81-705: 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
162-583: A Special Area of Conservation and is an SSSI of European importance. The stretch of river between Thorncroft Manor (1 km [0.6 mi] south of Leatherhead ) and River Lane in Fetcham has been designated a Local Nature Reserve. Although much of the surrounding land has been taken by residential and commercial development, this section of the Mole supports 20 different mammal species, 20 butterfly species and 15 species of dragonfly . The geology of
243-434: A navigational context, if one were floating on a raft or other vessel in the main stream, this would be the side the tributary enters from as one floats past; alternately, if one were floating down the tributary, the main stream meets it on the opposite bank of the tributary. This information may be used to avoid turbulent water by moving towards the opposite bank before approaching the confluence. An early tributary
324-520: A Grand Imperial Ship Canal from Deptford to Chichester passing through the Mole Gap, however he was unable to attract sufficient financial interest in his scheme. Today the Mole is navigable for the 400 m (440 yd) from the confluence with the River Thames to Molember Weir at East Molesey where there is a private mooring facility. The river may be paddled all year by canoe between Brockham and Fetcham with an appropriate licence from
405-664: A flat-bottomed dug-out canoe were found at the confluence of the Mole and Thames in 1877 by a local boatman. The canoe is preserved at the Pitt Rivers Museum in Oxford . The Mole Gap and North Downs do not appear to have been settled until the late Stone Age: A large axe, typical of a "rough-out" axe produced during the Neolithic period, which was discovered in 1952 during building work in Westhumble, A flint mine of
486-447: A larger stream ( main stem or "parent" ), river, or a lake . A tributary does not flow directly into a sea or ocean . Tributaries, and the main stem river into which they flow, drain the surrounding drainage basin of its surface water and groundwater , leading the water out into an ocean. The Irtysh is a chief tributary of the Ob river and is also the longest tributary river in
567-414: A left tributary which is called Right Fork Steer Creek. These naming conventions are reflective of the circumstances of a particular river's identification and charting: people living along the banks of a river, with a name known to them, may then float down the river in exploration, and each tributary joining it as they pass by appears as a new river, to be given its own name, perhaps one already known to
648-476: A lower flood plain, which broadens as the river turns in an axehead meander. At the east end of Painshill Park, the flood plain narrows into a trench about 60 m (66 yd) wide, in which the river runs northwards for 6.5 km (4.0 mi) towards Hersham, where the river enters the flood plain of the River Thames. Between Cobham and Esher, the Mole's historic courses have deposited gravel on top of
729-612: 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 (1.2 mi) 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
810-407: A particular place is used for the datum. The First Geodetic Levelling of England and Wales (1840–1860) needed to define a datum plane from which to specify spot heights. At first it was specified as a horizontal plane 100 feet below an arbitrary benchmark on St John's Church, Liverpool. Subsequently, however, it was redefined as mean sea level (MSL). To establish MSL, tidal observations were taken at
891-542: 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
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#1732773313352972-472: A smaller 50-metre (160 ft) 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
1053-457: A survey in 1958, the geologist C. C. Fagg identified 25 active swallow holes between Dorking and Mickleham ; the majority were only a few centimetres in diameter and were located in the vertical banks of the river below the water line. 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. A few much larger swallow holes were also observed separated from
1134-410: A third stream entering between two others is designated the middle fork; or the streams are distinguished by the relative height of one to the other, as one stream descending over a cataract into another becomes the upper fork, and the one it descends into, the lower ; or by relative volume: the smaller stream designated the little fork, the larger either retaining its name unmodified, or receives
1215-479: A wide variety of coarse fish species. The geographical distribution of many species of invertebrate in the river reflects the geology of the catchment area. Viviparid snails and water scorpions (of the genus Nepidae ) are commonly found where the river runs over the London Clay . Crayfish are common in areas associated with high alkalinity , particularly around Brockham, and the tributaries which run over
