The rockfall occurred on 28 July 1954 on the American Falls side of the site.
Cracks were first noticed by a nine-year old child the day before it collapsed.

The rockfall occurred on 28 July, 1954 on the American Falls side of the site.
It appears that cracks at the site were first noticed by a nine-year old child the day before the failure.
The 1954 Prospect Point Rockfall at Niagara Falls is shown in this amazing video below:
The rockfall left a large debris pile, which then became a tourist attraction in its own right – and was then removed by the U.S. army in 1969.
In the aftermath of the collapse, the Niagara Frontier State Parks Commission undertook blasting to remove rock left in a precarious position. Thereafter the site was deemed to be safe.
According to scientists, the rockfall had a mass of about 168,000 tonnes (185,000 tons). More history news on Strange Sounds and Steve Quayle. [Youtube]