How did this happen ? Whats the explanation to this ?
Maintenance basically
Erosion control efforts have always been of extreme importance. Underwater weirs redirect the most damaging currents, and the top of the falls have also been strengthened. In June 1969, the Niagara River was completely diverted away from the American Falls for several months through construction of a temporary rock and earth dam (clearly visible in the photo at right).[37] While the Horseshoe Falls absorbed the extra flow, the U.S. Army Corps of Engineers studied the riverbed and mechanically bolted and strengthened any faults they found; faults that would, if left untreated, have hastened the retreat of the American Falls. A plan to remove the huge mound of talus deposited in 1954 was abandoned owing to cost, and in November 1969, the temporary dam was dynamited, restoring flow to the American Falls. Even after these undertakings, Luna Island, the small piece of land between the main waterfall and the Bridal Veil, remained off limits to the public for years owing to fears that it was unstable and could collapse into the gorge at any time.
https://en.wikipedia.org/wiki/Niagara_Falls#Impact_on_industry_and_commerce
http://www.theatlantic.com/national...iagara-falls-looks-like-without-water/282991/
http://photoblog.nbcnews.com/_news/2010/12/16/5661928-niagara-falls-without-water-as-seen-in-1969