1296-514: Is a tributary that joins the main stem river closer to its source than its mouth, that is, before the river's midpoint ; a late tributary joins the main stem further downstream, closer to its mouth than to its source, that is, after the midpoint. In the United States, where tributaries sometimes have the same name as the river into which they feed, they are called forks . These are typically designated by compass direction. For example,
1377-785: Is first recorded in the Red Book of Thorney in AD 983 as Emen and in the AD 1005 Cartulary of the Abbey of Eynsham as both Emen and Æmen . Variations in the name are recorded throughout the Middle Ages and the river appears as Amele in the Domesday Book of 1086, and subsequently as Emele in 12th- and 13th-century Court Rolls. This name is probably derived from the Old English word æmen meaning misty or causing mists , and
1458-559: Is found between Box Hill and Leatherhead. The Mole is one of only three locations in England where the river shingle beetle ( Meotica anglica ) is found. A nationally scarce species which is locally common on the River Mole is the greater dodder ( Cuscuta europaea ), a parasitic plant . The Mole catchment area includes twelve Sites of Special Scientific Interest (SSSIs) that include wetland habitats, The Mole Gap forms part of
1539-521: Is measured at a fourth gauging station, located at Castle Mill (39 m [128 ft] above OD). At Pixham the Mole meets the Pipp Brook , a tributary draining the northeastern slopes of Leith Hill. Between Dorking and Leatherhead the Mole cuts a steep-sided valley (the Mole Gap) though the North Downs , carving a 170-metre-high (560 ft) river cliff on the western flank of Box Hill and
1620-705: Is used. In common with much of the rest of the Weald, the earliest evidence of human settlement along the Upper Mole is from the Mesolithic Period (20,000–7000 BC). Mesolithic sites at Wonham, Flanchford and Sidlow. Finds at Wonham include arrowheads and a plano-convex knife. The Lower Mole appears to have been settled during the same period and a flint axe dating from Mesolithic period found on spit of land close to River Mole in Cobham in 1965. Remains of
1701-597: The American River in California receives flow from its North, Middle, and South forks. The Chicago River 's North Branch has the East, West, and Middle Fork; the South Branch has its South Fork, and used to have a West Fork as well (now filled in). Forks are sometimes designated as right or left. Here, the handedness is from the point of view of an observer facing upstream. For instance, Steer Creek has
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#17327733133521782-693: The British Canoe Union . During the Second World War several sites along the course of the Mole were fortified for the defence of London against invasion from the south. GHQ Line B ran along the North Downs from Farnham and Guildford, crossing the Mole Gap to the north of Dorking and following the river to Horley. Between Betchworth and Box Hill, the north bank of the River Mole was stabilised and made steeper to prevent wheeled vehicles from crossing. At Boxhill Farm, where access to
1863-726: The House of Lords , involving a member of the South-East Strategic Committee of the Thames Conservancy (succeeded by the Environment Agency ) and four others. In this it was said, "...I can see no future for [The Mole]. 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
1944-538: 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 running side by side in a northeasterly direction, merging 400 m (440 yd) before the confluence with the River Thames, on the reach above Teddington Lock . (For the purposes of
2025-562: The Victoria Dock, Liverpool , over a short period in 1844. By the time of the Second Geodetic Levelling (1912–1921) the importance of stability was better appreciated and so it was decided to use Fundamental Bench Marks (FBMs) installed in solid rock, rather than on buildings as before. To measure average MSL around Great Britain three tide gauges were employed: at Dunbar , Newlyn and Felixstowe . However, it
2106-473: The Weald Clay provide an excellent habitat for stoneflies , caddisflies , fast swimming mayflies and riffle beetles . The beautiful demoiselle ( Calopteryx virgo ) disappeared from the River Mole during the 1960s owing to deteriorating water quality, but has since recolonised. The white-legged damselfly ( Platycnemis pennipes ) is also found along the river, and the downy emerald ( Cordulia aenea )
2187-470: The Wey Gap is significantly higher than might be expected from natural rainwater percolation alone. It has been suggested that a proportion of the excess water originates from the Mole Gap. At Leatherhead the river leaves the chalk bedrock, moving onto London Clay . The river meanders across an alluvial plain between 400 and 800 m (440–870 yd) wide towards Cobham, where it begins to descend to
