The volume of water the Iguazu river carries when it goes through the falls fluctuates during the year. In several occasions during this millennium the volume was so low that most of the falls dried up.
The Brazilian organization responsible for the Iguazu river monitors the volume of water going down the river course:
The interesting bit of data is the column labeled vazão (m3/s). The anual average for the river at the falls is 1,200 m3/s. During August 2006, the level dropped to 200 m3/s. During the rainy season, it can reach the incredible amount of 8,000 m3/s. And during occassional floods, the figure can rocket up to unbelievable 11,000 m3/s, as it did on June 2013. From 900 m3/s upwards you can be sure the display at the falls will be fantastic. A value much lower than that should make you reconsider your trip.
destination: Iguazu Falls