Monday, 13 July 2009

Somerset Rivers:The River Parret / Pendredan *****


The River Parrett /Pendredan is a river flowing through the counties of Dorset and Somerset in South West England. It has its source in the Thorney Mills springs in the hills around Chedington in Dorset, and flows north west through Somerset and the Somerset Levels to its mouth at Burnham-on-Sea where it flows into the Bridgwater Bay Nature Reserve on the Bristol Channel.
The main tributaries include the Rivers Tone, Isle and Yeo. The River Cary drains into the Parrett via the King's Sedgemoor Drain. The River Parrett drains an area of over 652.5 sq mi (1,690.0 km2), comprising around 50% of the land area of Somerset. The 37 miles (59.5 km) long river is tidal for 27 miles (43 km) up to Oath. Because the fall of the river, between Langport and Bridgwater is only 1 foot per mile (20 cm/km), it is prone to frequent flooding in winter and high tides. Many approaches have been tried since the early 19th century to reduce the incidence and effect of floods.
From the medieval period the river served the Port of Bridgwater, enabling cargoes to be transported inland. The coming of the railways led to a decline and commercial shipping now only docks at Dunball. The river, and man's influence on it, have left a legacy of bridges and industrial artefacts. The River Parrett Trail has been established along the banks of the river.


Course
The River Parrett is 37 miles (59.5 km) long, flowing roughly south to north from Dorset through Somerset. Its source is in the Thorney Mills springs in the hills around Chedington, 2.5 miles (4.0 km) from that of the River Axe in nearby Beaminster, which runs in the opposite direction, to the English Channel at Axmouth in Devon. The two rivers give their name to Parrett and Axe Parish Council, which administers those two villages.

The river near the A303 at South Petherton

From its source, the Parrett runs north through South Perrott and under the Salisbury to Exeter railway line, before passing to the west of North Perrott and Haselbury Plucknett. It then runs through fields between Merriott to the west and West Chinnock and Chiselborough to the east. The underlying geology is a thin layer of Fuller's earth clay over Yeovil sands; the resulting light soil made the area important for the production of flax and for market gardening. Passing under the A303 road to the east of South Petherton, the river flows between East Lambrook and Bower Hinton west of Martock and then towards Kingsbury Episcopi, through Thorney and Muchelney, passing the remains of Muchelney Abbey before entering Langport. Below Thorney Bridge the river's banks have been raised to mitigate flooding.
The Parrett then flows west through the Somerset Levels past Aller, close to the Aller and Beer Woods and Aller Hill biological Sites of Special Scientific Interest (SSSI). At the deserted medieval village of Oath the Lock marks the river's tidal limit. The lock was built when it was realised that those at Stanmoor and Langport would not provide the depth of water specified in the Act of Parliament of 4 July 1836 authorising the construction of the River Parrett Navigation. It has since been replaced by a sluice gate to control flooding.