2268-651: 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 . 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 (good) or better. In 2002 this figure was 60%. Investment in the Sewage Treatment Works in
2349-464: 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
2430-489: The London Clay. The depth of the deposits generally varies from 2.5 to 7 m (8–23 ft); the lower layers are generally highly compacted and cemented together with brick-red iron oxide, whereas the upper layers are loosely packed with angular flints and sand. Remains of a further gravel terrace, containing cherts and flints to a depth of 4 m (13 ft), line the east side of St George's Hill . The river
2511-471: 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
River Mole - Misplaced Pages Continue
2592-542: 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
2673-597: 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 m (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 m/s (49 cu ft/s). The Mole passes Horley to
2754-413: The Mole is south of Gatwick Airport (57 m [187 ft] above OD ). The mean flow is 0.33 m/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
2835-410: 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 , (which drains Ashtead ) joins to
2916-460: The Mole was used for navigation in the past: In the late 13th century, Thorncroft Manor (south of Leatherhead) purchased a shout , a type of boat up to 16 metres (52 ft) in length used to carry produce to market and it has been suggested that stone cut from quarries in Reigate was transported to London via the river. During the 17th century, two bills came before parliament to make sections of
2997-616: The Pipp Brook. Zander have been caught in the Lower Mole below Dorking since the 1980s. In the Mole Gap between Dorking and Leatherhead the river supports populations of chub, dace, barbel , 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
3078-529: 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
3159-414: The bed and banks. The amount of water lost from the river is significant and in very hot summers the channel can become dry between Mickleham and Thorncroft Manor; this was recorded most recently in 1949, 1976 and 2022. At Leatherhead , the river leaves the chalk and flows across impermeable London Clay . At this point, the water table rises enough for the water to flow back into the main river channel. In
3240-471: The canalisation of a short stretch of the River Mole between Betchworth and Dorking to facilitate the movement of chalk from quarry to market. In 1810 the engineer John Rennie proposed a canal linking the River Medway to Portsmouth which was to have a branch to London following the Mole for much of its length. Between 1825 and 1828 the architect and civil engineer Nicholas Wilcox Cundy proposed
3321-409: 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
River Mole - Misplaced Pages Continue
3402-534: The cutting at the south end of Box Hill & Westhumble railway station , suggesting that even in its early history, the river had swallow holes. The author Daniel Defoe , who attended school in Dorking and probably grew up in the village of Westhumble , described the swallow holes in the River Mole in his book A tour thro' the Whole Island of Great Britain (first published in 1724): ...the current of
3483-416: The designation big . Tributaries are sometimes listed starting with those nearest to the source of the river and ending with those nearest to the mouth of the river . The Strahler stream order examines the arrangement of tributaries in a hierarchy of first, second, third and higher orders, with the first-order tributary being typically the least in size. For example, a second-order tributary would be
3564-478: 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 , 1 km (0.6 mi) northeast of Dorking . A mean flow of 3.74 m/s (132 cu ft/s)
3645-402: The entry to the Mole Gap. Above Meath Green (near Horley ), the terrace corresponds to the flood plain of the river, however from this point downstream to Brockham, the river cuts a narrow trench no more than 150 m (160 yd) wide. A similar trench is runs up several of the Mole's tributaries, notably along Deanoak Brook which follows a local strike . The presence of the trench protects
3726-443: The impermeable Weald Clay . From the source to Dorking , the river drains an area of 340 km (130 sq mi), of which approximately 60% is on Wealden or Atherfield Clay, 20% is on Tunbridge Wells Sand and 20% is on greensand . Brickearth deposits are common in the valley around Betchworth and east of Dorking. The upper Mole catchment is dominated by a single broad terrace, which runs continuously from Tilgate Forest to
3807-413: The land from flooding, rendering it suitable for agriculture . Between Dorking and Leatherhead the Mole cuts a steep-sided valley through the North Downs, creating a 170-metre-high (560 ft) 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
