High and dry

 作者:恽猴     |      日期:2019-03-08 08:05:03
By Dan Falk in Toronto NIAGARA Falls almost ran dry when the level of the water in Lake Erie fell, say geologists in Colorado. Troy Holcombe and his colleagues at the University of Colorado in Boulder used detailed sonar maps to examine the bed of Lake Erie. They found evidence of a previous shoreline, submerged between 10 and 15 metres below the current lake level, which dates back about 5000 years. That is below the level of the outflow channel, where the waters of Lake Erie now flow into the Niagara River and ultimately tip over the Niagara escarpment. Scientists think that water first flowed over Niagara about 12 000 years ago, as melting glacier ice found new routes across the landscape. Today, more than 500 million cubic metres of water pours over the falls every day. Holcombe agrees that there definitely were falls between 12 000 and 10 000 years ago, though they were less ample than they are now. “You had about one-tenth of the water entering the Lake Erie basin as you have today,” Holcombe says. But at one point between 10 000 and 5000 years ago, the lake must have dropped below the level needed to sustain the falls, Holcombe told a meeting of the International Association for Great Lakes Research in Cleveland, Ohio, last week. The the falls returned with a vengeance, he says. As the glaciers retreated from Ontario, the land began to rise, making it difficult for water to escape to the north. Instead, it started flowing southwards into Lake Erie. The water level in the lake probably didn’t take more then about five years to regain its present level—which is no time at all in geological terms—so features from the previous, lower shoreline, such as sand bars and river deltas,