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.
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Upcoming ueue, submitted by
ninigoat.
My guess then is some sort of ice-jam/log-jam further up river that severely restricted the flow of water at the time, thereby making the freeze-up more likely in relatively normal albeit cold temperatures.
Anyway, whenever I see those photos, I always theorized that ice chunks formed and perhaps blocked up areas, restricting water flow, which would have encouraged more ice to form, and perhaps allowed for a total freeze up in certain conditions.
I didn't think about it till you mentioned water temperature, which the two hydroelectric power plants could affect how cold the river water manages to get.
You're quite right. The water does have to go somewhere. But in this case the somewhere would be the Canadian falls. The falls pictured is the American side. The water flow to the American falls squeezes through a comparatively tight section with Goat Island. It would be very easy to have that section blocked by ice and other debris and the niagara river otherwise flowing fast and dumping over the Canadian side.
http://urbanlegends.about.com/library/bl_niagara_falls_frozen2.htm