Starting at the confluence of Paraná and Iguaçu rivers, it passes through the Iguaçu Falls and follows the thalweg of that river to the mouth of Santo Antônio River, then running upstream the course of this river until its source. From there the boundary runs 25.1 km (15.6 mi) by land until reaching the source of the Peperi-Guaçu River and from there along the channel of that river to its confluence with the Uruguay River, then running downstream the course of the Uruguay to the mouth of river Quaraí.