The Uruguay River (Spanish: Río Uruguay, Spanish pronunciation: [uɾuˈɣwai]; Portuguese: Rio Uruguai, Portuguese pronunciation: [uɾuˈɡwaj]) is a river in South America. It flows from north to south and makes boundary with Brazil, Argentina, and Uruguay, separating some of the Argentine provinces of the Mesopotamia from the other two countries. It passes between the states of Santa Catarina and Rio Grande do Sul in Brazil; forms the eastern border of the provinces of Misiones, Corrientes, and Entre Ríos in Argentina; and makes up the western borders of the departments of Artigas, Salto, Paysandú, Río Negro, Soriano, and Colonia in Uruguay.
The river measures about 1,600 kilometres (990 mi) in length and starts in the Serra do Mar in Brazil, where the Canoas River and the Pelotas River are joined, at about 200 m above mean sea level. In this stage the river goes through uneven, broken terrain, forming rapids and falls. Its course through Rio Grande do Sul is not navigable.
Unusual feature of Uruguay River is a submerged canyon along most of its length. This canyon has formed in dry period during the Ice Age and its depth in some locations reaches 100 m. Canyon is visible only at Moconá Falls - unusual, up to 12 m high and more than 2 km wide waterfall, which has formed on a rim of this canyon 1 215 km from the mouth of Uruguay.
Together with the Paraná River, the Uruguay forms the Río de la Plata estuary. It is navigable from around Salto Chico. Its main tributary is the Río Negro, which is born in the south of Brazil and goes through Uruguay 500 km until its confluence with the Uruguay river, which is located 100 km north from the Uruguay's confluence with the Río de la Plata, in Punta Gorda (Colonia Department, Uruguay).
The river is crossed by five international bridges (from north to south): Integration Bridge and Paso de los Libres-Uruguaiana International Bridge, between Argentina and Brazil; and the Salto Grande Bridge, General Artigas Bridge and Libertador General San Martín Bridge between Argentina and Uruguay.
The drainage basin of the Uruguay River has an area of 365,000 square kilometres (141,000 sq mi). Its main economic use is the generation of hydroelectricity and it is dammed in its lower portion by the Salto Grande Dam and by the Itá Dam upstream in Brazil.