The Niagara Falls public library has this information:

From June to November 1969 the American Falls were dewatered. This action cut back the normal flow of 60,000 gallons a second to almost nothing. Most of the diverted water was either sent over the Horseshoe Falls or diverted to the Robert Moses generating plant's upriver intakes. The action enabled Canadian and US power companies and the US Army Corps of Engineers to do on-the-spot inspections and aerial photographs of the river bed's rock formation. This was all part of a plan to reduce erosion to the Falls.

Must have been an amazing sight to see. I've posted some pictures of the falls as they normally run just for comparison.

Barcroft Media via Fame Pictures In 1969 the falls went quiet as the Niagara river was temporarily diverted. The dam can be seen in the upper right.

Barcroft Media via Fame Pictures Niagara falls without water as seen in 1969.

Kike Calvo / AP file The falls as they normally run, as seen in June, 2009. A natural phenomenon, Niagara Falls sees between four and six millions cubic feet of water go over its crest every minute. The Falls are 188 feet high, with the deepest section of the Niagara River just at their base-going down 170 feet.

Barcroft Media via Fame Pictures While the water was diverted, large amounts of fallen rock could be seen at the base of the falls.