In 1969 the US Army Corps of Engineers shut down the American section of Niagara Falls for maintenance. Parts of the rock had been breaking off, putting Niagara's grandeur in jeopardy.
A 600 foot (180 m) temporary rock dam was erected to divert all water to the Canadian side of the falls, leaving the ordinarily enormous and awe-inspiring American Falls dry. For about six months tourists flocked to the falls to seize the once-in-a-lifetime opportunity to stroll along the sheer stone drop off.
In November 1969 the Corps of Engineers finished their erosion-prevention work and blew up the temporary dam, reinstating the misty, roaring American Falls majesty.
This animated GIF shows the 1969 dry falls in contrast to the typical flow. For reference, the distance from the ledge to the top of the rock pile ranges between 70 to 110 feet (21–34 m).