Which river is the principal tributary of the Rio de la Plata by discharge?

The Paraná River is the principal tributary of the Río de la Plata by discharge. It carries most of the freshwater entering the great South American estuary, ahead of the Uruguay River and smaller tributaries.

The Paraná rises in Brazil from the confluence of the Paranaíba and Grande rivers, then flows through or along Brazil, Paraguay, and Argentina for roughly 4,880 kilometers when its headwaters are included. Near its lower course, it receives the Paraguay River before spreading into the Paraná Delta.

The Paraná and Uruguay rivers meet at Punta Gorda to form the Río de la Plata, which opens into the Atlantic Ocean between Argentina and Uruguay. The Paraguay River is a major tributary of the Paraná, not the principal direct tributary of the Río de la Plata, a distinction that can cause confusion.

The river is also an important transport and energy corridor. The Itaipu and Yacyretá dams generate enormous amounts of hydroelectric power, while the Paraná–Paraguay waterway links inland South America with Atlantic ports.