雅思阅读素材来源,雅思阅读素材积累：RunawayDevilsLake

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　　Devils Lake is where I began my career as a limnologist in 1964, studying
the lake's neotenic salamanders and chironomids, or midge flies. Back then, the
lake covered about 80 square kilometers, had a maximum depth of about 3 meters
and held about 130,000 acre-feet of water. The lake has since risen 13 meters,
from a surface elevation of 430 meters above mean sea level to 443 meters.
Estimated lake volume is now 4.1 million acre-feet, or about 32 times greater
than it was in 1964, and about 370 times greater than it was in 1940 when the
lake stood at a record low elevation of 427 meters.

　　The Devils Lake Basin is an endorheic, or closed, basin covering about
9,800 square kilometers in northeastern North Dakota. The basin is at the
epicenter of an unprecedented wet period in the lake's modern-day history going
back to 1867, when the lake's surface elevation was first measured. Basin
climate has become substantially wetter since 1990, with the years 1990 through
2009 ranking as the wettest 20-year period in more than a century. The National
Weather Service has referred to this trend as "the new climate" for the Devils
Lake region, cautiously predicting that the current weather pattern may continue
for several decades and possibly intensify. Indeed, the agency has warned that
the region faces the strong possibility of an "unprecedented fourth consecutive
major spring flood threat in 2012."

　　Rising lake waters have flooded much of the region, engulfing hundreds of
homes and farmsteads, more than 650 square kilometers of productive farmland,
major highways and bridges, state parks, Native American tribal lands,
historical landmarks and more than half a million trees. Submerged too is the
North Dakota Biological Station, a two-story limnological facility established
in 1909 to study the lake's unusual ecology and biogeochemistry. Portions of
U.S. Highway 281 are now underwater, which has forced the relocation of this
principal north-south highway several kilometers to the west. Other roads and
highways are either extremely hazardous or simply impassable because of
encroaching floodwaters. Amtrak and the BNSF Railway may have to reroute their
trains over more southern lines as rising waters threaten to wash out roadbeds
and bridges. The small town of Minnewaukan, once located 13 kilometers west of
the lake, is now partly underwater, and many of its 300-plus residents have been
forced to abandon their homes. Only a handful of people remain in Churchs Ferry
and nearby Penn, communities established more than a century ago. The city of
Devils Lake, North Dakota's eleventh largest city with about 7,000 residents,
sits behind a U.S. Army Corps of Engineers levee that protects the community
from storm-generated waves that reportedly reach 2 meters or more in height.
Without the levee, 3 to 4 meters of water would now cover parts of the city. To
date, efforts by federal, state and local governments to control flooding and
protect communities exceed $1 billion, a cost that is rising as fatefully as
lake waters.

　　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.

　　Water Supply and Overflows

　　Devils Lake receives nearly all of its water from surface runoff and direct
precipitation. Most surface-water runoff originates from a chain of remnant
lakes located a few kilometers north of Devils Lake, although many of these
smaller lakes have now merged with Devils Lake as the water levels rise. (By
September 2007, for example, Devils Lake and all of the lakes to the
east—including the two Stump lakes—had completely merged.) Total annual inflows
ranged from near zero during the drought-stricken 1930s to nearly 400,000
acre-feet in 1993. Inflows, averaging 65,500 acre-feet annually between 1950 and
1993, rose to 317,000 acre-feet annually between 1993 and 2000, a fivefold
increase. The years 1993 to 1995 contributed 24 percent of all inflow to Devils
Lake between 1950 and 1995.

　　If Devils Lake rises approximately two additional meters and begins
overflowing, as scientists predict it will, lake waters will enter the Sheyenne
River. The Sheyenne, which originates 50 kilometers west of the river's juncture
with the Tolna Coulee outlet, meanders on an easterly course that lies about 15
kilometers south of the Devils Lake Basin. After turning south, the river is
impounded by a Corps of Engineers dam (Bald Hill Dam) located 20 kilometers
north of Valley City, a town of about 6,300 residents. The dam's narrow
reservoir (Lake Ashtabula) extends 43 kilometers upstream and contains about
71,000 acre-feet of water at full capacity. After passing through Valley City,
the river joins the Red River of the North near the city of Fargo. The Red River
flows northward before emptying into Canada's Lake Winnipeg.

　　Like climate predictions in general, predictions about when the current
lake will overflow are rife with uncertainty. For example, in a report published
in 2008, the U.S. Geological Survey predicted that the probability of the lake
exceeding 443 meters between years 2008 and 2015 was only 10 percent, but the
lake reached that elevation in 2011. Also predicted was a 50-percent probability
that the lake would not exceed an elevation of 442 meters between 2008 and 2040.
In fact, the lake had reached 442 meters by June 2009. Recent computer
simulations predict that the probability of the lake overflowing by 2030 is only
15 to 20 percent, even with planned man-made outlets in operation. That scenario
may prove to be far too optimistic, however, given that precipitation totals
during water year 2011 (October 1–September 30), which are forecast to continue,
raised the lake 0.7 meters.

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