rotten spray ice
The main falls are not froze. This the stuff splashed on the side then frozen outside the main flow.
Neither the river nor the falls ever freezes solid, mind you. The water continues to flow beneath the ice at all times, albeit reduced to a mere trickle on rare occasions when ice jams block the river above the falls.
Historically, when this blanket of ice has spanned the entire Niagara River, the phenomenon has been known as the “ice bridge.” Just as you see in the photos, people used to stroll and frolic on and around the frozen falls and even walk across the ice bridge, though no one has been allowed to do the latter since 1912, when the bridge unexpectedly broke apart and carried three tourists to their deaths.
http://urbanlegends.about.com/library/bl_niagara_falls_frozen2.htm