The flooded Southlake Moor

The river then crosses Southlake Moor, another SSSI which forms part of an extensive grazing marsh and ditch system. When conditions in the River Parrett are suitable, the moor can be deliberately flooded in winter by opening a sluice in the river's floodbank. Some 96 species of aquatic and bankside vascular plant species have been recorded on Southlake Moor, including the Greater water-parsnip (Sium latifolium). When the moor is flooded it can be occupied by large numbers of wildfowl; up to 22,000 Wigeon (Anas penelope), 250 Bewick's Swan (Cygnus bewickii) and good numbers of Pochard (Aythya ferina), Teal (Anas crecca) and Tufted Duck (Aythya fuligula). Signs of Otters (Lutra lutra) are regularly seen on the muddy banks of the River Parrett. The ditches on the east side of the site are populated by Palmate Newts (Triturus helveticus).
The next major landmark along the river's course is Burrow Mump, an ancient earthwork owned by the National Trust. It is a natural hill of Triassic sandstone capped by Keuper marl, standing at a strategic point where the River Tone and the old course of the River Cary join the River Parrett. It probably served as a natural outwork to the defended royal island of Athelney at the end of the 9th century.
The river then arrives in Burrowbridge, where the old pumping station building was once a museum. It then flows north, passing Langmead and Weston Level SSSI, where four nationally rare species of terrestrial and aquatic invertebrates have been recorded, and on past the pumping station at Westonzoyland, which is now an Industrial Heritage museum of steam powered machinery and land drainage, and houses most of the equipment from the Burrowbridge pumping station.
Further downstream the river passes the village of Huntworth before flowing under the M5 motorway at Dunwear. As it enters Bridgwater it passes under Somerset and Hamp bridges. Bridgwater Castle had a tidal moat up to 65 feet (20 m) wide in places, fed by water from the river. Under an 1845 Act of Parliament the Port of Bridgwater extends from Brean Down to Hinkley Point in Bridgwater Bay, and includes parts of the River Parrett (to Bridgwater), River Brue and River Axe. Although ships no longer dock in the town of Bridgwater, in 2006 90,213 tonnes of cargo were handled within the port authority's area, compared to more than 200,000 tons (approximately equivalent to metric tonnes) in 1878, most of which were stone products through the wharf at Dunball.
When the Bridgwater and Taunton Canal was opened in 1827 it joined the Parrett by a lock at Huntworth, where a basin was constructed, but in 1841 the canal was extended to a floating harbour in Bridgwater, and the Huntworth link was filled in. The canal and river were not re-connected at this point when the canal was restored, because the tidal Parrett, at this point, is a salt water river laden with silt, whereas the canal contains fresh water. Not only is there a risk of silt entering the canal, but the salt water cannot be allowed to contaminate the fresh, as the canal is still used for the transport of drinking water for Bridgwater's population. Silt was dredged from the river over a 2 miles (3.2 km) stretch between Somerset Bridge and here to make Bath bricks, an early cleaning material.

Dunball Wharf.
To the right is Dunball Clyce where the King's Sedgemoor Drain flows into the River Parrett

The King's Sedgemoor Drain drains into the River Parrett at Dunball, next to the wharf, via a clyse. The clyse has been moved from its original position and it now obstructs the entrance to a small harbour adjacent to the wharf.
Dunball wharf was built in 1844 by Bridgwater coal merchants. It was formerly linked to the Bristol and Exeter Railway by a rail track which crossed the A38. The link was built in 1876 by coal merchants, and was originally operated as a horse-drawn tramway. It was removed as part of the railway closures made by Dr. Beeching in the 1960s. Dunball railway station, which had opened in 1873, was closed in 1964. The wharf was used during the Second World War to bring Welsh coal to the nearby Royal Ordnance Factory, ROF Bridgwater. The wharf is now used for landing stone products, mainly marine sand and gravels dredged in the Bristol Channel.

The river near Pawlett showing Hinkley Point power stations A and B

The village of Combwich lies on Combwich Reach, where the River Parrett flows to the sea. In the Domesday book it was known as Comich, which means "The settlement by the water", from the Old English cumb and wic. It was here, or in the immediate vicinity, that Hubba, the Danish raider, was defeated and killed by Earl Oddune of Devon in 878. It was also the site of an ancient ferry crossing, and served as a port for the export of local produce and the import of timber from the 15th century. Until the 1930s, when the creek silted up, it also served the local brick and coal yard. Brick and tile making was first recorded in the village in 1842.

Hinkley Point, Brant/Brent Knoll just showing through the mist

The Steart peninsula has flooded many times during the last millennium. The most severe recent floods occurred in 1981. By 1997, a combination of coastal erosion, sea level rise and wave action had made some of the defences distinctly fragile and failure prone. As a result, the Environment Agency produced the Stolford to Combwich Coastal Defence Strategy Study in 2002, to examine options for the future.


