The Sado estuary is smaller than the Tagus estuary, 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. The Sado River has pluvial discharge regime with average annual values of 10 m³/s. A number of small streams also discharge in the estuary, Marateca Stream being the most important, with a flow in order of 1 m³/s.