Engineers once shut down Niagara Falls' water flow for three days

For several months in 1969, the torrent of water rushing over American Falls, one of three waterfalls that makes up Niagara Falls, was reduced to little more than a trickle.

American Falls is recognizable for the immense rock pile, or talus, at its base, the result of a series of natural rockslides over the years. In the late 1960s, concerns were growing that further rockslides could erode the falls completely.

To study the geological composition of the falls and forestall their potential destruction, a joint American-Canadian commission decided to dewater them for five months.

Over three days in June 1969, more than 1,200 trucks dumped nearly 28,000 tons of rocky fill into a cofferdam upstream of the falls, diverting the flow of the Niagara River away from American Falls and toward the much larger Horseshoe Falls.

With the falls dry for the first time in millennia, the US Army Corps of Engineers began their investigation.

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1969: When Niagara Falls temporarily ran dry
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1969: When Niagara Falls temporarily ran dry
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
General view of the de-watered Niagara Falls in 1969 in New York. Niagara Falls is to be 'turned off' for the first time in over 45 years so that two new bridges can be built. These amazing pictures show the last time the colossal waterfall was switched off in 1969 for an erosion study. The new walkways will replace two 155-year-old pedestrian bridges. Niagara Falls straddles the border between New York, USA and Ontario, Canada however only the American side will be turned off as part of the proposed plan. (Photo credit Russ Glasson / Barcroft USA / Barcroft Media via Getty Images)
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Two corpses were discovered — for some observers, a surprisingly low number given the falls' history of accidents and suicides.

As engineers bored into the riverbed to map and probe the stresses, faults and pressures of the rock, excited tourists scampered across the dry expanse, collecting coins thrown into the water decades ago.

Instruments were planted to monitor rock movements in several locations, steel bolts and cables were installed to stabilize the rocks around Luna Island and Bridal Veil Falls and drainage holes were drilled to relieve hydrostatic pressure at several points.

As for the accumulated talus at the base of the falls, the popular opinion was to leave it where nature left it. Though the engineers determined it would be feasible to remove it, they agreed that it would be a waste of effort for a purely aesthetic goal.

In November 1969, the cofferdam was slowly removed, and American Falls roared once more.

They could be dry again soon, however — two aging bridges above the falls need to be replaced, an operation which would likely require dewatering.

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