Bridgwater Bay

The mouth at Burnham-on-Sea is a nature reserve where it flows into Bridgwater Bay on the Bristol Channel. It consists of large areas of mud flats, saltmarsh, sandflats and shingle ridges, some of which are vegetated. It has been designated as a Site of Special Scientific Interest since 1989, and is designated as a wetland of international importance under the Ramsar Convention. The risks to wildlife are highlighted in the local Oil Spill Contingency Plan. In addition to the rivers Parrett, Brue and Washford, several of the man-made drainage ditches, including the River Huntspill from the Somerset Levels, and the Cannington Brook from the "Pawlett Hams", also empty into the bay.

Etymology
The name means 'The barge river' from the Latin paradie 'barse'.

History
The River Parrett was established as the border between Wessex and Dumnonia in 658 AD following the defeat of the West Welsh (Dumnonia) in the Battle of Peonnum, at Penselwood, in the same year. This natural border endured for almost a century until further fighting between Anglo-Saxons and the West Welsh in the mid 8th century, when the current borders of Devon (West Welsh) and Somerset (Anglo-Saxon) were established.
A ford, usable only at low tide, and later a ferry operated across the mouth of the river at Combwich, it is thought, since Roman times. The crossing lay on the route of a Saxon herpath; and in the 15th century was regarded as part of the King's Highway. Records relating to the costs of the ferry exist for 1589 and 1810; and the White House Inn, a licensed victualler, on the Pawlett bank traded from 1655 to 1897. The river crossing has fallen out of use, and the former White House Inn was demolished round about 1930.
In the medieval period the river was used to transport Hamstone from the quarry at Ham Hill for the construction of churches throughout the county.

Port of Bridgwater

Bridgwater Docks

Bridgwater was originally part of the Port of Bristol; however in 1348 the Port of Bridgwater was created, covering 80 miles (130 km) of the Somerset coast line, from the Devon border to the mouth of the River Axe. Historically, the main port on the river was at Bridgwater; the river being bridged at this point, with the first bridge being constructed in 1200 AD.

West Quays

Quays were built in 1424; with another quay, the Langport slip, being built in 1488 upstream of the town bridge. The river was navigable, with care, to Bridgwater town bridge by 400–500 tonne vessels. By trans-shipping into barges at the town bridge the Parrett was navigable as far as Langport and (via the River Yeo) to Ilchester. After 1827, it was also possible to transfer goods to Taunton via the Bridgwater and Taunton Canal.
Combwich Pill, a small creek near the mouth of the river, had been used for shipping since the 14th century; and a wharf in the 18th century was used for the unloading of coal and tiles. From the 1830s, with the development of the brick and tile industry in the Bridgwater area, Combwich wharf was used by two brickyards to import coal and to export tiles to Wales and parts of Gloucestershire. This traffic ceased in the 1930s; and in 1950 the wharf was taken over by the CEGB to bring in materials for the construction of Hinkley Point nuclear power station.
Since 1845, when the Port of Bridgwater Act was passed, the mouth of the river as far as the first bridge has been under the jurisdiction of the Port of Bridgwater. Sedgemoor District Council acts as the Competent Harbour Authority for the port, and has provided pilotage services for all boats over 98 feet (29.9 m) using the river since 1998, when it took over the service from Trinity House. Pilotage is important because of the constant changes in the navigable channel resulting from the large tidal range, which can exceed 39 feet (11.9 m) on spring tides. Most commercial shipping travels upriver as far as Dunball wharf, which handles bulk cargoes.
Marine sand and gravel accounted for 55,754 tonnes of the total tonnage of 90,213 using the Port facilities in 2006, with salt products accounting for 21,170 tonnes in the same year, while the roll-on roll-off berth at Combwich is used occasionally for the transfer of heavy goods for the two Hinkley Point nuclear power stations. Combwich Pill is the only site where recreational moorings are available in the estuary.
Trade on the river upstream of Bridgwater had developed during the 18th Century, with 20 ton barges operating between Bridgwater and Langport, while smaller barges carrying 6 or 7 tons operated on the upper reaches between Langport and Thorney, and along the River Yeo to Long Load bridge and Ilchester. The channel below the junction with the River Tone had been improved as a result of Acts of Parliament passed in 1699 and 1707, for making and keeping the River Tone navigable from Bridgewater to Taunton, and a third act with a similar purpose was passed in 1804. Traffic on the higher reaches was hindered by shoals in the river, and by the Great Bow bridge at Langport, which consisted on nine small arches, none of them big enough for navigation. All cargoes heading upstream had to be off-loaded from the bigger barges, carried to the other side of the bridge, and reloaded into the smaller barges. Traffic above Langport was sporadic, as the water levels were often inadequate, with the boats having to wait several days for the right conditions to proceed.
The abortive Ivelchester and Langport Navigation scheme had sought to avoid the bridge, by making the Portlake Rhine navigable, rebuilding Little Bow Bridge in the centre of Langport, and making a new cut to Bicknell's Bridge. Seven locks, each with a small rise, were planned but the scheme foundered in 1797, due to financial difficulties. After the cessation of hostilities with France at the beginning of the 19th century, there was renewed interest in canal building in Somerset, with the Bridgwater and Taunton Canal being authorised in 1824, the Glastonbury Canal in 1827 and the Chard Canal in 1834. With the prospect of the Chard Canal in particular damaging trade on the Parrett, four traders from Langport including Stuckey and Bagehot, who together operated a river freight business, commissioned the engineer Joseph Jones to carry out a survey which was then put before parliament. It was supported by Brunel and a large quantity of documentary evidence. Objections from local landowners were handled by including clauses to ensure that surplus water would be channelled to the Long Sutton Catchwater Drain by culverts, siphons and sluices, and the Act of Parliament was passed on 4 July 1836.

