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Grazing marsh is a British Isles term for flat, marshy grassland in polders. It consists of large grass fields separated by fresh or brackish ditches, and is often important for its wildlife.
Grazing marshes were created from medieval times by building sea walls (earth banks) across tidal mudflats and salt marsh to make polders (though the term "polder" is little used in Britain). Polders in Britain are mostly drained by gravity, rather than active pumping. The original tidal drainage channels were augmented by new ditches, and flap valves in the sea walls let water drain out at low tide and prevent the sea or tidal river from entering at high tide. Constructing polders in this way is called inning or reclaiming from the sea.
Grazing marshes have been made in most lowland estuaries in Britain, often leaving only the river channel and the lowest part of the estuary tidal. In a few cases (such as Newtown Harbour on the Isle of Wight, and Pagham Harbour in West Sussex) the sea walls have been breached, and the estuaries have returned to a tidal state. Grazing marshes have also been made on low-lying open coasts.
Many grazing marshes were inned in stages, and the old sea walls (called counter walls) may be found marooned far from the current sea wall. Land levels on either side of a counter wall often differ by several metres. Paradoxically, the lower side is the land inned earlier, because sediment continued to build up on the side that remained tidal.
Wintering wildfowl are characteristic of grazing marshes, often including large flocks of Eurasian wigeon, brent goose, white-fronted goose and Bewick's swan. Many of these birds are hunted by predators such as peregrine and marsh harrier.
In spring, waders such as common redshank, Eurasian curlew, snipe, and northern lapwing breed. [1]
The ditches often have a range of salinity, depending on how close to the sea wall they are. The more saline ditches host specialist brackish-water plants and animals. These include, for example, the rare brackish amphipod Gammarus insensibilis and sea club-rush (Bolboschoenus maritimus). Fresher ditches may support rare animals, such as the great silver water beetle (Hydrophilus piceus) and the great raft spider (Dolomedes plantarius), and a wide range of pondweeds ( Potamogeton and relatives).
The grassland vegetation usually has a fairly small number of species, but those present are often scarce elsewhere, such as sea arrowgrass (Triglochin maritimum), divided sedge (Carex divisa) and strawberry clover (Trifolium fragiferum).
Many grazing marshes have been converted into arable land, often using pumped drainage to lower the water levels enough to grow crops, though most are used for grazing cattle. [2] The low ditch levels and agricultural runoff combine to remove much of the aquatic wildlife, although the arable fields may still be used by some wintering wildfowl.
Some areas of grazing marsh and other polder land have been used to recreate tidal habitats by a process of managed retreat.
Many of the larger areas of grazing marsh bear nature conservation designations, including Site of Special Scientific Interest, Special Protection Area, Special Area of Conservation and Ramsar Site.
A polder is a low-lying tract of land that forms an artificial hydrological entity, enclosed by embankments known as dikes. The three types of polder are:
The Halvergate Marshes are an area of grazing marsh in the east of the English county of Norfolk. They form part of the area of The Broads and lie between the River Bure and the River Yare, bordering Breydon Water on the east. The marshes cover an area of around 2,642 hectares An area of 1,432.7-hectare (3,540-acre) is a biological Site of Special Scientific Interest. Some areas are also in the Breydon Water Local Nature Reserve, the Broadland and Breydon Water Ramsar sites, The Broads Special Area of Conservation, and The Broads and Breydon Water Special Protection Areas.
Berney Marshes and Breydon Water RSPB reserve is a nature reserve managed by the Royal Society for the Protection of Birds (RSPB). Berney Marshes is situated south of the River Bure while Breydon water is the combined estuary of the River Waveney and the River Yare inland from Great Yarmouth in Norfolk, England.
The River Stour(, rhymes with "flour") is a river in Kent, England that flows into the North Sea at Pegwell Bay. Above Plucks Gutter, where the Little Stour joins it, the river is normally known as the Great Stour. The upper section of the river, above its confluence with the East Stour at Ashford is sometimes known as the Upper Great Stour or West Stour. In the tidal lower reaches, the artificial Stonar Cut short cuts a large loop in the natural river.
Ouse Washes is a linear 2,513.6-hectare (6,211-acre) biological Site of Special Scientific Interest stretching from near St Ives in Cambridgeshire to Downham Market in Norfolk. It is also a Ramsar internationally important wetland site, a Special Protection Area for birds, a Special Area of Conservation and a Nature Conservation Review site, Grade I. An area of 186 hectares between March and Ely is managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire and another area near Chatteris is managed by the Royal Society for the Protection of Birds. The Wildfowl & Wetlands Trust manages another area near Welney.
Bridgwater Bay is on the Bristol Channel, 5 kilometres (3.1 mi) north of Bridgwater in Somerset, England at the mouth of the River Parrett and the end of the River Parrett Trail. It stretches from Minehead at the southwestern end of the bay to Brean Down in the north. The area consists of large areas of mudflats, saltmarsh, sandflats and shingle ridges, some of which are vegetated. It has been designated as a Site of Special Scientific Interest (SSSI) covering an area of 3,574.1 hectares 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.
