This is a rare image which was purportedly taken in 1911 offer postcard views of Niagara Falls completely frozen over. It circulated by email from aprox. 2003.
The flow of water was stopped completely over both 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!
Niagara had frozen again in 1936.