After merging with the Iguazu, the Paraná then becomes the natural border between Paraguay and Argentina. The river continues its general southward course for about 291 miles (468 km) before making a gradual turn to the west for another 510 miles (820 km), and then encounters the Paraguay River, the largest tributary along the course of the river.
Before this confluence the river passes through a second major hydroelectric project, the Yacyretá Dam, a joint project between Paraguay and Argentina. It is the second largest dam in the world. The massive reservoir formed by the project has been the source of a number of problems for people living along the river, most notably the poorer merchants and residents in the low-lying areas of Encarnación, a major city on the southern border of Paraguay. River levels rose dramatically upon completion of the dam, flooding out large sections of the city's lower areas.
From the confluence with the Paraguay River, the Paraná again turns to the south for another approximately 510 miles (820 km) through Argentina, making a slow turn back to the east near the city of Rosario for the final stretch of less than 311 miles (500 km) before merging with the Uruguay River to form the Río de la Plata and emptying into the Atlantic Ocean.
During this part of its course, downstream from the city of Diamante in the province of Entre Ríos, it splits into several arms and forms the Paraná Delta, a long flood plain which reaches up to 39 miles (60 km) in width.