The RNES (Natural Reserve of the Sado Estuary) was established on  October 1st  in 1980.

Its creation was due primarily to the botanical and fauna interest of this vast body of water, with specific objectives of maintaining the natural vocation of the estuary, development activities consistent with the balance of the estuarine ecosystem, the correct use of resources, the defense cultural or scientific order of values and the promotion of outdoor recreation.

It covers an area of 23,160 hectares consisting on estuarine waters, flood plains, dunes and sea and river beaches, encompassing enclave of Alcacer do Sal municipalities, Grândola, Palmela and Setúbal.

The reserve is included in the list of International Importance Wetlands (Ramsar Convention), since May 7, 1996, and is a Special Protection Area (SPA) for birds since 23 September 1999. More than 80% of the area of reserve is also covered by the Sado site of the Natura 2000 network.

Alcácer do Sal downstream develops a wide estuary (Sado Estuary) separated from the ocean by the Troia peninsula. It is on of the few Portuguese rivers flowing from south to north, as the Mira River (in Beja).

In the Sado estuary dwells a population of dolphins (Bottlenose dolphins-nosed).

The catchment area of the river Sado has an area of 7692 km² entirely Portuguese. The estuary covers an area of approximately 160 km², with an average depth of 8m with a maximum of 50m. The flow is forced mainly by the tide.

The estuary is the habitat of a large community of bottlenose dolphins; there are 31 members of the pod, each of whom has been named (2007).

The river is dammed in several places, chiefly for irrigation of rice, maize, and other vegetables.

In its course, the river crosses the city of Alcácer do Sal.