Setúbal is a Portuguese city, capital of the Setúbal district (since 1926) and seat of the diocese (since 1975), with 91,000 inhabitants in its urban perimeter. It is the seat of a municipality with 230.33 km² in area and 116 330 inhabitants (2017), subdivided into 5 parishes. The municipality is limited to the west by the municipality of Sesimbra, to the northwest by Barreiro, to the north and east by Palmela and, to the south, the Sado estuary separates it from the municipalities of Alcácer do Sal and Grândola. The Troia peninsula, belonging to Grândola, is located in front of the city, between the Sado estuary and the coast of the Atlantic Ocean.

The Sado (formerly called Sádão) is a Portuguese river, which rises at 230m altitude, in the Serra da Vigia in Ourique and travels 180 kilometers until it flows into the Atlantic Ocean near Setúbal. On its route it passes through Panoias, Alvalade and Alcácer do Sal, the mouth in front of Setúbal. Downstream from Alcácer do Sal to the mouth, a large estuary is separated from the ocean by the Troia peninsula. It is one of the few Portuguese rivers that flows from south to north, such as the Côa river that originates in the municipality of Sabugal and will flow into the Douro River, near Vila Nova de Foz Côa as well as the Mira River (Odemira, Alentejo). A population of dolphins (bottlenose dolphins) inhabits the Sado estuary, which has resisted the invasion of their habitat by man (maritime traffic to the Mitrena shipyards, to the port of Setúbal and due to fishing and the recreational dock, in addition to ferry between the banks). In 2013, the population consists of 28 elements. The Sado River does not have a large flow due to several factors, two of which stand out: the most arid climate in the Alentejo, where its source is located; and the small gap between the height of the spring and the altitude of the mouth. The hydrographic basin of the river Sado has an area of ​​7692 km², the hydrographic basin being the largest area entirely Portuguese. The estuary occupies an area of ​​approximately 160 km², with an average depth of 8m with a maximum depth of 50m. The flow is forced mainly by the tide. The average annual flow of the river is 40m³ / s with strong seasonal variability - ranging from daily values ​​below 1m³ / s in summer to above 150m³ / s in winter.