The three great rivers that form the La Plata system - the Paraguay, Parana and Uruguay - have their sources in the highlands of Brazil and flow southward through a great continental depression, two of them forming eastern boundary lines, and one of them, the Parana, flowing across the eastern part of the republic. The northern part of Argentina, therefore, drains eastward from the mountains to these rivers, except where some great inland depression gives rise to a drainage having no outlet to the sea, and except, also, in the " mesopotamia " region, where small streams flow westward into the Parana and eastward into the Uruguay.