Some years later, in 1911, thick ice at the falls again attracted visitors. An amazing photograph exists that shows people standing on the falls. You can find it at this Niagara website. If you scroll down to the section entitled "Do the Falls Freeze over in the Winter?" near the end of the article, you will see the photograph. It's not known how authentic the image is (some sources say it's been altered), but it certainly seems to demonstrate what the scene may have looked like in 1848.
Humans, rather than nature, intervened instead in 1969, when the flow over a part of the falls was shut off by engineers to study the rock pile at the bottom of the American Falls. You can see what that looked like in the same article.
Original sources: Infoplease.com, NiagaraFallslive.com