雅思阅读材料：Runaway Devils Lake

　　Ancient Lake Minnewaukan

　　Devils Lake owes its existence to a continental glacier that covered much of North America during the Pleistocene Epoch. Carving a basin as it advanced over the landscape, the glacier deposited excavated materials along its leading edges, leaving terminal moraines marking the farthest extent of glacial ice sheets. Near the end of the Pleistocene, roughly 11,000 years ago, the glacier began its retreat. As the glacier withdrew, glacial meltwaters poured into the basin, creating a vast proglacial lake dammed by morainal deposits. Native Americans called this lake Minnewaukan, meaning, among other possible interpretations, Bad Spirit Water. Recent flooding has perhaps given credence to a legend told by those Native Americans, claiming that the lake once overflowed and flooded the entire world.

　　Based on abandoned beaches, or strand lines, geologists estimate that the ancestral lake reached a maximum surface elevation of between 444 and 445 meters. At that elevation, the lake covered about 1,050 square kilometers, held about 5 million acre-feet of water and had a maximum depth of around 50 meters. A natural outlet called Tolna Coulee, which allowed water to flow out of the basin and prevented the lake from rising and expanding further, controlled the maximum elevation. How often the lake has overflowed is uncertain, but geologists believe it has happened at least twice over the past 4,000 years, most recently around 2,000 years ago.

　　During the centuries that followed the lake’s origin, climate shifts caused water levels to fluctuate between 6 and 12 meters every few hundred years. Sediment analyses by geologist Edward Callender, published in his 1968 University of North Dakota doctoral thesis, indicated that the lake might have been completely dry 6,500 years ago. After the lake last rose to its maximum elevation and began overflowing, water levels continued to fluctuate in response to alternating dry and wet periods. A persistently dry climate 500 to 600 years ago held levels at relatively low elevations for perhaps as long as 200 years. Wetter conditions followed, raising the lake to levels that prevailed until the late 1800s. Levels then began dropping precipitously, falling to the lowest-recorded elevation by 1940 before rising again.

　　Whether Lake Minnewaukan was completely dry at times or not, periodic drawdowns during dry conditions reduced its immense volume to numerous remnant lakes scattered across the south-central region of the basin. Nonindigenous people who settled the region beginning in the mid-1800s named the largest and most prominent of these remnants "Devils Lake," perhaps because of the lake’s highly saline, undrinkable water, or perhaps in tribute to Sioux warriors whose canoes were often capsized in the lake’s treacherous, storm-tossed waters.

　　In 1964, Devils Lake consisted of three principal basins called West Bay, Main Bay and East Bay. West Bay then was essentially dry and Main Bay covered about 53 square kilometers. The Rock Island State Military Reservation separated East Bay—which covered about 27 square kilometers—from Main Bay. According to T. E. B. Pope of the U.S. Bureau of Fisheries, Main Bay and East Bay had become isolated during the 1890s after lake levels dropped about 6 meters during the previous 25 to 30 years. Besides Devils Lake, other major lakes nearby included Pelican Lake to the west and, to the east, East Devils Lake, Swan Lake, West Stump Lake and East Stump Lake, in that order.