Niagara falls are about 12,000 years old. It once was seven miles down stream from where it is today and as the layer of rock continues to change from the continuing pressure of water. The falls will change again in time moving up stream once more. Here are some more info:
American Falls
Height 180 ft (56 m)
Width 1060 ft (320 m)
Horseshoe Falls
Height 170 ft (54m) Width 2200 ft (675m)