River Mole

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The Mole near Fetcham

The River Mole is one of the two major rivers within Surrey. It is a tributary of the River Thames, entering it at East Molesey.

The Mole rises in Baldhorns Copse near Rusper, just over the border in Sussex, south of Gatwick Airport and the nascent brook flows northwest into Surrey, skirting Gatwick Airport and through Surrey from north to south for the rest of its course of 50 miles to the Thames at East Molesey.

The Mole breaks through the North Downs between Dorking and Leatherhead, where it cuts a steep-sided valley, known as the Mole Gap, through the chalk.[1] Much of the catchment area lies on impermeable rock (including Wealden Clay and London Clay), meaning that the river level responds rapidly to heavy rainfall.[1]

The Mole is navigable for just the 400 yards up from the mouth with the River Thames, to Molember Weir at East Molesey where there is a private mooring facility.[2] During the close fishing season the river may be paddled by canoe between Brockham and Fetcham with an appropriate licence from the British Canoe Union.

Name of the river

The common belief it that the river Mole is named from its running underground in the Mole Gap, where it breaks through the chalk of the North Downs, but this is unlikely: it is more likely to be a back-formation from Molesey (Mul's island).[3]

The name of the river is first recorded in the Red Book of Thorney in 983 AD as Emen and in the 1005 AD Cartulary of the Abbey of Eynsham as both Emen and Æmen.[4][5] Variations in the name are recorded throughout the mediæval period and the river appears as Amele in Domesday Book and subsequently as Emele in twelfth and thirteenth century Court Roles.[6] This early name is probably derived from the Old English word æmen meaning misty or causing mists.[5] The name of the River Ember probably has its origins in this name.[6]

The use of the name Mole for the river does not appear until the sixteenth century, first occurring as Moule in Harrison's Description of Britain of 1577. The antiquarian William Camden uses the Latinized form Molis in the 1586 edition of Britannia and Michael Drayton is the first to use Mole in his poem Poly-Olbion published in 1613.[6] 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).[3] In John Rocque's 1768 map of Surrey, the name Moulsey River is used.[7]

The river runneth under

In very hot summers the river channel can become dry between Dorking and Leatherhead (as it did during the 1976 drought).[8][9] Furthermore, the chalk in this stretch is full of swallow-holes, and in times of low flow, the whole flow of the river can disappear into these holes.

This behaviour has given the Mole a reputation amongst writers as an underground river. In John Speed's 1611 map of Surrey this stretch of the river near Dorking is denoted by a series of hills accompanied by the legend "The river runneth under".


The Mole at the foot of Box Hill

The river has captured the imagination of several authors and poets, for the sweetness of its banks and most pointedly for its reputation as an underground river, which has spurred the romantic imagination.

In The Faerie Queene (first published in 1590) Edmund Spenser wrote of the river:

And Mole, that like a nousling mole doth make
His way still under ground till Thamis he overtake.[10]

In Poly-Olbion (first published in 1612) the poet Michael Drayton described the journey taken by the River Thames to the sea:

As still his goodly traine yet every houre increast,
And from the Surrian shores cleer Wey came down to meet
His Greatnes, whom the Tames so gratiously doth greet
That with the Fearne-crown'd Flood he Minion-like doth play:
Yet is not this the Brook, entiseth him to stay.
But as they thus, in pompe, came sporting on the shole,
Gainst Hampton-Court he meets the soft and gentle Mole.
Whose eyes so pierc't his breast, that seeming to foreslowe
The way which he so long intended was to go,
With trifling up and down, he wandreth here and there;
And that he in her sight, transparent might appeare,
Applyes himselfe to Fords, and setteth his delight,
On that which might make him gratious in her sight.[11]

But Tames would hardly on: oft turning back to show,
For his much loved Mole how loth he was to go.
The mother of the Mole, old Holmsdale, likewise beares
Th'affection of her childe, as ill as they do theirs:
But Mole respects her words, as vaine and idle dreames,
Compar'd with that high joy, to be belov'd of Tames:
And head-long holds her course, his company to win.
Mole digs her selfe a path, by working day and night
(According to her name, to shew her nature right)
And underneath the Earth, for three miles space doth creep:
Till gotten out of sight, quite from her mothers keep,
Her foreintended course the wanton Nymph doth run;
As longing to imbrace old Tame and Isis son...[12]

Drayton writes in the appendix to Song XVII:

This Mole runnes into the earth, about a mile from Darking in Surrey, and after some two miles sees the light againe, which to be certaine hath been affirmed by Inhabitants thereabout reporting triall made of it.

