I can't imagine how cold it would have to be to freeze such a large, fast flowing river and falls. These pictures were taken in 1911 before there was a dam in place, so the water would have been much higher and much faster.
Link
From the Upcoming ueue, submitted by ninigoat.
Link
From the Upcoming ueue, submitted by ninigoat.
http://urbanlegends.about.com/library/bl_niagara_falls_frozen2.htm