3888-529: The local area is complex, since the river leaves the chalk of the Mole Gap at this point and flows over gravel and clay, creating a patchwork of different habitats including scrub , woodland , hedges , banks, and meadows as well as the water itself. The river also provides a corridor for wildlife through the centre of Leatherhead. The West End Common forms part of the Esher Commons , owned and managed by Elmbridge Borough Council . The River Mole forms
3969-610: The main river by a channel of about a metre. About six of these larger swallow holes were found to the west of the Burford Bridge Hotel , along the course of the A24 Mickleham Bypass during its construction in 1936. Initially the surveyors tried 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
4050-403: The main stream. Distributaries are most often found in river deltas . Right tributary , or right-bank tributary , and left tributary , or left-bank tributary , describe the orientation of the tributary relative to the flow of the main stem river. These terms are defined from the perspective of looking downstream, that is, facing the direction the water current of the main stem is going. In
4131-469: 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
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#17327733133524212-447: The most diverse fish population of any river in England. 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, Horley, was enhanced to create a gravel spawning area to encourage chub and dace in addition to roach . In 1974 zander , a non-indigenous coarse fish native to Europe, were introduced legally to Old Bury Hill Lake which supplies
4293-620: The name of the River Ember probably has its origins in this name. The name Mole does not appear until the 16th 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. In John Speed 's 1611 map of Surrey this stretch of
4374-453: 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 11-metre (35 ft) diameter waterwheel raises water 5 metres (16 ft) from the river to feed the ornamental lake in the park. From Painshill Park the river flows northeastwards to the Thames, passing to
4455-415: 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 at Hampton Court Palace it is possible to catch
4536-464: The people who live upon its banks. Conversely, explorers approaching a new land from the sea encounter its rivers at their mouths, where they name them on their charts, then, following a river upstream, encounter each tributary as a forking of the stream to the right and to the left, which then appear on their charts as such; or the streams are seen to diverge by the cardinal direction (north, south, east, or west) in which they proceed upstream, sometimes
4617-521: 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: The marsh frog ( Pelophylax ridibundus , a non-native species introduced from Europe in the 1930s) is now commonly found in
4698-536: The remainder of this article, the River Mole and the River Ember are treated as a single entity.) 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
4779-482: The result of two or more first-order tributaries combining to form the second-order tributary. Another method is to list tributaries from mouth to source, in the form of a tree structure , stored as a tree data structure . Ordnance Datum An ordnance datum ( OD ) is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps . A spot height may be expressed as above ordnance datum ( AOD ). Usually mean sea level (MSL) at
4860-441: The river navigable . The first, in 1663, was passed by the House of Commons but was defeated in the House of Lords . During the following year (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 combined lower section of the River Wey . In 1798 William Marshall advocated
4941-481: 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
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#17327733133525022-491: The river from the north bank was required for the herd of dairy cows, a row of twelve concrete cylinders were cast as an anti-tank measure. Gun mounts were also installed to protect both Boxhill and Deepdene bridges and several pillboxes were installed. An anti-tank ditch was dug from the Stepping Stones eastwards across the fields belonging to Bradley Farm (now Denbies vineyard ). The river crossing at Sidlow Bridge
5103-473: 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
5184-400: 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). In John Rocque 's 1768 map of Surrey, the name Moulsey River
5265-586: 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 . At Leatherhead,
5346-480: The same period has been discovered at East Horsley along with Neolithic flakes of flint at Fetcham and Headley Heath . Significant Bronze Age finds include a bronze sword found close to the river north of Amberley Farm near Charlwood and a small hoard of weaponry consisting of two palstave axes and a scabbard chape was discovered in 2003 in Norbury Park close to Ham Bank. It is not clear to what extent
