The Days Niagara Falls Went Dry... March 29-31, 1848"

The flow of water was stopped completely over American Falls & Horseshoe Falls on March 29th 1848. "Due to an ice jam in the upper river for several hours. This is the only known time to have occurred. The Falls did not actually freeze over, but the flow was stopped to the point where people actually walked out and recovered artifacts from the riverbed".
Even during the coldest months, "the river waters continue to flow, carrying Lake Erie and Niagara River ice over the cataract and into the great basin below the Falls. Some of these blocks of ice are quite large and able to temporarily withstand the erosive forces of the raging waters. When the temperatures are right, some of the falling water freezes instantly on the block ice and form great ice falls in front of the actual waterfalls. Such formations grow further forming castle turrets and other outcrop patterns as the mist settles on them. On occasion, a ice bridge forms across the Niagara River within the Great Gorge below the Falls as a result of the laying down of these ice blocks.
On six reported occasions (1883, 1896, 1904, 1909, 1936 and 1947), the water flow over American Falls has been totally blocked by ice and ceased to fall. But only once has this happened to the much larger Horseshoe Falls (originally known as the Canadian Falls)