The Lagoon of Óbidos is the most extensive lagunar system in the coast of Portugal. It has an approximate area of 6.9 square kilometres and an average depth of two meters, with a quota that goes from half a meter to five meters. The Lagoon of Óbidos extends through two branches: Braço do Bom Sucesso towards West, and the Braço da Barrosa facing East.

It also borders the municipality of Caldas da Rainha (North) with the Parish Councils of Foz do Arelho and Nadadouro, and the municipality of Óbidos (South) with the Parish Councils of Vau and Saint Maria. On the South side is Poça do Vau and on the West side it is connected to Poça das Ferrarias. It has a maximum length of 6 km and a width that oscillates between 1 and 1.5 km, with a preferential NW-SW orientation.

As a coastal lagoon system, it has a high value due to its productive characteristics, namely in terms of biodiversity of ecosystems and fauna and floristic species, but also of birdlife (where migratory movements of several birds can be observed).

It communicates with the ocean through a tide bar (“Aberta” in local toponymy) artificially maintained, which ensures the exchange of water and sediments between the lagoon and oceanic spaces.

Of the entire Lagoon area, the Braço da Barrosa is undoubtedly the most important place from an ecological point of view, since it is important for several different biological groups, it has one of the few remaining grove spots in the lagoon, as well as some emptying areas with relatively well preserved vegetation, which defines good conditions for birdlife and is also important for ichthyo fauna, providing satisfactory conditions for the nursery function.