John Milton (c. 1562–1647) described the river as:

sullen Mole that runneth underneath

- echoed by Alexander Pope (1688–1744) in Windsor Forest (1713):

:And sullen Mole that hides his diving flood

Robert Bloomfield (1766–1823) writes the following lines about the Mole Valley in his 1806 poem Wild Flowers:

Sweet Health, I seek thee! Hither bring
Thy balm that softens human ills;
Come on the long drawn clouds that fling
Their shadows o'er the Surry-Hills.
Yon green-topt hills, and far away
Where late as now I freedom stole,
And spent one dear delicious day
On thy wild banks romantic Mole.

Ay there's the scene! Beyond the sweep
Of London's congregated cloud,
The dark-brow'd wood, the headlong steep,
And valley paths without a crowd!
Here Thames I watch thy flowing tides,
Thy thousand sails am proud to see;
But where the Mole all silent glides
Dwells Peace - and Peace is wealth to me.[13]

Extract from The River Mole or Emlyn Stream by Mary Uniacke (writing under her maiden name Mary Drinkwater-Bethune), which was published in 1839.

Who may count back that forgotten time
When first the waters forced an outlet here:
When the foundations of these stedfast hills
Were shaken, and the long imprisoned stream
Flowed through the yawning chasm? That awful day
Yet leaves its trace. The waters find their way,
Now laughing in the sun - now swallowed up
In caverns pervious to their course alone,
They leave their channel dry, and hide awhile
Their silent flow; like bitter tears, unshed
From the dim eye, before a careless world
Unheeding of our grief; but swelling still
In the full heart, which leaves unsoothed, unseen,
And broods o'er ruined hopes, and days gone by.


Map of the River Mole (dark blue)
River Mole at Baldhorns Park, half a mile or so downstream of the source at Rusper

The drainage area of the River Mole is 198 square miles and forms 5% of the River Thames basin above the latter's tidal limit. Annually the catchment area receives 30 inches of rain each year. The Mole catchment reaches a maximum elevation of 869 feet above sea level at Leith Hill to the southwest of Dorking (Surrey's county top).[14] 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.


Upper Mole

The River Mole rises in Baldhorns Copse half a mile to the south of the village of Rusper in Sussex. The brook flows initially southwards for half a mile or so to a small lake at Baldhorns Park, before flowing 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 lies within the catchment 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 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 on the mile-long stretch immediately north of the runway were reinstated in 1999, in a £1.2 million project to facilitate airport expansion.[15][16]

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 2 miles upstream of the confluence with the Mole, which discharges a substantial amount of the river's flow at that point. The Mole passes Horley to the west, flowing north towards Sidlow and entering a largely rural area. The Earlswood Brook, a tributary draining the urban areas to the south of Reigate and Redhill, joins the Mole at Sidlow.

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 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. Approximately 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.[17] The river leaves the Wealden clay at Brockham, flowing briefly across the greensand and gault clay to Pixham, half a mile northeast of Dorking. At Pixham the Mole meets the Pipp Brook, a tributary draining the northern slopes of Leith Hill.

Mole Gap

The Mole at Dorking

Between Dorking and Leatherhead the Mole cuts a steep-sided valley known as the Mole Gap though the North Downs, carving a 560-foot high river cliff on the western flank of Box Hill and a smaller 165-foot cliff at Ham Bank in Norbury Park. The sudden change from impermeable Wealden clay to permeable chalk and the increased gradient of the river (which drops 50 feet in the six-mile stretch between Brockham and Leatherhead, compared to 10 feet in 12 miles between Horley and Brockham) allow the water table to drop below the bed of the river. Water is able to flow out of the river through swallow holes in the bed and banks, decreasing the volume of water carried in the river channel, and creating the underground stream so beloved of poets.

The course of the river north of the village of Westhumble was partially straightened when the Epsom to Horsham railway was built in 1837, with the removal of a small meander.[18] The meander was reinstated in 1997, in an attempt to create a local nature reserve, although it has since become blocked by silt. The entirety of the Mole Gap lies within the Surrey Hills "'Area of Outstanding Natural Beauty".[19]

Lower Mole

Painshill Park Waterwheel

At Leatherhead, the Mole leaves the chalk and turns northwestwards to flow across impermeable London Clay 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.[8] The aquifer at Fetcham is the only one in the entire catchment area.[20] At Cobham the river swings round in a pronounced axe-head meander, passing Painshill Park to the south. A waterwheel 35 feet in diameter raises water 16 feet from the river to feed the ornamental lake in the park.[21]

From Painshill Park the river flows northwestwards towards the Thames, passing West End Common. Here there are long flood defences, not on the riverbank so as to spoil the pretty river, but discreetly placed inland, in response to heavy flooding in September 1968.

