= https://web.archive.org/web/20091229075037/http://www.tvenergy.org/pdfs/betchworth-estate.pdf |archive-date= 29 December 2009 |url-status= live }}</ref> The river leaves the [[Weald Clay]] at Brockham, passing Betchworth Castle <ref>{{National Heritage List for England|num=1017996|desc=Betchworth Castle|access-date=16 June 2013}}</ref> and flowing briefly across [[greensand]] and [[Gault Clay]] to [[Pixham]], {{convert|1|km|mi|1|abbr=on}} northeast of [[Dorking]]. A mean flow of {{convert|3.74|m3/s|cuft/s|abbr=on}} is measured at a fourth gauging station, located at Castle Mill ({{cvt|39|m|disp=sqbr}} above OD).<ref name=CEH/> At Pixham the Mole meets the [[Pipp Brook]], a tributary draining the northeastern slopes of Leith Hill.<ref group=note>The Pipp Brook drains the northernmost heavily wooded slopes of Leith Hill (which include Squire's Great Wood and Abinger Forest), as well as areas of [[Wotton, Surrey|Wotton]], [[Westcott, Surrey|Westcott]] and Dorking.</ref>

===Mole Gap===
Between Dorking and [[Leatherhead]] the Mole cuts a steep-sided valley (the Mole Gap) though the [[North Downs]], carving a {{convert|170|m|ft|adj=mid|-high}} river cliff on the western flank of Box Hill and a smaller {{convert|50|m|adj=on}} cliff at Ham Bank in [[Norbury Park]]. The sudden change from [[permeability (earth sciences)|impermeable]] [[Weald Clay]] to [[permeability (earth sciences)|permeable]] [[chalk]] and the increased gradient<ref group=note>The Mole descends {{cvt|15|m|disp=sqbr}} in the {{cvt|10|km|0|disp=sqbr}} stretch between Brockham and Leatherhead, compared to {{cvt|3|m|disp=sqbr}} in {{cvt|12|mi|disp=sqbr|order=flip}} between Horley and Brockham.</ref> 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 hole]]s 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 [[Sutton & Mole Valley Lines|Epsom to Horsham railway]] was built in 1837, with the removal of a small meander north of [[Westhumble]].<ref>Shepperd R (1982) The Manor of Wistomble in the Parish of Mickleham ''Westhumble Association''</ref> The meander was reinstated in 1997, in an attempt to enhance this area of the Park, although it has since become blocked by [[silt deposit|silt]]. The entirety of the Mole Gap lies within the [[Surrey Hills AONB|Surrey Hills Area of Outstanding Natural Beauty]].<ref>{{cite web |url= http://www.surreyhills.org/ |title= Surrey Hills Area of Outstanding Natural Beauty |year= 2010 |publisher= Surrey Hills Partnership |access-date= 9 February 2010 |archive-url= https://web.archive.org/web/20100126045834/http://www.surreyhills.org/ |archive-date= 26 January 2010 |url-status= live }}</ref>

===Lower Mole===
[[File:Painshill-Waterwheel.jpg|thumb|upright|Painshill Park Waterwheel (on its [[Leat|millstream]] channel)]]
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.<ref name=Wooldridge_Hutchings>{{cite book |title= London's Countryside : Geographical Field Work for Students and Teachers of Geography |url= https://archive.org/details/londonscountrysi0000wool |url-access= registration |last1= Wooldridge |first1= SW |last2= Hutchings |first2= GE |year= 1957 |publisher= Methuen |location= London |pages=[https://archive.org/details/londonscountrysi0000wool/page/89 89–90]}}</ref> The aquifer at [[Fetcham]] is the only one in the entire catchment area.<ref name=CAMs/>  The next major tributary, [[The Rye (brook)|The Rye]], (which drains [[Ashtead]]) 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 Park|Painshill Landscape Garden]], where a {{convert|35|ft|m|order=flip|adj=on}} diameter [[waterwheel]] raises water {{convert|5|m|ft}} from the river to feed the ornamental lake in the park.<ref>{{National Heritage List for England|num=1030126|desc=The Water Wheel|grade=II|access-date=16 June 2013}}</ref><ref>{{cite web |url=http://www.painshill.co.uk/The-Park/The-Hamilton-Landscapes.aspx |title=The Bramah Waterwheel |publisher=Painshill Park |access-date=23 February 2010 |archive-url=https://web.archive.org/web/20100607121017/http://www.painshill.co.uk/The-Park/The-Hamilton-Landscapes.aspx |archive-date=7 June 2010 |url-status=live }}</ref><ref group=note>The undershot wheel at Painshill Park was restored in 1987 and is located at the end of an artificial [[leat]], rather than on the main river channel.</ref>

From Painshill Park the river flows northeastwards to the Thames, passing to the west of [[Esher Commons]] and then forming the boundary between [[Hersham, Surrey|Hersham]] and [[Esher]], where the mean flow of the river is {{convert|5.43|m3/s|cuft/s|abbr=on}}.  In response to [[Great Flood of 1968|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.<ref name=Riddell>{{cite conference |title= Channel Protection, Operation and Maintenance in Flood Alleviation Schemes |year= 1984 |conference= Channels and Channel Control Structures |author= Riddell KJ |editor= Smith KVH |book-title= Proceedings of the 1st International Conference|publisher= Springer-Verlag |location= Heidelberg, Germany |volume= 4|pages= 53–71 |isbn=0-905451-21-X}}</ref> Finally the Mole splits into two branches at the [[Island Barn Reservoir]] near [[Molesey]]: the northern (and smaller) branch continues as the River Mole and the southern branch is known as the [[River Ember]].<ref name=FASLRM>WE Foster WB Harris (1988) Flood Alleviation Scheme for the Lower River Mole ''Proceedings of the Institution of Civil Engineers'' 84 235-263</ref> The two rivers flow either side of the reservoir, before running side by side in a northeasterly direction, merging {{cvt|400|m|yd}} before the [[confluence]] with the River Thames, on the reach above [[Teddington Lock]].<ref name=FASLRM/> (For the purposes of the remainder of this article, the River Mole and the River Ember are treated as a single entity.)

Prior to the [[Last Glacial Period|last ice age]], the River Thames followed a more northerly route to the [[North Sea]], from [[Reading, Berkshire|Reading]] via [[Marlow, Buckinghamshire|Marlow]], [[Chorleywood]], [[St Albans]], [[Hertford]] and along the present [[Suffolk]]–[[Essex]] border.<ref>Wooldridge SW (1938) The glaciation of the London Basin and the evolution of the lower Thames drainage system ''Quarterly Journal of the Geology Society'' 94 627-668</ref> 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, London|Richmond]] to meet the proto-Thames near [[Ware, Hertfordshire|Ware]] in [[Hertfordshire]]. Today the Mole and Wey are less than {{convert|2|km|mi|abbr=on}} apart at their closest point near Painshill Park. During the [[Middle Pleistocene]] period,<ref group=note>The Middle Pleistocene period was {{circa|781,000–126,000}} [[before present|years ago]]</ref> 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. [[Meltwater|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]].<ref>Bridgland and Gibbard (1997) Quaternary River Diversions in the London Basin and the Eastern English Channel {{lang|fr|Géographie physique et Quaternaire|italic=yes}} 51 (3) 337–346</ref>