Existe uma seção turismo completa no site do governo para a represa . Sobre a questão dos vertedouros, diz (grifo meu):
Watch the spillway in action between the months of December and February when the steel floodgates are open during the rainy season. The opening of the spillway does not follow a preset schedule. It is directly linked to the level of the lake. Those lucky enough to watch the water draining out through the three chutes have the chance of beholding an unforgettable display. The outflow of Itaipu's spillway is equivalent to 40 Iguazu Falls. That deluge slides down a 30 meter toboggan to form a 10 meter overflow, and then finally goes back to the rocky bed of the Paraná River, at 40 meters deep.
A represa como um todo parece bem legal, mesmo sem isso, mas eu diria que se você estiver visitando na estação das chuvas, talvez você possa tentar ir quando estiver chovendo muito?