Niagara Falls is the collective name for three waterfalls that straddle the international border between Canada and the United States. From largest to smallest, the three waterfalls are the Horseshoe Falls, the American Falls and the Bridal Veil Falls. The Horseshoe Falls lie mostly on the Canadian side and the American Falls entirely on the American side, separated by Goat Island. The smaller Bridal Veil Falls are also on the American side, separated from the other waterfalls by Luna Island.
To dewater the Niagara’s American Falls the army had to build a 600ft (182 m) dam across the Niagara River, which meant that 60,000 gallons of water that flowed every second was diverted over the larger Horseshoe Falls which flow entirely on the Canadian side of the border. The dam itself consisted of 27,800 tons of rock, and on June 12, 1969, after flowing continuously for over 12,000 years, the American Falls stopped. The completed dewatering of the American Falls was made easier because only 10% of the water follows that route.
While the Horseshoe Falls absorbed the extra flow, the U.S. Army Corps of Engineers studied the riverbed and mechanically bolted and strengthened any faults they found; faults that would, if left untreated, have hastened the retreat of the American Falls. A plan to remove the huge mound of talus deposited in 1954 was abandoned because of the high cost. For a portion of that period, while workers cleaned the former river-bottom of unwanted mosses and drilled test-cores in search of instabilities, a temporary walkway was installed a mere twenty feet from the edge of the dry falls, and tourists were able to explore this otherwise inaccessible and hostile landscape.
Finally, in November 1969 in front of 2,650 onlookers, the temporary dam was dynamited, restoring flow to the American Falls. Even after these undertakings, Luna Island, the small piece of land between the main waterfall and the Bridal Veil, remained off-limits to the public for years owing to fears that it was unstable and could collapse into the gorge.
Today, erosion of the American Falls is estimated at 3 – 4 inches every 10 years (used to be on average 4 feet a year). The water flow which is regulated at a minimum level of 10% of the estimated 100,000 cubic feet per second during the summer (50,000 cubic feet per second during winter) is insufficient to cause major erosion.