North of Esher, 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 flowing side by side in a north easterly direction, merging 450 yards before the confluence with the River Thames.

Meeting of Mole and Thames at Hampton Court

The Mole discharges into the River Thames at Molesey.


The River Mole has the most diverse fish population of any river in Britain.[22] The Gatwick Stream is dominated by coarse fish such as brown trout, brook lamprey, and eel. In 2003, the upper River Mole near Meath Green Lane, Salfords, was enhanced to create a gravel spawning area to encourage chub and dace in addition to roach.[20] In 1974 zander, a non-indigenous coarse fish native to Europe, were introduced legally to Old Bury Hill Lake which supplies the Pippbrook. Zander have been caught in the Lower Mole below Dorking since the 1980s.[23]

The marsh frog (a non-native species introduced from Europe in the 1930s) is now commonly found in the upper Mole and its tributaries around Newdigate and Gatwick. There is no evidence that the presence of the frogs has had a deleterious effect on indigenous amphibians.[24] A second non-native species, the edible frog was introduced to a site at Newdigate in the early 1900s. It has been recorded in tributaries of the River Mole at Capel and Brockham.[25]

In the Mole Gap between Dorking and Leatherhead the river supports populations of chub, dace, barbell and brown trout. Both barbel and brown trout are extremely sensitive to water quality and pollution.

Below Leatherhead the river has historically supported larger predatory fish including chub, perch, pike, and eels. However, in recent years, chub and eel numbers have begun to decline.

North of Esher the old river channel is dominated by floating pennywort, a highly invasive weed, which cuts off all light to the river bed, reducing oxygen levels and resulting in a poor habitat for fish. The Ember flood relief channel has a diverse fish population, including chub, dace, roach, bleak, large pike and barbel.[20] At the confluence of the Mole and the River Thames it is possible to catch brown trout and flounder.[20]

The Mole underground

Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a 560-foot high river cliff on the western flank of Box Hill. The bedrock is permeable chalk and the water table lies permanently below the level of the riverbed, allowing water to drain out of the river through swallow holes in the bed and banks.[26] The amount water lost from the river is significant and in very hot summers the channel can become dry between Mickleham and Thorncroft Manor, (recorded most recently in 1949 and 1976).[9][27][28]

At Leatherhead, the river leaves the chalk and flows across impermeable London Clay. It is at this point that the water table rises sufficiently, enabling the water to flow back into the main river channel.[26]

In a survey in 1958, the geologist C C Fagg, identified twenty five active swallow holes between Dorking and Mickleham, finding that most were only a few inches in diameter and were located in the vertical banks of the river below the waterline.[29] Most holes were difficult to observe in times of normal or heavy flow and were susceptible to silting up as new holes were continually being formed.[29] A few much larger swallow holes were also observed separated from the main river by a channel of about a yard.[29][30] An area to the west of the Burford Bridge Hotel containing half a dozen of these larger swallow holes was located along the projected course of the A24 Mickleham Bypass. During the construction of the bypass in 1936. Initially the surveyors attempted to fill the holes with rubble to prevent the foundations of the new road subsiding. However this proved to be impractical and they were instead covered by concrete domes (up to 60 feet in diameter) each fully supported by the surrounding chalk and provided with a manhole and access shaft to allow periodic inspections.[30][31] During the late 1960s the domes were reopened and inspected and the alluvium in the largest swallow hole was observed to have subsided by 5 feet under the centre of one of the domes.[31] When the Dorking to Leatherhead railway was constructed in 1859, a fossilised swallow hole was discovered in the cutting at the south end of Box Hill and Westhumble railway station, suggesting that even in its early history, the river possessed swallow holes.[29]

The author, Daniel Defoe (who attended school in Dorking and probably grew up in the village of Westhumble)[32][33] described the swallow holes in the River Mole in his book A Tour Through the Whole Island of Great Britain (first published in 1724):

.. the current of the river being much obstructed by the interposition of those hills, called Box Hill ... it forces the waters as it were to find their way through as well as they can; and in order to do this, beginning, I say, where the river comes close to the foot of the precipice of Box-Hill, called the Stomacher, the waters sink insensibly away, and in some places are to be seen (and I have seen them) little channels which go out on the sides of the river, where the water in a stream not so big as would fill a pipe of a quarter of an inch diameter, trills away out of the river, and sinks insensibly into the ground. In this manner it goes away, lessening the stream for above a mile, near two, and these they call the Swallows.