Construction
The act allowed the proprietors, of which 25 were named, to raise £10,500 in shares and £3,300 by mortgage, with which to make improvements to the river from Burrow Bridge to Langport, to reconstruct the restrictive bridge at Langport, and to continue the improvements as far as Thorney. The River Isle, which joined the Parrett at Muchelney, was to be improved for its first mile, and then the Westport Canal was to be constructed from there to Westport. Locks were planned at Stanmoor, Langport and Muchelney, with a half-lock at Thorney. An extra lock was added at Oath, when tests revealed that the depth of water would not meet that specified in the Act without it. Costs were considerably higher than expected, and a second act of parliament was obtained in 1839, to allow an extra £20,000 to be raised.
The section below Langport was completed and opened on 28 October 1839; the section to Thorney and the Westport Canal were completed in August 1840. The Langport bridge was not finished until March 1841, at a cost of £3,749. £500 was received from the Langport Corporation, and a special bridge toll was operated from March 1841 until January 1843 to recoup costs. The total cost of the works was £38,876, and no dividends were paid until 1853, as all profits were used to repay the loans which had been taken out. There are no records of traffic, but it has been estimated at 60,000–70,000 tons per year, based on the toll receipts and the knowledge that the Stuckey and Bagehot boats carried about three quarters of the total tonnage.
Decline
The Bristol and Exeter Railway opened in late 1853, and the effects on the navigation were immediate, with receipts dropping from £1,440 in 1853 to £673 by 1857. The Company paid its final dividend in 1872. In 1875, parts of Westmoor were flooded, as a result of the Company being unable to repair the culvert under the river at Huish bridge, and Mr Thomas Mead opened the Langport lock gates to lower the upstream water levels. The Company had no option but to stop collecting tolls, and the gates were still open in 1877. On 1 July 1878 the Somersetshire Drainage Act was passed by Parliament, and it provided for the transfer of the navigation to the Drainage Commissioners at no cost, with options to abandon any or all of the navigation, but the Commissioners chose to abandon it all, despite petitions from users of the Westport Canal to keep their section open. Some boats continued to use the river to reach Langport and beyond until the early years of the 20th century. There is still a public right of navigation as far as Oath Lock, but very few private boats use the river, largely due to the fierce tides in the estuary and a lack of moorings along its route.

