Human intervention
So, why did engineers need to drain the Falls in the first place? Well, they had discovered that the waterfall held the secret of its own demise. As it poured over the edge of the cliff, the water was destroying the rocks on the edge of the Falls. In fact, since it formed, Niagara Falls has moved 7 miles due to eroding rocks.
When water has been flowing that fast for thousands of years, that’s a lot of rocks that have been thrown down to the bottom of the cliffs. These loose rocks are called scree, and they were beginning to pile up at the bottom of the Falls. Some engineers were worried that the Falls would collapse completely within a few decades.