18 miles downstream from Iguaçu Falls, Rio Iguaçu converges with the Paraná River. This intersection creates the borders of three countries. The gazebo at Marco das Três Fronteiras is in Brazil. In the upper left is Hito Tres Fronteras in Argentina. The riverbank on the right is Paraguay. Río Paraná is the second longest river in South America. It flows for over 3,000 miles before merging with the Uruguay River and finally entering the Atlantic Ocean.