Almost every year, the Great Lakes freeze over. They do not freeze solid, however: and underneath the ice hundreds of thousands of gallons of water per second continue to flow from Lake Superior and Lake Michigan to Lake Huron, down through the St. Clair river system into Lake Erie, and finally, in twinned 50 metre drops, into Lake Ontario. In the winter, long spectacular sheets of icicles and frozen spray form over and around this drop, lit by dazzling light displays; but underneath it all, the water keeps right on flowing, continuing to tear away at centuries of dolostone and shale.
For Niagara Falls to freeze completely, this flow must be utterly interrupted. The natural flow of the Great Lakes would have to freeze, within a very short space of time and at its tightest point of kinetic energy, into a massive ice dam capable of holding back nearly 20% of the world's fresh water, thousands of times the water pressure held back by Boulder Dam.
Written documents and one photograph suggest this may have come close to happening in 1848.
Toward the end of March 1848, east winds associated with a major storm system piled together ice at the east end of Lake Erie, until suddenly water pressure jammed the blocks of ice together across the top of the Niagara River, almost completely blocking the flow of water over the falls for nearly two days and shutting down the mills relying on the continuing flow of water. Multiple eyewitness reports mention that the usual thunderous sound of the falls was completely absent, replaced by an eerie silence. Parts of the lower riverbed lay exposed: many onlookers retrieved artifacts from the War of 1812, while the operators of the Maid of the Mist boats took the opportunity to blast away some rocks which had been a hazard to navigation. The temperature in some documents is recorded as being quite warm for the season, up to ten degrees above freezing, which, together with the wind, would have contributed to the suddenly high volume of lake ice jammed in the Niagara River. This is probably the event pictured in the famous sepia-toned photograph.
Far more common is the formation of an ice bridge across the Niagara River, created by large floating blocks of ice which jam together to build a surface-solid bridge of ice. Parts of this bridge can be up to 16 metres thick, although the average is much closer to one metre. (By comparison, reliable ice highways over the frozen lakes of the far north are opened to heavy truck traffic when the ice is only half a metre thick.) Until 1912 tourists would often walk out onto the natural wonder of this ice bridge, and even small confectionary stands would be built right on top of it. In the winter of 1911-12, while the Keokuk and Hamilton Water Power Company were building their dam, such an ice bridge formed to an average depth of over a metre, which created serious subsequent flood issues for the dam builders, both during the initial breaking of the bridge and later during the height of the flood season.
Because of a tilt in the river bed, the flow of water over the American Falls part of Niagara Falls is only about a tenth of the total flow over Niagara Falls. Thus, while neither the river nor the falls themselves freeze solid, it is relatively common in winter for the American Falls to ice over.
Although winters in the lower Great Lakes region have become generally warmer, the full flow of the Great Lakes also no longer passes solely through the falls. In fact, between 1/2 and 3/4 of the total flow is now diverted to various hydroelectric projects. However, ice jams across the upper Niagara River have become far less common since the New York State Power Authority has begun stringing an ice boom between the cities of Buffalo and Fort Erie during the winter months. This ice boom serves to hold back 98% of Lake Erie ice from the Niagara system so that it won't clog the narrower waterways closer to the falls and the constantly spinning hydroelectric turbines: making it virtually impossible that Niagara Falls will ever come close to freezing over again.