Once upon a lake: the life, times, and demise of the world's largest lake.

During the last ice age, sea levels fell more than 100 meters. The ocean water didn't disappear, of course. It just ended up someplace else. A significant decrease in global temperatures permitted snow in some places to accumulate faster than it melted. Glaciers formed at high altitudes and scoured mountain valleys. Existing ice masses, like those in Greenland and Antarctica, thickened. In the higher latitudes of Asia, North America, and Europe, ice sheets hundreds of meters thick grew to smother vast regions.

When Earth's average temperature began to rise and the ice started to melt, the water long sequestered on continents once again sought the sea. Along many edges of the ice sheets, meltwater could flow directly into the ocean. In the interiors of continents, however, the ice sheet itself blocked the water's downhill progress. Immense lakes formed, deepened, and spread across the landscape until they reached a spillway or broke through a weak spot along the ice sheet's edge to scour a new route to the ocean.

During the waning days of the ice age, North America's Lake Agassiz was the largest body of fresh water in the world. At times it held more water than that in all the world's lakes today. Lake Agassiz strongly influenced North American climate, but scientists have also linked several major surges of fresh water from the lake to sudden global climate changes. The last and largest of these discharges was associated with a 400-year-long cold spell and raised sea levels about a half-meter. It also may have been the source of the flood stories included in the Bible and the Babylonian Epic of Gilgamesh.

A LAKE IS BORN Between 18,000 and 20,000 years ago, at the height of the last ice age, the Laurentide Ice Sheet covered Canada east of the Rockies. At its highest point, over what is now Hudson Bay, the ice was nearly 5 kilometers thick and weighed so much that it depressed the underlying terrain more than 1km. Tonguelike lobes of ice spilled into the Midwest as far south as Iowa and central Illinois, and their meltwater fed tributaries of the Mississippi River and ultimately flowed into the Gulf of Mexico.

Then, about 13,000 years ago, the southern edge of the ice sheet retreated past the divide that separates the Mississippi watershed from areas that drain northward. After that time, meltwater pooled against the ice sheet on the northern slope of the divide. Thus, Lake Agassiz was born. The lake was named after Louis Agassiz, the 19th-century Swiss geologist who first proposed that massive ice sheets once covered large areas of continents.

The size, shape, and depth of the lake changed significantly--and sometimes rapidly over its lifetime, says David W. Leverington, a geologist at the Smithsonian Institution's Center for Earth and Planetary Studies in Washington, D.C. Scattered remnants of Lake Agassiz' ancient beaches stretch across the Canadian landscape like partially scrubbed bathtub rings.

The northern edge of Lake Agassiz followed the fluctuating position of the ice sheet. The locations and altitudes of spillways largely determined the depth of the lake, Leverington explains. The water would rise until it reached an outlet, at which point the lake would overflow. By carbon-dating sediments deposited in the lake's spillways or in marshes left high and dry by sudden drops in water, scientists can now chronicle the lake's changing profile.

In its earliest stages about 13,000 years ago, Lake Agassiz' overflow spilled down the Mississippi River. Some of the oldest beaches stretched along the lake's southern and western shores, from an outlet in North Dakota northward into Manitoba. Leverington and his colleagues at the University of Manitoba in Winnipeg estimate that the lake then covered more than 134,000 square kilometers. At that time, the researchers say, what is now Winnipeg was submerged beneath about 200 m of icy water.

Then, a new spillway opened along the eastern edge of the lake. A sudden 100-m drop in lake level sent 9,500 cubic kilometers of fresh water--about 85 percent of Lake Agassiz' volume at the time--flowing through the Great Lakes and the St. Lawrence River into the North Atlantic. This abrupt change in the routing of melt-water coincided with the onset of a 1,600-year worldwide cold spell known as the Younger Dryas, says Peter U. Clark, a geophysicist at Oregon State University in Corvallis.

The period's cooler temperatures are recorded in ice cores in Greenland, lake sediments in Japan, deposits drilled from the ocean floor off the northern coast of South America, and elsewhere. Many of these climate indicators suggest that the Atlantic's influx of fresh water from Lake Agassiz interrupted the ocean's transport of heat from the equator to high latitudes. Many researchers suggest that this heat transport, called the thermohaline circulation, drives Earth's climate.

BACK AND FORTH In the Atlantic Ocean today, the Gulf Stream conveys warm, salty water north from the equator along the ocean's surface. Off Greenland, the water cools, becomes denser than the ocean layers beneath, and sinks to the bottom to form what scientists call North Atlantic Deep Water.

The sudden rerouting of Lake Agassiz' meltwater into the North Atlantic 13,000 years ago provided a one-two punch to this briny conveyor belt. First, says Clark, the fresh water that arrived in the initial flood was much lighter than the ocean's salt water and therefore couldn't cool enough to sink. That, in turn, stifled the flow of warm water to the North Atlantic.

