The Sado estuary is smaller than the Tagus estuary (see Figure 1), but it is still a large estuary (about 150 km2). It has also large intertidal areas (about 1/3 of its surface) and a more complex bathymetry, with two channels in the lower estuary separated by a set of sand banks and 3 main basin in the upper part (Marateca, Alcácer Channel and Comporta). The main fresh water discharge is the Sado River, entering through Alcácer Channel. A number of small streams also discharge in the estuary, Marateca Stream being the most important.
Figure 1 - Sado Estuary.