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, England.

It is internationally significant for wintering and breeding wildfowl and waders, especially teal, pintail, Eurasian wigeon, shoveler, pochard and Bewick's swans.

After the last glaciation between 100,000 and 10,000 years ago the sea level in eastern England was about 30 metres (98 ft) lower than at present.

Coastal woodland was drowned by the returning sea and slowly degraded to peat overlying deposits of marine clays and creating the Fens.

[4] The Environment Agency sets the trigger level for the sluices, allowing higher levels in the Great Ouse in summer than in winter.The enclosed area of washland runs from Earith northeast to Downham Market where it links via the New Bedford River to the tidal Great Ouse and hence to the sea.

As the peat underlying the Fens has dried out through drainage, it has shrunk and lowered the level of the washlands, making flooding more frequent.

[6][8][9] The Ouse Washes Landscape Partnership scheme (OWLP) was a £1 million, 3-year project supported by the Heritage Lottery Fund which ran from 2014 to 2017.

[11] RSPB Ouse Washes is a nature reserve, managed by the Royal Society for the Protection of Birds at Welches Dam.

[16] Increased summer flooding led to declines in the numbers of breeding waders from the 1970s onwards, and to counteract this areas of former farmland adjacent to the washes were acquired and converted to wet grassland.

[6] The Ouse Washes are the most important site in England and Wales for breeding snipe, and also hold good populations of lapwings and redshanks and oystercatchers.

Hobbies, marsh harriers and barn owls all breed around the washes, as do yellow wagtails, corn buntings and tree sparrows, and kingfishers nest in artificial banks.

The number of Bewick's swans reaching the UK has fallen dramatically in the current century, as milder winters encourage them to remain in continental Europe, but several hundred still visit the washes.

High waters levels prevent wigeon grazing on grass, and the extension areas were designed with this species in mind, so that they could feed when the main site was flooded.

Gulls roost overnight, as do hen harriers, and common cranes also spend the night on the washes after foraging for maize stubble on the arable farmland.

[6] Large numbers of migrants may seek temporary refuge on the marshes, including 2 million sand martins (1968) 5,000 common snipe (1979), 500 ruffs (1989) and 130 red-breasted mergansers (1956).

[6] Apart from the grassland, there are small patches of trees and osier, but the main interest lies in rare aquatic plants such as greater water-parsnip and fringed water lily, although the diversity and numbers of such species has reduced due to nutrient deposition by floodwater.

The areas neighbouring the washland, used for intensive arable farming for 50 years, have now been reseeded with grasses and associated plants such as meadow buttercup, tufted vetch, ribwort plantain.

The new areas outside the washes are kept wet through a closely spaced network of ditches filled from ground aquifers by submersible pumps, and protected by fox- and badger-proof fences.

[6] It is anticipated that a combination of lower flows in the Great Ouse and higher sea water levels will make it harder to manage drainage, although more areas of shallow water might benefit potential colonising breeders including the glossy ibis, black-winged stilt and various heron and egret species.