During an extended winter cold snap a hardened crust of ice can accumulate over parts of the falls. The phenomenon has been known as the «ice bridge.
Niagara Falls gets cold every year. The average temperature in Niagara Falls in January is between 1 and 8 degrees. Naturally, it being that cold, ice floes and giant icicles form on the falls, and in the Niagara River above and below the falls, every year. The ice at the base of the falls, called the ice bridge, sometimes gets so thick that people used to build concession stands and walk to Canada on it.
It happened in 1848, 1886 and 1911.