There have only been 3 times that we know of where the Falls have either completely ran dry or were reduced to a trickle.
Accounts of the time the Falls ran dry are incomplete at best. According to newspapers of the time, sometime on March 29th or early on March 30th 1848 the water over the falls stopped for almost 2 days.
For several days the wind had been blowing across Lake Erie at gale force causing ice to jam up at the mouth of the Niagara River and effectively blocking the flow of water. Residents woke to an eerie silence on the morning of March 30th. Mills that had depended on the water flow had to shut down.
Over 5000 people came to see the dry falls. The dry river bed gave up its treasures that had been there for years, things like muskets, bayonets, tomahawks and other things that had lay under the water since the battle of 1812.
People made a show of walking across the river. If they had tried to cross while water was flowing it would have been a certain death. Soldiers form the U.S. Cavalry rode their horses down the river bed putting on an exhibition. It was a historical event, nothing like it had ever been known and had never happened again.
The owners of the Maid of the Mist boat were able to blast away boulders that had endangered the boat. But there was a downside. Many of the people believed the silencing of the Falls was a doomsday event, churches on both sides were filled and special services were held. As the Falls remained silent, fear escalated among the people. On the evening of March 31st, the wind shifted, residents heard a loud groan from upstream, followed by the normal flow of water over the Falls. The return of the water eased the fear of the residents.
The wind had a lot to do with the Falls running dry. A strong wind can raise the water level in one end of Lake Erie by up to 6 feet. It would be the same thing that happens with storm surges during a hurricane but on a smaller scale. Normally the river can only hold about 2% of the ice flow from Lake Erie, with the gale force winds they had at that time, it didn't take long for the ice to cause a jam.
The second time the Falls were reduced to a trickle was in 1953 when remedial work was done on the Falls. They had built coffer dams over a portion of Horseshoe Falls to do the work that would allow a more even flow of water and lessen the erosion over the Falls.
The U.S. Army Corp of Engineers built a series of coffer dams over American Falls in 1969 to study the rock formations at the crest of the falls and see how difficult it would be to remove some of the rocks at the base. They finally decided to let nature take its course and restored the flow of water over the falls.