Spain and Portugal share four important Iberian rivers, namely from North to South: the Minho- Limia, Douro, Tagus and Guadiana. They all discharge into the Atlantic Ocean. The catchment areas of these basins total 44.8% of the Iberian Peninsula and represent about 46% of the annual surface discharge. These shared rivers account for 740 km of the 1214 km border between the two countries. Very few countries in the world have four shared rivers with four different configurations (see Figure 1), occupying so much of their territory.