In 1969, the US Army Corps of Engineers dumped 27,000 tons of rock to dam the Niagara River and stop the American Falls.
They were assessing a growing pile of boulders at the bottom out of concern the falls could become rapids.
But in the end, engineers found the boulders were necessary to prop up the face of the falls.
"They tamed it and now they will unleash it."
According to The New York Times in 1969, that was how a tourist characterized the historic moment that the Niagara River was released to gush down over the American Falls once again after it had been cut off for six months.
Engineers had blocked it off so they could examine the boulders at the base of the falls and see whether they could be removed.
While it was blocked off, they discovered two bodies as well as countless coins. They also found it wasn't feasible to move all of the rock.
Here's why, after 12,000 years, the famous falls were "turned off."
About 12,000 years ago, Niagara Falls formed when a new channel — now known as the Niagara River — linked up between Lake Erie and Lake Ontario. The falls are at a crook where the Niagara River turns to the north by about 90 degrees.
Niagara Falls are divided up between Canada's Horseshoe Falls, the American Falls, and the Bridal Veil Falls. The most impressive of the falls is undoubtedly Horseshoe Falls.
An aerial view of the Horseshoe Falls, one of three parts of Niagara Falls.
In 1842, author Charles Dickens described his visit there. He wrote, "When I felt how near to my Creator I was standing, the first effect, and the enduring one — instant lasting — of the tremendous spectacle, was Peace."
Tourists observe the American Falls in the summer of 1753.
For centuries, people visited the falls and some even jumped in. Some strange incidents occurred as well, including a hotel owner who sent a ship of animals down the falls, killing everything except a goose.
The American Falls and the Bridal Veil Falls are at the left with Horseshoe Falls at the right in 1890.
The US Army Corps of Engineers had dumped 27,000 tons of rock upstream to dam the Niagara River. The plan was to spend six months assessing the pile of boulders at the bottom of the falls and whether it was feasible to remove them.
The American Falls were cut off to just a trickle on June 13, 1969.
The pile of boulders had been building up at the base of the falls for years. At times, it happened rapidly, like in 1931 when about 76,000 tons of rock fell to the base or in 1954 when 185,000 tons fell to the base.
On the first weekend of June, more than 100,000 people showed up to see the waterless falls. It was double the number of people who had gone on that weekend the previous year.
An aerial view of the trickle of water from the American Falls as water from the Niagara River was diverted to the Canadian Falls.
Bob Olsen/Toronto Star via Getty Images
Tourists asked two questions: "Why did they turn it off?" and "How long will it be off?"
But after early hopes that tourism wouldn't be impacted, it soon became clear that when people came to look at the bare cliff, they only stayed for a short stint.
For the next five months, engineers examined the bed of boulders, before later concluding in 1974 that the boulders were actually necessary to prop up the falls.
Workers are seen atop the dried up American Falls, which had been dried up to just a trickle in 1969.
AP
The American Falls International Board, which wrote the report, stated it was against their removal.
"It seems quite wrong to make the falls static and unnatural, like an artificial waterfall in a garden or a park, however grand the scale," according to the report.
But it hadn't been a waste of time.
While the American Falls was stopped, engineers bolted and anchored the rocks to make the area as stable as possible.
On November 25, 1969, at 9:55 a.m., a crowd of several hundred people gathered to watch a crane dig a hole in the dam, and, according to The New York Times, water began to flow "as though from a leaky faucet."
The American Falls on December 10, 1969, after it was back to normal.
After an hour, "the rocks became a churning rapids and water cascaded over the falls spewing forth a fine white mist," according to The New York Times. The American Falls were back.
Niagara Falls in November 1969 after the flow of water was released.
AP
Not that everyone was impressed. One bystander told The Times: "Big deal, so it starts flowing again."
Another person said, "Isn't that something? They tamed it and now they unleash it."
Another man was even less patient about the work. He said the project was "damn-fool shenanigans — if God wants the falls to become a rapids that's the way it should be."
And that man may still end up being right. The American Falls continues to change and erode, though less than it used to. It now loses about 4 inches every decade in comparison to what used to be about 13 feet per year.
The American Falls as seen in 2014.
Roberto Machado Noa/LightRocket/Getty Images
In around 15,000 years, the falls are expected to simply disappear.
It's also likely that the falls could be turned off again at some point. There are plans to repair several bridges linking New York to an island called Goat Island. But there's been no confirmed date for when that'll happen.
Spectators observe the American Falls and steel arch bridge from Goat Island, Niagara Falls, New York in 1895.
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