The basins of the Parana and Paraguay are separated by low mountain ranges extending north from the sierras of Paraguay.
The northern slope of this great plateau is drained by the AraguayaTocantins, Xingu, Tapajos and Guapore-Mamore-Madeira, which flow northward, and, except the first, empty into the Amazon; the southern slope drains southward through a multitude of streams flowing into the Parana and Paraguay.
The Argentine " mesopotamia," between the Parana and Uruguay rivers, belongs in great measure to this same region, being partly wooded, flat and swampy in the north (Corrientes), but higher and undulating in the south (Entre Rios).
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.
The largest of the rivers through which Argentina drains into the Plata system are the Pilcomayo, which rises in Bolivia and flows south-east along the Argentine frontier for about 400 m.; the Bermejo, which rises on the northern frontier and flows south-east into the Paraguay; and the Salado del Norte (called Rio del Juramento in its upper course), which rises on the high mountain slopes of western Salta and flows south-east into the Parana.