[8] For 1,205 kilometres (749 mi), to its confluence with the San Antonio River, the Iguazu flows west through Paraná State, Brazil.

[2] Continuing west, the river drops off a plateau, forming Iguazu Falls, which are accessible via the Rainforest Ecological Train.

It empties into the Paraná River at the point where the borders of Argentina, Brazil, and Paraguay join, an area known as the Triple Frontier.

[11][12] This also means that, except for the threatened Steindachneridion melanodermatum in the lower part, large migratory fish known from much of the Paraná River Basin are naturally absent from Iguazu.

[10][13] Almost 30 introduced species are found in the river where about one-third originate from other continents (such as carp, largemouth bass, tilapia and African sharptooth catfish) and the remaining from elsewhere in South America (such as dorado, Cichla kelberi, pacu, Brycon hilarii, Prochilodus lineatus and Odontesthes bonariensis).