The water in the Niagara River moves very fast, moving an average of 212,000 cubic feet of water per second. It's moving far too fast and pushing way too much water to ever freeze.
In late December an ice bridge forms at the base of the falls and over portions of the lower Niagara River. The Niagara River is fed by Lake Erie which is a huge lake, but not very deep. The ice goes over the Falls and freezes to the edges and keeps building on itself until the river is covered in ice anywhere from 40 to 80 feet thick. This ice rises on a layer of air, as the water keeps flowing beneath it.
Since records have been kept, American Falls has frozen a total of six times. The flow of water over this Fall is rather small, 10,000 cubic feet per second and in the winter that is reduced to around 8,000. This amount of water is barely able to cover the rock face and during harsh winters, allows the American Falls to freeze over.
In the 1960's an ice boom was placed at the mouth of the River and since then it has basically eliminated the possibility of the Falls freezing over.
The longest American Falls was ever frozen was 15 days, beginning on February 7th 1936. American Falls now receives about 10% of the flow of water to the Falls.
Due to the possibility of the ice bridge collapsing or moving, the public is not allowed to walk across the ice bridge. The mist formed as the water flows over the Falls freezes on contact and creates a winter wonderland of shining crystals along the shoreline.