Uruguay River, a river in South America. It is about 1,000 miles (some 1,600 km) long. From its source in the coastal mountains of southern Brazil, the river flows westward to Argentina. It then runs southwest and south, forming the boundary between Brazil and Argentina, and between Uruguay and Argentina. Just north of Buenos Aires it joins the Paraná River, forming the Río de la Plata, a broad estuary. From the estuary, the Uruguay River is navigable for large ships for 130 miles (209 km). Small vessels can sail 60 miles (97 km) farther upstream to Salto Grande Falls, where a hydroelectric plant is operated by Uruguay and Argentina.