5427-493: 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
5508-553: 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
5589-507: The springs at the southern end of Fetcham Mill Pond, which have never been known to run dry. A survey in March 1883 estimated that the Fetcham springs were producing about 3.6 million imperial gallons (16,000 m) every day. A second survey in 1948 estimated that the same springs were yielding about 5 million imperial gallons (23,000 m) a day. The water table in the chalk of
5670-466: The surrounding chalk and provided with a manhole and access shaft to allow periodic inspection. In the late 1960s the domes were reopened and inspected and the alluvium in the largest swallow hole was observed to have subsided by 1.5 metres (4 ft 11 in) under the centre of one of the domes. When the Dorking to Leatherhead railway was constructed in 1859, a fossilised swallow hole was discovered in
5751-492: The then Prime Minister Clement Attlee , replacing those destroyed during the Second World War as an anti-invasion measure. The location is popular with anglers and families, but swimming is strongly discouraged as the water is polluted in places. The stones give their name to the pub in the nearby village of Westhumble . Tributary A tributary , or an affluent , is a stream or river that flows into
5832-426: 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. A second non-native species, the edible frog ( Pelophylax kl. esculentus ) was introduced to a site at Newdigate in the early 1900s. It has in 2008 been recorded in tributaries of the River Mole at Capel and Brockham . The River Mole has
5913-606: The west bank of the Mole, west of the Island Barn Reservoir. The Heath is an area of rough scrubland and, despite its name, is a reclaimed landfill site hence its large but artificial mound and small plateau. The site is a rich habitat for birds including the redshank and little ringed plover . The Mole rises south of Rusper in West Sussex, where an outcrop of the Hastings Beds sandstone dips below
5994-457: The west of Esher Commons and then forming the boundary between Hersham and Esher , where the mean flow of the river is 5.43 m/s (192 cu ft/s). 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
6075-496: The west, flowing north towards Sidlow and entering a largely rural area. 0.7 km (0.4 mi) south of Sidlow the mean flow is measured as 2.21 m/s (78 cu ft/s) at Kinnersley Manor gauging station (48 m [157 ft] above OD). 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 (Reigate STW) discharges up to 118,500 m (4,180,000 cu ft) per day into
6156-457: The western boundary of the common, flowing past a steep sandy area known locally as The Ledges . Plant species typical of ancient woodland are found in this area, including bluebells ( Hyacinthoides non-scriptus ), marsh marigolds ( Caltha palustris ) and golden saxifrages ( Chrysosplenium sp.). large bittercress ( Cardamine amara ) and the non-native Himalayan balsam ( Impatiens glandulifera ). Molesey Heath Local Nature Reserve lies on
6237-433: The world with a length of 4,248 km (2,640 mi). The Madeira River is the largest tributary river by volume in the world with an average discharge of 31,200 m /s (1.1 million cu ft/s). A confluence , where two or more bodies of water meet, usually refers to the joining of tributaries. The opposite to a tributary is a distributary , a river or stream that branches off from and flows away from
6318-518: Was found that the measured difference between the Dunbar and Newlyn stations was 0.81 feet (0.247m), far larger than could be accounted for by error. The difference was real. Accordingly, it was decided not to use average MSL and fix on one site: MSL Newlyn. Newlyn has certain practical advantages: it is set in granite bedrock, is far from major rivers, and it better represents deep ocean sea levels. The difference between ODL (Liverpool) and ODN (Newlyn)
6399-643: Was found to be 0.13 feet. It took some time for the changed definition — Liverpool to Newlyn — to work through the system: by 1950, some 40% of the lower secondary and tertiary levellings were still using the Liverpool datum. But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum. Newlyn Tidal Observatory was given grade II listing on 11 December 2018. Tunnel datum
6480-456: Was heavily defended and a line of pill boxes was constructed on the north bank of the river. Concrete anti-tank dragon's teeth were built on both sides of the river, a short distance upstream of the bridge, as an obstacle to armoured vehicles . The North Downs Way crosses the river at Box Hill via seventeen hexagonal stepping stones , which are frequently submerged after heavy rainfall. The current stones were dedicated in September 1946 by
6561-461: Was the oldest, dating from Saxon times . The drainage area of the Mole is 477 km (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
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