Romney Marsh is a sparsely populated wetland area in the counties of Kent and East Sussex in the south-east of England. It covers about 100 square miles (260 km2). The Marsh has been in use for centuries, though its inhabitants commonly suffered from malaria until the 18th century. Due to its location, geography and isolation, it was a smuggler's paradise between the 1600s and 1800s. The area has long been used for sheep pasture: Romney Marsh sheep are considered one of the most successful and important sheep breeds. Criss-crossed with numerous waterways, and with some areas lying below sea level, the Marsh has over time sustained a gradual level of reclamation, both through natural causes and by human intervention.
A salt marsh, salt marsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments. Salt marshes play a large role in the aquatic food web and the delivery of nutrients to coastal waters. They also support terrestrial animals and provide coastal protection.
Blacktoft Sands RSPB reserve is a nature reserve in the East Riding of Yorkshire, England. It is managed by the Royal Society for the Protection of Birds, which leases the site from Associated British Ports.
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Montrose Basin is a nearly circular tidal basin which makes up part of the estuary of the River South Esk and which sits just inland of the town of Montrose in Angus on the east coast of Scotland. The basin is protected by a number of designations, it is managed by the Scottish Wildlife Trust as a Wildlife Reserve, as well as being designated as a Local Nature Reserve, Site of Special Scientific Interest, a Special Protection Area and a Ramsar Site.
A reedbed or reed bed is a natural habitat found in floodplains, waterlogged depressions and estuaries. Reedbeds are part of a succession from young reeds colonising open water or wet ground through a gradation of increasingly dry ground. As reedbeds age, they build up a considerable litter layer that eventually rises above the water level and that ultimately provides opportunities in the form of new areas for larger terrestrial plants such as shrubs and trees to colonise.
A tidal marsh is a marsh found along rivers, coasts and estuaries which floods and drains by the tidal movement of the adjacent estuary, sea or ocean. Tidal marshes experience many overlapping persistent cycles, including diurnal and semi-diurnal tides, day-night temperature fluctuations, spring-neap tides, seasonal vegetation growth and decay, upland runoff, decadal climate variations, and centennial to millennial trends in sea level and climate.
Located in northern California, the Suisun Marsh has been referred to as the largest brackish water marsh on west coast of the United States of America. The marsh land is part of the San Francisco Bay tidal estuary, and subject to tidal ebb and flood. The marsh is home to many species of birds and other wildlife, and is formed by the confluence of the Sacramento and San Joaquin rivers between Martinez and Suisun City, California and several other smaller, local watersheds. Adjacent to Suisun Bay, the marsh is immediately west of the legally defined Sacramento-San Joaquin Delta as well as part of the San Francisco Bay estuary.
A tidal river is a river whose flow and level are caused by tides. A section of a larger river affected by the tides is a tidal reach, but it may sometimes be considered a tidal river if it had been given a separate and another title name.
Oare Marshes is a 71.4-hectare (176-acre) Local Nature Reserve north of Faversham in Kent. It is owned and managed by Kent Wildlife Trust. It is part of The Swale Nature Conservation Review site, Grade I, National Nature Reserve, Ramsar internationally important wetland site, Special Protection Area under the European Union Directive on the Conservation of Wild Birds, and Site of Special Scientific Interest.
Blake's Pools are a 4 hectare nature reserve owned by Environment Agency and leased by the Avon Wildlife Trust, on the banks of the Congresbury Yeo close to its mouth, near Kingston Seymour, Somerset, in South West England.
Brackish marshes develop from salt marshes where a significant freshwater influx dilutes the seawater to brackish levels of salinity. This commonly happens upstream from salt marshes by estuaries of coastal rivers or near the mouths of coastal rivers with heavy freshwater discharges in the conditions of low tidal ranges.
Classification of wetlands has been a problematical task, with the commonly accepted definition of what constitutes a wetland being among the major difficulties. A number of national wetland classifications exist. In the 1970s, the Ramsar Convention on Wetlands of International Importance introduced a first attempt to establish an internationally acceptable wetland classification scheme.
Hurst Castle and Lymington River Estuary is a 1,077.3-hectare (2,662-acre) biological and geological Site of Special Scientific Interest near Lymington in Hampshire. It is a Nature Conservation Review site and two areas are Geological Conservation Review sites. Three areas are local nature reserves, Boldre Foreshore, Sturt Pond and Lymington and Keyhaven Marshes; the latter site is managed by the Hampshire and Isle of Wight Wildlife Trust. Part of it is North Solent National Nature Reserve. It is part of Solent and Southampton Water Ramsar site and Special Protection Area. Parts of it are in Solent Maritime and Solent and Isle of Wight Lagoons Special Areas of Conservation.