Second, drainage from Lake Agassiz supplemented the meltwater already reaching the North Atlantic from the eastern lobes of the Laurentide Ice Sheet. The flow of fresh water from the St. Lawrence River rose for the next several decades to approximately double what it had been before the break.

From climate records, scientists can't distinguish which of these two components--the initial flood or the long-term doubling of St. Lawrence River discharge--had a bigger effect on Earth's climate, says Clark. He and his colleagues are now constructing computer models that may answer the riddle.

Regardless of these factors' relative contributions, the net result was significant global cooling. That big chill, in turn, caused the ice sheet to grow, extend southward again, and shut off the fresh water to the North Atlantic, rerouting overflow once more into the Mississippi. The thermohaline circulation then returned to again warm the globe.

This switch in climate unfolded in a geological eye blink. It took less than a century for the Younger Dryas cold spell to take hold, and only about a decade for Earth's average global temperatures to warm again.

Soon after the Younger Dryas ended, another dramatic shift in Lake Agassiz' drainage cooled world climate for an extended period known as the Preboreal Oscillation. About 11,350 years ago, water levels in Lake Agassiz dropped about 50 m in no more than 3 years, says Timothy G. Fisher, a geologist at Indiana University--Northwest in Gary. This time, the meltwater spilled from a northwestern outlet into the Mackenzie River in northwestern Canada and made its way to the Arctic Ocean. From there it flowed to the North Atlantic to once again interrupt thermohaline circulation.

Here's the chain of events: The influx of fresh water into the Arctic Ocean boosted the amount of ice that formed there, says Fisher. The pack ice thickened and covered a larger area, which cooled the region by reflecting more sunlight back into space. Large chunks broke off that extra ice at the top of the world and floated into the North Atlantic. When that ice melted, the relatively fresh water atop the saline ocean again interfered with the formation of North Atlantic Deep Water and led to a worldwide chilling. Fisher and his colleagues reported their analysis in the April Quaternary Science Reviews.

The Preboreal Oscillation cool spell lasted no more than 250 years, says Fisher. Carbon dating of sediments recently dug from the bottoms of Big Stone Lake and Traverse Lake in Minnesota suggest that Lake Agassiz' northwestern outlet was active for no more than 400 years. Fisher reports this finding in an upcoming issue of Quaternary Research.

LAST HURRAH The final major shift in Lake Agassiz drainage occurred when the center of the Laurentide Ice Sheet collapsed over Hudson Bay about 8,400 years ago. At the time, Lake Agassiz had merged with another glacial lake to stretch 2,000 km with an area of more than 841,000 [km.sup.2].

Before the collapse, the lake drained southeastward through an outlet that led to the St. Lawrence River. At spots along the northern edge of the ice sheet, which defined the lake's northern edge, the water was more than 500 m deep. When the sheet's mass of ice suddenly gave way, at least 163,000 [km.sup.3] of water-about 30 percent more water than is contained in all of the world's lakes today--spilled northward through Hudson Bay into the North Atlantic.

That discharge of water coincided with the onset of a 400-year dip in global temperatures, says James T. Teller, a geologist at the University of Manitoba. In addition to its effect on Earth's climate, the meltwater surge would have raised global sea levels by about 0.5 m over a year or so. Although that may not sound like much, Teller notes that this jump would have been superimposed on sea levels that were already gradually rising in response to the melting of ice sheets elsewhere.

Rising sea levels are particularly problematic where seashores are nearly flat. For example, many areas just offshore of today's continents--the so-called continental shelves--have a slope of about 1 in 2,000. In these places, says Teller, for every 0.5-m rise in sea level, the shoreline would move 1 km inland. On the almost-level floor of the Persian Gulf--which is no lower than 100 meters below current sea level and has a slope of about 1 in 20,000--the shoreline would move about 10 km.

During Lake Agassiz' demise, a young civilization may have inhabited ground that is now the floor of the Persian Gulf, Teller noted at a recent conference in London on environmental catastrophes since the last ice age. The Tigris and Euphrates Rivers, which now meet the Persian Gulf at the Iraqi shoreline, then flowed another 1,000 km across the flat, dry basin before they reached the sea. Even modest changes in sea level from the final drainage of Lake Agassiz half a world away would have forced people who lived beside those rivers to flee their flooded lands.

Teller suggests that such large-scale inundations from a half-meter rise in sea level over about a year could have been the source of the stories of massive floods recorded in Babylonian history and the Bible.

THREE EXITS--At various times, Lake Agassiz drained northwest through the Mackenzie River, south through the Mississippi River, and eastward through the St. Lawrence River. Arrows show drainage routes for those outlets.

ANCIENT BEACHFRONT--South of Wahpeton, N.D., slightly elevated terrain--marked by stripes of tan, drought-stressed vegetation--are remnants of dunes that lined Lake Agassiz' beaches.

GONE SOUTH--Sediments drilled from sites in Minnesota's Traverse Lake (yellow arrow) and Big Stone Lake (white arrow) suggest that Lake Agassiz' southern spillway was active at least twice.