The Sado River (formerly called Sádão) is a Portuguese river, which rises at 230m altitude, in the Serra da Vigia and travels 180km until it flows into the Atlantic Ocean. It is one of the few rivers in Europe that flows from south to north. The watershed of the river Sado has an area of 7640 km2.
The estuary occupies an area of approximately 160km2, with an average depth of 8m, the maximum being 50m.
Its route passes through the interior of the municipality of Grândola (6Km approx.) In the parish further east -Azinheira de Barros and São Mamede do Sadão, a name associated with the old name of Rio. from Alentejo, for approximately 15 km.
Although most of them are located in the municipalities of Setúbal, Palmela, and Alcácer do Sal, the Sado Estuary Nature Reserve also covers part of the municipality of Grândola. It consists of a succession of sea and river, sandbanks and mudflats, marshes and freshwater ponds, scrub and forest, agricultural areas and pastures, beaches and dunes. Presenting considerable values from the malacological, ichthyological, or ornithological point of view, this Reserve is an important nursery for restocking the coastal zone and presenting itself as an area of great relevance to waders, anatids and mammalogical species.