From Mashable.com:
For several months in 1969, the torrent of water rushing over American Falls, one of three waterfalls that makes up Niagara Falls, was reduced to little more than a trickle.
American Falls is recognizable for the immense rock pile, or talus, at its base, the result of a series of natural rockslides over the years. In the late 1960s, concerns were growing that further rockslides could erode the falls completely.
To study the geological composition of the falls and forestall their potential destruction, a joint American-Canadian commission decided to dewater them for five months.
Over three days in June 1969, more than 1,200 trucks dumped nearly 28,000 tons of rocky fill into a cofferdam upstream of the falls, diverting the flow of the Niagara River away from American Falls and toward the much larger Horseshoe Falls.
With the falls dry for the first time in millennia, the US Army Corps of Engineers began their investigation.