This week, we were treated to some beautiful images of Niagara Falls mostly iced over, thanks to the Polar Vortex. This happens somewhat regularly, but it's gorgeous nonetheless.

What you see in this post is a much rare occurrence: the dewatering of one part of Niagara Falls, which occurred in the summer of 1969 and continued into the late fall.

To understand these images, it helps to know a couple things. There are three sets of waterfalls at Niagara Falls: American and Bridal Veil Falls on one side of Goat Island and the larger Horseshoe Falls on the other. The dewatering project only stopped water flowing to American and Bridal Veil Falls. And the vast majority of the water actually goes over Horseshoe.

Here's a map:

Still, it was a massive project. Engineers built a 600-foot cofferdam from the American shoreline out to Goat Island using 28,000 tons of earth and rock. They did it to investigate the geology of the falls, which had seen very substantial rockfalls, leading to a buildup of boulders and debris at the base of the falls. Ultimately, after a lot of public debate, the International Joint Commission, which manages the falls, decided not to change the falls, and the dam was removed on November 25, 1969.