Bridges and structures
Much of the history of the river has been defined by its bridges. The Drove bridge is the first and the newest road bridge to cross the river, and marks the end of the Port of Bridgwater. The bridge, which has a span of 184 feet (56 m), was constructed as part of the Bridgwater Northern Distributor road scheme (1992), and provides a navigable channel which is 66 feet (20 m) wide with 8.2 feet (2.5 m) headroom at normal spring high tides. Beyond this is the telescopic bridge built in 1871 to the design of Francis Fox, the engineer for the Bristol and Exeter Railway. It carried a railway siding over the river to the docks, but had to be movable, to allow boats to proceed upriver. An 80-foot (24 m) section of railway track to the east of the bridge could be moved sideways, so that the main 127-foot (39 m) girders could be retracted, creating a navigable channel which was 78 feet (24 m) wide. It was manually operated for the first eight months, and then powered by a steam engine, reverting to manual operation in 1913, when the steam engine failed. The bridge was last opened in 1953, and the traverser section was demolished in 1974, but public outcry at the action resulted in the bridge being listed as a Scheduled Ancient Monument. It was later used as a road crossing, until the construction of the Chandos road bridge alongside it, and is now only used by pedestrians. Parts of the steam engine were moved to Westonzoyland Pumping Station Museum in 1977.

Bridgwater Town Bridge

The next bridge is the town bridge. There has been a bridge here since the thirteenth century, when Bridgwater was granted a charter by King John. The present bridge was designed by R. C. Else and G. B. Laffan, and the 75-foot (23 m) cast iron structure was completed in 1883. It replaced an earlier bridge, which was the first cast iron bridge to be built in Somerset when it was completed in 1797. The stone abutments of that bridge were reused by the later bridge, which formed the only road crossing of the river in Bridgwater until 1958. Above the bridge there were two shoals, called The Coals and The Stones, which were a hazard to barge traffic on the river, and bargees had to choose when to navigate the river carefully, to ensure that there was sufficient water to carry them over these obstructions. In March 1958 a new reinforced concrete road bridge, the Blake Bridge, was opened as part of a bypass to take traffic away from the centre of Bridgwater. It now carries the A38 and A39 roads.

1826 bridge at Burrowbridge

Before 1826, the bridge at Burrowbridge, just below the junction with the River Tone, had consisted of three arches, each only a little wider than the barges which used the river. They restricted the flow of water in times of flood, and made navigation difficult. The bridge was highlighted in a report made by William Armstrong in 1824, as a factor which would prevent the River Tone Navigation competing with the new Bridgwater and Taunton Canal, then being built. An Act of Parliament was obtained in 1824 by the Turnpike Commissioners, authorising the construction of a new bridge and the removal of the old. A design for a 70 ft (21 m) single span bridge in cast iron was dropped because of the cost of cast iron at the time, and instead a stone bridge was built, which was completed in 1826. This is the longest single span masonry road bridge in the county, and was also the last toll bridge in Somerset, until it was 'freed' in 1946. Just below the bridge there was a shoal of rocks and stones, which was also mentioned in Armstrong's report, but no action was taken to remove it. Except on spring tides, Burrowbridge was the normal upper limit for barges riding the incoming tide. Above here, horses were used to pull the boats, either towards Langport or along the River Tone towards Taunton.
Stanmoor lock was constructed after the junction with the River Tone, but all traces of it have gone. Oath lock no longer functions as a lock, but the sluice is used to regulate the river levels. Below Langport, the river is crossed by a lattice girder bridge, carrying the Taunton to Westbury railway line, which approaches the crossing on multi-arched viaducts. This is followed by the derelict remains of the Langport lock and sluice.

Great Bow Bridge at Langport

At Langport, the Great Bow bridge is a three-arched bridge, constructed under the terms of the Parrett Navigation Act of 1836. Completed in 1841 at a cost of £3,749, it replaced the previous medieval bridge, with its nine tiny arches, all too small to allow navigation. A bridge at this site was first mentioned in 1220. The medieval bridge consisted of a total of 31 arches, of which nine crossed the river, and 19 of the original arches were located by ground-penetrating radar in 1987, buried beneath the road which runs from Great Bow bridge to Little Bow bridge.

