3-in-1 falls
You might not be aware that Niagara Falls isn’t actually one waterfall. It’s three different waterfalls: American Falls, Horseshoe Falls, and Bridal Falls. The most famous of these, and the one you’ll usually see on TV, is the Horseshoe Falls. American and Bridal Falls can be visited from the American side, and Horseshoe from the Canadian side.
The problem that engineers faced when they were trying to block the Falls in 1969 was that if you just blocked the flow from one of the Falls, the water would still cascade down the other two, making any work on the bed impossible.