The Day Niagra Falls Went Dry

 In 1965 a local newspaper editorial was published discussing the destruction of
the American Falls (one of three waterfalls that together make up the Niagara Falls).
It was believed that the destruction was being caused by previous rock debris that
had gone over the falls and that the continued erosion would eventually lead to the
American Falls’ destruction.

To address this concern, the International Joint Commission created a board called
 the American Falls International Board in 1967. After two years of deliberation on
what to do, the Board appointed the task to the Albert Elia Construction Company of
Niagara Falls, New York. Their job was to build a 183-meter cofferdam that stretched
 from mainland to Goat Island. This would block water to the falls so workers could
 clear the debris from the top of the falls.

 Once complete, they would then remove the cofferdam and restore waterflow to the
American Falls.

The cofferdam took thirty workers three days of working two 11-hour shifts in
mid-June of 1969. The cofferdam consisted of 1,264 truckloads of boulders and fill
 and weighed 27,800 tons. A fence was also erected. With the falls now dry, workers
 set to remove debris while scientists performed geological studies to determine
ways to slow and prevent future erosion.

Source: http://www.niagarafrontier.com/dewater.html