The Warehouse in Langport was built in the late 18th century of English bond red brick, with Flemish bond extensions. It has clay plain tile roofs with hipped ends. It was built by the Parrett Navigation Company, a trading Company owned by Vincent Stuckey and Walter Bagehot, on the banks of the River. When the river became unnavigable, the usefulness of the building waned and it was eventually abandoned. The Somerset Trust for Sustainable Development (STSD) purchased the site, designated as a brown field site, in February 2003, and worked with Somerset Buildings Preservation Trust (SBPT), English Heritage and local councils to redevelop it into a craft, heritage learning and small business centre, with the surrounding land being used for an eco-friendly housing development. It is a grade II listed building. The newest bridge across the Parrett is Cocklemoor Bridge, a pedestrian footbridge located close to the Great Bow bridge, that was erected in 2006 and forms part of the River Parrett Trail.

Flood prevention Monk's Leaze Clyce.
This sluice regulates the flow of water between the River Parrett and the Sowy River (the River Parrett Relief Channel).The waters of the Severn Estuary, which are heavily laden with silt, flow into the lower reaches of the Parrett and River Tone on each tide. This silt can rapidly gather on the banks of the rivers reducing the capacity and performance of the channel, and increasing the risk of flooding of surrounding land.




Westonzoyland Pumping Station Museum

The river is technically a highland carrier, as it is embanked and the water level is often higher than that of the land through which it flows. Water from the surrounding countryside does not therefore drain into the river naturally, and drainage schemes have relied on pumping to remove the water. The pumping station at Westonzoyland was built in 1830, the first mechanical pumping station on the Somerset Levels. It was designed to drain the area around Westonzoyland, Middlezoy and Othery, and the success of the drainage system led to the formation of other drainage boards and the construction of other pumping stations. The pump at Westonzoyland originally comprised a beam engine and scoop wheel, which is similar to a water wheel, except that it is driven round by the engine and lifts water up to a higher level. After 25 years, there were problems pumping the water away as the land surface had dropped as it dried out. A better method was sought, and in 1861 a replacement pump was installed. The engine was built by Easton and Amos of London, to a design patented in 1858 by Charles Amos. It is a twin cylinder, vertical condensing engine, driving a centrifugal pump. A similar engine was on display at the Great Exhibition of 1851 and was shown to be able to lift 100 tons of water per minute, to a height of 6 feet (1.8 m). The Westonzoyland pump lifts water from the rhyne (pronounced "reen") into the River Parrett. The pump operated until 1951, when a new diesel powered pumping station, capable of pumping 35 tons per minute at any state of the tide, was built adjacent to the old one.
In the 1960s the Somerset River Authority was established. They undertook engineering works for drainage, including pump and river works, at the Parrett, King's Sedgemoor Drain and River Brue systems. They tried to ensure that agricultural lands benefitted from a potable water supply in the groundwaters from the Quantock Hills to the coastline.
Various measures including sluice gates, known locally as Clyse, have been deployed to try to control the risk of flooding. Completed in 1972, the Sowy River is a 7.5 miles (12.1 km) embanked channel which starts at Monks Leaze Clyse below Langport, and carries excess water from the river to the Kings Sedgemoor Drain, from where it flows to the estuary by gravity, rejoining the Parrett near Dunball wharf. Construction of the channel, together with improvements to the Kings Sedgemoor Drain and the rebuilding of the clyse at Dunball, to create a fresh water seal which prevents salt water entering the drain from the river, cost £1.4 million. The scheme has resulted in less flooding on Aller Moor.

The sluice at Oath Lock in summer, with the gates lowered. Oath Lock cottage is off to the right.

