According to experts, Niagara Falls has eroded 7.1 miles over the last 12,300+ years. Until 1942, the mean rate of erosion at the American Falls was 5 feet and the Horseshoe Falls at more than a foot per year. Since then erosion has slowly reduced and now is at an all-time low.
This stark reduction can be attributed to anti-erosion and water diversion efforts by both the U.S. and Canadian governments. Essentially, both the American and Canadian sides of the falls divert water away from Niagara Falls to their respective hydroelectric power plants which reduce regular flow by 50-75%. Although this takes year-round, more water is diverted away at night and throughout non-peak tourist seasons.
Once the sun goes down between May and October the water flowing over Niagara Falls is reduced from 100,000 cubic feet per second to 50,000. Between November and April an additional 50,000 cubic feet per second is diverted by both the New York State Power Authority (U.S. side) and Hydro One (Canadian). This means that 3/4 of the possible water that could go over Niagara Falls is diverted away from it, leaving only 1/4 to go over the brink each day.
In total, erosion of the American Falls is estimated at 2-4 inches every 10 years and the Horseshoe Falls at less than a foot per year. Although the Horseshoe Falls is eroding at a quicker rate than the American Falls, geologists believe increased water diversion efforts are reducing erosion to only one foot every ten years.