Not all of the water removed from the river by the swallow holes, is returned to the channel at Leatherhead. The chalk aquifer also feeds the springs at the southern end of Fetcham Mill Pond, which have never been known to run dry.[26] A survey carried out in March 1883, estimated that the Fetcham springs were producing approximately 3.6 million gallons of water every day.[35] A second survey performed in 1948, estimated that the same springs were yielding approximately five million gallons of water a day.[36] The water table in the chalk of the Wey Gap is significantly higher than might be expected from natural rainwater percolation alone and it has been suggested that a proportion of the excess water originates from the Mole Gap.[37]


Domesday Book listed twenty mills on the River Mole in 1086,[38][39] of which Sidlow Mill was the oldest, dating from before the Norman Conquest.[38][39]

Upper Mole

Horley Mill was first mentioned in a deed of the early thirteenth century. The most recent mill was demolished in 1959, although the mill house still stands.[39]

The first mill at Sidlow was built during Anglo-Saxon times. The final mill on the site was demolished in 1790, however remains of the mill leat are still visible.[39]

Mention is made of a mill at Brockham in 1634 and remains of the mill race are still visible.[39]

Lower Mole

Slyfield Mill near Stoke d'Abernon is first mentioned in Domesday Book. It was used for fulling woollen cloth and milling corn.[39]

Cobham Mill
The River Mole where it runs separately from the River Ember

Five of the mills mentioned in Domesday Book were in the Elmbridge Hundred.[38]

Downside Mill, Cobham was the mill of the manor of Downe. Until the Dissolution of the Monasteries it was owned by Chertsey Abbey.[39] It has been used for many purposes including the processing of corn, paper, iron, tinplate and flock and the generation of electricity. The present building dates from the 18th century but it is inaccessible to the public.

Cobham Mill, downstream of Leatherhead, consisted of two mills used for grinding corn. In 1953 the larger mill was demolished by Surrey County Council to allieviate traffic congestion on Mill Road. The remaining red brick mill dates from the 1822 and was in use until 1928. It was restored to full working order by the Cobham Mill Preservation Trust, and is now open to the public from 2 pm to 5 pm on the second Sunday of each month (between April and October).

Esher Mill was at the end of Lower Green Road where there is now an industrial estate. It was used to process corn, brass wire, iron, paper, linoleum, and books. For many years there may have been two mills on the site for corn grinding and industrial use. There were a series of fires over a century and after the last in 1978 the buildings were demolished.

East Molesey Upper Mill was associated with the manor of Molesey Matham. It was used to produce gunpowder from the time of the Commonwealth until about 1780. The island where it stood now forms part of the ornamental gardens of a housing development called "The Wilderness".

East Molesey Lower Mill, also known as Sterte Mill, was associated with the manor of Molesey Prior. During the Commonwealth it was used for gunpowder manufacture, but after the restoration of the monarchy in 1660 it reverted to corn milling. An old timber structure was replaced by a brick building in the 1820s which can be seen from the bridge over the Ember in Hampton Court Way.

In addition there was Ember Mill, which stood on the banks of the old course of the River Ember near Hampton Court Way.


The Mole, notwithstanding its great length and size by the time it reaches the Thames, is a fairly self-reliant river in that it has no major tributaries. The biggest are the Isfield Brook, Gatwick Stream, Earlswood Brook, Pipp Brook and The Rye.


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  10. The Fairie Queen, book 4, canto 11, verse 32
  11. Poly-Olbion, Song XVII lines 20-32
  12. Poly-Olbion, Song XVII lines 47-50, 53-57, 59-64
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  20. Cite error: Invalid <ref> tag; no text was provided for refs named CAMs
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  22. Environment Agency
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  34. Daniel Defoe's Tour Through the Whole Island of Great Britain, 1726
  35. JW Grover (1887) Chalk springs in the London basin, illustrated by the Newbury, Wokingham, Leatherhead and Rickmansworth Water Works Proceedings of the Institution of Civil Engineers 90
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