In the 1970s a study was commissioned by Wessex Water to investigate the likely effects of construction of a tide-excluding barrier, aimed at stopping the silt, just upriver of Dunball Wharf on the hydraulic, sedimentary and pollutant regime of the estuary. Results showed that a site further upriver could be viable.
The area around the estuary, known as Parrett Reach, around the Steart Peninsula has flooded many times during the last millennium. The most severe recent floods occurred in 1981. By 1997, a combination of coastal erosion, sea level rise and wave action had made some of the defences distinctly fragile and at risk from failure. As a result in 2002 The Environment Agency produced the Stolford to Combwich Coastal Defence Strategy Study to examine options for the future.
Following summer floods of 1997 and the prolonged flooding of 1999/2000 the Parrett Catchment Project was formed, partly funded by the European Union Regional Development Fund, by 30 organisations, including; British Waterways, Campaign to Protect Rural England, The Countryside Agency, Department for Environment, Food and Rural Affairs, Environment Agency, Kings Sedgemoor and Cary Vale Internal Drainage Board (now part of Parrett Internal Drainage Board), Levels and Moors Partnership, National Farmers Union, Sedgemoor, Somerset County Council, South Somerset District Council, Taunton Deane and Wessex Water. They aim to tackle twelve areas, which, when combined, will make a significant contribution to reducing the adverse effects of flooding. These include the conversion of arable land, adoption of the Sustainable Drainage Systems (SuDS) approach to controlling rainwater runoff from developed areas, dredging, raising riverbanks and improving pumping facilities. Further studies of the possible beneficial effects of woodland in reducing flooding have also been undertaken

The Severn Estuary (Welsh: Môr Hafren) is the estuary of the River Severn, the longest river in Great Britain. Its high tidal range means it has been at the centre of discussions in the UK regarding renewable energy.GeographyDefinitions of the limits of the Severn Estuary vary. A narrower definition adopted by some maps is that the river becomes the Severn Estuary after the Second Severn Crossing near Severn Beach, South Gloucestershire and stretches to a line from Lavernock Point (south of Cardiff) to Sand Point near Weston-super-Mare. A wider definition is that the estuary extends upstream to Aust, the site of the Severn Bridge. The estuary is about 2 miles (3.2 km) wide at Aust, and about 9 miles (14 km) wide between Cardiff and Weston-super-Mare.
The estuary forms the boundary between Wales and England in this stretch. On the northern side of the estuary are the Caldicot and Wentloog Levels, on either side of the city of Newport; and, to the west, the city of Cardiff together with the resort of Penarth. On the southern, English, side, are Avonmouth, Portishead, Clevedon, and Weston-super-Mare. Denny Island is a small rocky island of 0.24 hectares (0.6 acres), with scrub vegetation, approximately three miles north of Portishead. Its rocky southern foreshore marks the boundary between England and Wales, but the island itself is reckoned administratively to Monmouthshire, Wales.

Map of the Bristol Channel and Severn estuary (shown here as "Mouth of the Severn")

The estuary has one of the highest tidal ranges in the world — about 15 metres (49 ft). The estuary's funnel shape, its tidal range, and the underlying geology of rock, gravel and sand, produce strong tidal streams and high turbidity, giving the water a notably brown coloration.
The tidal range also results in the estuary having one of the most extensive intertidal wildlife habitats in the UK, comprising mudflats, sandflats, rocky platforms and islands. These form a basis for plant and animal communities typical of extreme physical conditions of liquid mud and tide-swept sand and rock. The estuary is recognised as a wetland area of international importance and is designated as a Ramsar site.
West of the line between Lavernock Point and Sand Point is the Bristol Channel, which in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The islands of Steep Holm and Flat Holm are located close to that line, in the middle of the estuary.
Sometimes the term Severn Estuary is used to include the tidal upstream stretch between Gloucester and Aust. During the highest tides on the upper reaches of this stretch, the rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river currentArchaeologyThe archaeology of the Severn Estuary is richly varied and of considerable importance, reflecting both the varied nature of the topography and the importance of the river for both fishing and as a maritime waterway. The archaeological resource within the estuary is under threat from natural processes such as coastal erosion, exacerbated by the high tidal range and strong tidal currents, and from threats such as ongoing development pressure along the shoreline, marine aggregates extraction and new coastal defensive and realignment measures as well as proposed major infrastructure projects.

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