Designated in 1976

The Nature Reserve has a total area of 14 416 ha and includes a large portion of estuarine waters, mud flats surrounded by inlets, salt marshes, saltworks and farmland on alluvial deposits (river meadowlands). The maximum altitude is 12 m. The Nature Reserve covers parts of Vila Franca de Xira, Benavente and Alcochete municipalities and is part of the Tagus estuary, which covers a total area of 32 500 ha, making it the largest in Western Europe.
Strongly influenced by salt water brought in by the tides, the estuary banks host a characteristic floral community: the salt marsh. This region, which is highly productive in Polychaetae, molluscs and shellfish, is a nursery for various fish species. Nevertheless, it is the birds that give the Tagus estuary its status as the most important wetland in Portugal and one of the most important in Europe. Wintering birds sometimes number 120 000 individuals. Regular counts indicate that about 54% of Portugal’s waders and over 30% of the wintering populations of ducks and geese winter in this area, as well as the avocet Recurvirostra avosetta, of which the Reserve houses 25% of the total wintering population in Europe. Many other species testify to the biological wealth and value of this region for nature conservation, like the flamingo Phoenicopterus ruber, greylag goose Anser anser, dunlin Calidris alpina and black-tailed godwit Limosa limosa.
The largest estuary in Portugal and one of the most important along the Atlantic coast, the Tagus estuary is one of the ten most significant wetlands for migrating water birds in Portugal. The strong human pressure in the area gave rise to the need for a legal framework for this region, with broad coverage and respecting its importance in terms of nature conservation. Thus, under the terms of the Wild Birds Directive (Council Directive 79/409/EEC), the Special Protection Area (SPA) was created on 5 November 1994. With a total area of 45 020 ha around the Nature Reserve, the SPA includes the estuary area upstream to the line defined by the mouth of the river Trancão. It also includes the area all the way to the southernmost point of the Montijo peninsula, and the river meadowlands and wooded areas with cork oak Quercus suber. It aims to maintain human activities such as farming, salt extraction and fishing in a way that is compatible with the presence of wild birds and their habitats, in order to preserve the ecological conditions and freedom from disturbance needed by migrating birds.
An estuary with the features and size of the river Tagus has been attractive to humans since early times, serving as a communications route, as a trading post and harbour, as a rich fishing and hunting area and for farming in its rich alluvial soil. The human presence in the area dates from the Mesolithic period. People from many countries and regions arrived here in the past, of whom the Arabs and the Romans were the most important. Beginning in the 15th century, this region was used as a logistic centre for the Portuguese maritime discoveries. The largest population centre in Portugal, Lisbon and the other towns along the Tagus estuary have more than one and a half million inhabitants.