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The River Mole is a river in southern England, which rises in West Sussex near Gatwick Airport and flows north west through Surrey for 80 km (50 miles) to the River Thames near Hampton Court Palace. The river gives its name to the Surrey district of Mole Valley. 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, where it cuts through the North Downs (most recently in 1976). 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". The river's name is unlikely to have derived however 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). In medieval times the river was known as the Emlyn Stream.

Catchment area
The drainage area of the River Mole is 512 km² and forms 5% of the River Thames catchment area above Teddington. Annually the catchment area receives 761 mm rain each year, the greatest average level of rainfall is 800 mm rain around Crawley. 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 to the east of the West Sussex village of Rusper, initially flowing eastwards across the Wealden Clay. The first tributaries to join the young river drain the northernmost part of St Leonards Forest, between Horsham and Crawley, although the much of this area lies within the catchment of the River Arun. After skirting the Crawley suburb of Isfield, the river turns northwards to run under the runway of Gatwick Airport in a culvert. 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 1.5 km stretch immediately north of the runway were reinstated in 1999, in a £1.2M project to facilitate airport expansion.

The Mole enters Surrey to the west of Horley, where it meets its first major tributary, the Gatwick Stream, which drains Worth Forest to the southeast of Crawley. The river changes direction at this point to flow northwestwards towards Brockham. The tributaries joining from the west are typically second order streams draining the arable land between Horsham and Dorking, those which join from the west are generally larger and drain the outcrops of greensand around Outwood and Nuffield.

Mole Gap
The Mole leaves the Wealden Clay at Brockham, flowing briefly across the greensand and gault clay, before turning northwards to cut a steep-sided valley (the Mole Gap) though the North Downs. The sudden change from impermeable Wealden Clay to permeable chalk and the increased gradient of the river (which drops 15 m in the six mile stretch between Brockham and Leatherhead, compared to 3 m 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. 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. The meander was reinstated in 1999, in an attempt to create a local nature reserve, although it has since become blocked by silt.

Lower Mole
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. The aquifer at Fetcham is the only one in the entire catchment area. At Cobham the river changes direction again, flowing northwards, passing Esher to the west before splitting 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. The two rivers flow either side of the reservoir, before flowing side by side in a north easterly direction, merging 400 metres before the confluence with the River Thames, on the reach above Teddington Lock. (For the purposes of the remainder of this article, the River Mole and the River Ember are treated as a single entity.)

Fisheries
The Gatwick Stream is dominated by coarse fish such as brown trout, brook lamprey, and eel. In 2003, the River Mole at Meath Green was enhanced to create a gravel spawning area to encourage chub and dace in addition to roach. In the Mole Gap between Dorking and Leatherhead the river supports populations of chub, dace, barbel, 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. At the confluence of the Mole and the River Thames it is possible to catch brown trout and flounder.

Flora
Anationally scarce species which is locally common especially on the River Mole is the greater dodder Cuscuta europaea.

Mole Valley Corridor Local Nature Reserve
The Mole Valley Corridor has been designated a local nature reserve. The river's habitat has been enhanced as the river flows over a variety of different rock types: alluviums and gravel, chalk and clay. There are numerous different habitats: scrub, woodland, hedges, banks, and meadows as well as the water itself.

Ice Age
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 2 km (1.5 miles) apart at their closest point south of Esher.) During the mid-Pleistocene period (500 000 years before present), 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 into the valley of the Mole-Wey river, thus adopting its present route through London.

Swallow Holes between Dorking and Leatherhead
The underlying rock type in this part of Surrey is chalk and the water table lies permanently below the level of the riverbed. This allows water to drain out of the river through swallow holes in the river bed and banks. 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. In a survey in 1958, the geologist C.C. Fagg, identified twenty five active swallow holes between Dorking and Mickleham and classified them into four basic types: 1) Swallow holes at the bottom of depressions in the river bed.

2) Swallow holes in the bed of the river but on high spots near the banks [or on islands]

3) Swallows at varying levels in the vertical sides of the river... The openings into these are either into chalk or alluvium leading to chalk.

4) Swallows in depressions on the flood plain near to, but more or less detached from, the vertical banks. They are inaccessible to river water until it rises sufficiently to overflow into them. Most swallow holes are only a few centimetres in diameter, are of the third type and are difficult to observe in times of normal or heavy river flow. Old holes, particularly those in the river bed (types 1 and 2) are susceptible to silting up and new holes are continually being formed. Fagg continues: In 1948 [swallow hole] No. 2 could only be detected by water trickling between flints but in 1949, after the winter submergence, it was a gaping hole. The local historian and Westhumble resident Ronnie Sheppard describes a hole of the fourth type, which he recorded in 1947: ...at the foot of Ham Bank where the Mole takes a sharp turn to the north; it was about two feet in diameter and the water flowed down in a clockwise direction as if out of a bath. A year later, Fagg surveyed the same hole: At first it appeared that the river would have to rise to the level of the lip, before it could enter the pit containing the swallow, but water was found to enter at a lower level through a hidden channel in alluvium... Such a channel might be initiated by a moorhen or a burrowing animal. This channel was doubtless the original access to the swallow. An article published in The Times in April 1936 describes a low lying area of approximately half an acre, lying alongside and connected to the river, between the Burford Bridge and the southernmost railway bridge. There were four or five groups of swallow holes (again of the fourth type) at this site "around which the water swirled before it poured into the depths below." No. 1 comprises two big holes and several smaller areas round the brink; the southern measures across the top 80 x 80 ft. and is 10 ft. deep, the northern measures 70 x 70 ft.; No. 2 consists of two, 60 x 60 ft. and 30 x 30 ft. ; No. 3 is a straight down pipe 15 x 15 ft. and No. 4 is 90 x 88 ft. and 14 ft. deep." The A24 Mickleham Bypass was under construction at the time at which the article was written and it was decided 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 18 m in diameter) each fully supported by the surrounding chalk and provided with a manhole and access shaft to allow periodic indpections. A survey in the late 1960s showed that the alluvium in the largest swallow hole had subsided 1.5 m under the centre of the dome.

