A few days ago, my family stopped at Niagara Falls after returning from a geology conference in Madison, Wisconsin (we passed through Canada coming back).
They are impressive falls (two really - the American falls in the foreground and the horseshoe-shaped Canadian falls in the background - separated by Goat Island).
There's a lot of neat stuff I could write about concerning the geology of Niagara Falls but I'll just concentrate on one for today - the time they turned off the American Falls.
They actually slow down the falls each night. Ontario Hydro and the New York Power Authority pump tremendous amounts of water each night from the Niagara River upstream from the falls to top off their reservoirs. They do this at night because this action reduces the flow over the falls from 100,000 cubic feet per second (CFS) to 50,000 CFS (half). During the day, the flow is sort of left alone so the millions of tourists have impressive waterfalls to admire. By "sort of", I mean that water is still normally diverted for hydroelectric generation 24/7. Without this diversion, the flow over the falls would be a more awe-inspiring 200,000+ CFS!
Flow over Niagara Falls has actually been stopped (at least partially) three times in recorded history.
The first event was March 29-31, 1848. People living near the falls noticed an eerie quiet on the morning of the 30th. When they went to look, they saw a dry riverbed with fish and turtles flopping around. The falls had maybe 30-40 CFS - a mere trickle. People were able to walk on the riverbed and collected artifacts from the War of 1812 (muskets, bayonets, tomahawks, and the like). Special church services were held on both sides of the border for anxious people who didn't understand why the river suddenly vanished.
The next event was man-made. In 1953, some coffer dams were built exposing only a section of the Horseshoe Falls nearest the Canadian side (where the big tourist center is today). This allowed engineers to stabilize this section of the edge of the falls which were, of course, eroding as falls are wont to do.
The last time the falls were turned off was also a man-made event. The Army Corp of Engineers built a 600 foot coffer dams to Goat Island which turned off the American Falls (the water was diverted over the Canadian falls) starting on June 12, 1969. All part of the Corp of Engineers never-ending struggle to control nature and stop erosion (an ultimately impossible task - see John McPhee's The Control of Nature).
Tourist attendance at the falls topped all-time records to see this - they even allowed people to walk out onto the dry river bed. The Corp of Engineers injected concrete into the bedrock fractures and inserted a bunch of rock bolts over a period of about 5 months. Once "repaired", the falls were restarted on November 27.
In my next post, I'll talk a bit about erosion at Niagara Falls.