Did Niagara Falls ever stop flowing?

Yes! On March 29, 1848, Niagara Falls ran dry. A South-West gale blowing off of Lake Erie caused ice to jam and dam up at the mouth of the Niagara River causing the water flow over the Horseshoe Falls and American Falls to be reduced to a trickle. On the morning of March 30, 1848, the river flow returned to its normal rate.