Fagg (who was at the time a researcher at the Juniper Hall Field Centre) estimated that one swallow hole (of the first type) was able to carry 0.75 million gallons of water out of the river channel per day and that the daily average volume of the river at Dorking was 70 million gallons. As a rough estimate, the twenty five swallow holes which were active in 1957, were therefore able to reduce the flow of the river through the Mole Gap by approximately one quarter. The author, Daniel Defoe (who attended school in Dorking and probably grew up in the village of Westhumble) writing in his book A tour thro' 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. 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.

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.

Navigation
In March 1663, a bill was passed by the House of Commons, but was defeated in the House of Lords "to make navigable or otherwise passable, divers Rivers from Greenstead [ East Grinstead ], Arundell [ Arundel ], Petersfield, Darkin [ Dorking ] and Farnham... to London."

In 1664, an act was passed by both Houses of Parliament to make the River Mole navigable from Reigate to the River Thames, but was never executed. The only Surrey river to have been made fully navigable is the River Wey. The Mole is navigable for the 600 yards from the confluence with the Thames to Molember Weir, where there is a private mooring facility.

Crossings of the River Mole
The North Downs Way crosses the river at Box Hill via seventeen hexagonal stepping stones, which are frequently submerged after heavy rainfall. The location is popular both with anglers and families, although swimming is strongly discouraged. The stones give their name to the pub in the nearby village of Westhumble.

When the Burford Bridge was rebuilt in 1937, excavations revealed a "flint-surfaced approach to [a] ford at low level having all the signs of Roman workmanship" suggesting that Stane Street (which ran from London to Chichester via Dorking) crossed the river at this point. In Defoe's time, there was a footbridge at this point, but carts and waggons had to cross the river by a ford. There are three listed bridges in Leatherhead of which the 14 arch bidge, built in 1784 as a rebuild of an earlier medieval bridge is the oldest. The Shell Bridge links two small islands in the lanscaped gardens of Thoncroft Mannor, 0.5 km south of the town.

Watermills


The earliest recorded watermill on the river was at Cobham, downstream of Leatherhead, which was noted in the Domesday Book of 1086. The present red brick mill dates from the 1822 and was in use until 1928. In 1953 the main part of the mill was demolished by Surrey County Council to allieviate traffic congestion on Mill Road. The remaining building was restored to full working order by the Cobham Mill Preservation Trust, and is now open to the public from 2pm to 5pm on the second Sunday of each month (between April and October).

Hydroelectric power
In 2004, two 27.5 kW low-head hydro turbines were installed in an existing 18th century weir near Betchworth in Surrey, to to generate electricity. 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 situtated.

Pollution
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 spread downstream. The airport was fined £30,000 by Lewes Crown Court.

River diversions and flood relief schemes
The course of the river at Gatwick has been considerably altered since the airport opened in 1932 and it now flows under the main runway in a culvert. A 1.9 km section of river north of the runway and the west of the North Terminal was diverted in 1999 at a cost of £1.2M, to allow for airport expansion. Meanders were introduced into the course of the river and flood defences were improved. The impact on the river biodiversity has been positive and water draining from the airport surfaces is now held in a series of balancing ponds before being released into the river.

The River Mole originally flowed into the River Thames at the point where the present Hampton Court bridge now crosses the Thames (approximately 500 m upstream of the present confluence, on the reach above Teddington Lock).

However, during the early 1930s, when Hampton Court Way and the bridge were built, the River Mole was redirected to flow into the River Ember and both rivers now enter the Thames in a single widened and straightened channel once occupied only by the River Ember. There have been further alterations to the courses of these two rivers in a major flood prevention scheme since serious flooding in the area in 1947 and 1968.

Photograph of the River Ember Flood Relief Channel under construction in 1981.

Literature
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. It has been proposed that the following lines from Spenser's poem Colin Clouts come home againe (published in 1595) also refer to the river: Old father Mole, (Mole hight that mountain gray

That walls the Northside of Armulla dale)

He had a daughter fresh as floure of May,

Which gave that name unto that pleasant vale;

Mulla the daughter of old Mole, so hight

The Nimph, which of that water course has charge,

That springing out of Mole, doth run it downe right

To Butteuant....

In Poly-Olbion (first published 1619) 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..

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... He 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 (1562?-1647) described the river as sullen Mole that runneth underneath

In a similar vein, Alexander Pope (1688-1744) wrote in his poem Windsor Forest (first published 1713) And sullen Mole that hides his diving flood

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.

Tributaries
The order is from the mouth to the source:
 * Leigh Brook
 * Salfords Stream
 * Redhill Brook
 * Burstow Stream
 * Baldhorns Brook
 * Gatwick Stream
 * Stanford Brook
 * Holmwood Stream
 * Pipp Brook
 * The Rye
 * Bookham Brook

Distributary

 * River Ember