The thundering roar at the base of Niagara Falls is awesome indeed. On an average summer day, about 40 million gallons of water spill over the half-mile–wide Canadian portion of the cataract each minute. After falling over a cliff taller than a 16-story building, water pummels the rocks below, incessantly eroding the base of the cliff and triggering rockfalls. Before the 20th century, when engineers weakened the Niagara River by diverting some of its flow to produce hydroelectric power, the falls marched upstream an average of more than a meter per year.
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