[JackFromTexas]

In a landlocked lake, excess water goes downstream one way or another. What dam is closing the Niagara river?
If such an increase in water happens, then the Niagara Falls gets more water, more erosion, and, for a while, more spectacular.
Then all the water goes to the sea.

[jonascord]

The Niagara Power Authority controls the amount of water that goes over the Falls. Last time I was there it was running at about 50%, based on a balance between generated power needs and reducing the erosion of the lip of the Falls.

(Understand that the Niagara Gorge, 20+ miles of it, is the result of the water carving back.)

They have turned off the Falls completely, several times, off season, to clean things up, and recover bones wedged in the rocks.