Douro River basin covers a total area of 97 682 km2, which of them - 18 710 km2 – in Portugal (about 19%). The basin crosses Portugal with an E-W direction between 40º 20´ - 43º 10´ latitude north and 01º 43´ - 08º 40´ longitude west, occupying a considerable part of North Portugal with a dominant orientation east-west.
Douro River rises in Spain, in Urbion Mountain – about 1700 m high – and flows along approximately 927 km westward, until reaching the Douro Estuary at the Portuguese Coast near S. João da Foz, southward of O’ Porto. The river has a considerable annual average flow rate – 710 m3/s - and a pronounced seasonal variability. The River discharge into the estuary is controlled by Crestuma reservoir, located 16 km upstream of D. Luis Bridge.
Douro Estuary, represented in Figure 1, is a long (22 km) and narrow valley with minimum width of 135 m at D. Luis Bridge (about 6 km from the estuary mouth) and a maximum of 1300 m reached downstream of Arrábida Bridge. At it’s mouth the Cabedelo sand bank, perpendicular to estuary axis, is modulated by local hydrodynamics. In extreme high discharges situations (~ 10 000 m3/s) the bank is destroyed by the draining (Silva, 1995). After these extreme situations the sand bank is gradually reconstructed. The central part of Cabedelo reaches 5 m above the mean sea level (Peixoto et al., 1999). The upper part of the estuary is narrow and it depth is normally higher than 10 meters.
Figure 1 - River Douro Estuary.