Foz da Ribeira de Almargem consists of a relic lagoon of an estuary that existed at least until the Roman period and which, little by little, was filled by sand and river alluvium.

When full, the length of the lagoon reaches 830 meters, with a maximum width of 185 meters and a depth that never exceeds 5 meters. Next to the sea, in its southern terminal portion, it consists of a relatively high sandy barrier that prevents the flow of fresh water to the sea. However, in situations of spring tides or sea storms, oceanic overtopping may occur, breaking the sandy barrier and enabling partial renewal of the lagoon water mass.

The human presence along the streams that give rise to this lagoon is ancient, and has shaped and limited the fauna and flora present in the place, through traditional agriculture with dryland orchards, vegetable gardens and grazing.