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Pearls Unstrung

For a while, the Great Lakes weren’t connected by rivers and Niagara Falls was just a trickle

1:08pm, August 14, 2009

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.

Niagara Falls is one of the last links in an impressive chain: Water flows from Lake Superior and Lake Michigan to Lake Huron, onward to Lake Erie, then down the Niagara River and over the falls to Lake Ontario and thence to the sea. Today the falls seem unstoppable, but scientists have learned that there was a time after the most recent ice age when Niagara Falls was a mere trickle and the Great Lakes were a little le

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