The slowing of the falls is virtually immensurable, since the power station takes its water well upstream of the falls, and the water thus channeled is controlled "locally" by diverters, not my changing the volume taken from the Niagara River.
There are several interesting things about the Niagara power station.
For example, it is America's (the U.S. and Canada combined) first hydroelectric power station. It is also the first non-experimental AC power generating station in the world.
The original frequency of the power generated at Niagara was 25 Hz (cps back then), not the modern 60 Hz.
Some of the internal equipment used within the station is still 25 Hz. There is one (count them, one) company in Buffalo that has special equipment to repair and rewind 25 Hz pump motors specifically for the Niagra power station.
The falls themselves do change volume for natual reasons, however. Seasonally, the flow is the least in the winter and the greatest in the spring. There are also other factors causing short-term flow changes. One must remember that substantially all the water flowing into Lakes Superior, Michigan, Huron, and Erie ultimately passes over the falls, except for the very small amount diverted to the power station. This volume is so great that man simply has no real effect upon it.
Even the water consumed upstream of the falls for industrial, biological, and utilitarian use is ultimately returned to the flow upstream of the falls, and passes over.
