Magazine article Science News

Pearls Unstrung: For a While, the Great Lakes Weren't Connected by Rivers and Niagara Falls Was Just a Trickle

Magazine article Science News

Pearls Unstrung: For a While, the Great Lakes Weren't Connected by Rivers and Niagara Falls Was Just a Trickle

Article excerpt


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 fails 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 less great.

During the ice age, which began about 100,000 years ago, a kilometers-thick ice sheet smothered the region. And Niagara Falls--or the ice-covered cliff that would become the falls- was located several kilometers downstream of its current site. Sometime around 13,000 years ago, the ice retreated northward, leaving meltwater to accumulate in gouges that were left behind.

With the first flush of meltwater, lake levels rose and the falls raged. Studies show that as ice retreated and climate dried, however, the falls slowed to a trickle for several millennia, starting about 10,000 years ago. Scientists once thought that the falls slowed because the overflow from Lake Erie was rerouted to a different spillway when the landscape tilted and shifted as it was relieved of its icy burden. But now they are learning that some of the rivers connecting one lake to another simply disappeared during a long dry spell that started about 12,500 years ago.

In the last decade or so, scientists have uncovered clues that the water level in Lake Erie--and indeed, the levels in at least some of the other Great Lakes--fell well below all natural outlets, rendering those lakes isolated bodies of water.

New studies, including archaeological surveys and genetic analyses of fish, bolster the notion that today's submarine ridges and nearshore shallows were once land bridges and lakeside beaches.

Ups and downs

Today, even small fluctuations in lake levels can have a big effect on the region. In a good year, vessels from the United States, Canada and other nations transport more than 200 million tons of iron ore, coal and other cargo on the lakes, says Glen Nekvasil, vice president of the Lake Carriers' Association, based in Rocky River, Ohio. For every inch (2.5 centimeters) that lake levels drop, he says, the 65 vessels represented by the trade group must forgo carrying about 8,200 tons of cargo.

Instruments have tracked Lake Erie's water level only since the mid-1800s, but in that time the level has, according to modern standards, fluctuated substantially. From 1900 to today, the lake's surface altitude has varied by about 1.5 meters, says Gregory C. Wiles, a paleo-climatologist at the College of Wooster in Ohio. Although many people have suspected that human activity--dredging, engineering projects and the like--caused those variations, a study reported by Wiles and his colleagues in the March 6 Geophysical Research Letters hints that natural climate cycles are largely to blame.

Today, average precipitation over Lake Erie is about 99 centimeters per year, the researchers note. But evaporation steals about 90 centimeters of that water annually; the surplus water joins the incoming flow from the upper Great Lakes and exits Lake Erie via the Niagara River, says Wiles. Water level in the lake depends on the balance between income and outgo: In spring, when snowmelt is prodigious and temperatures--and therefore evaporation--are relatively low, the lake's level is typically at its high point for the year. …

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