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  3. Decline of Colorado River is Severe [A]The Colorado River, which flowed across the land with such gusto that it carved the majes

Decline of Colorado River is Severe [A]The Colorado River, which flowed across the land with such gusto that it carved the majes

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问题                     Decline of Colorado River is Severe
[A]The Colorado River, which flowed across the land with such gusto that it carved the majestic Grand Canyon in a merely five million years, once reigned as an unchallenged force in shaping and nurturing the Southwest. But then humans came along, and the mighty river has been reduced to a trickle in some places, even dying out in the desert during dry years before it reaches the Gulf of California.
[B]It doesn’t take a lot of smarts to figure out that if you take away the water, creatures and plants that depend on it will die. But now scientists have compiled an extensive record of what the Colorado was like before humans re-engineered the river, compared to what it’s like now.
[C]About 95 percent of the marine life in the river’s delta has been wiped out in less than 70 years. Where great beds of clams once flourished, ultimately providing the white sand that nature reshaped into large offshore islands, very little survives today. What’s astonishing is the scale of the impact. "It’s pretty shocking," says paleontologist(古生物学家)Michal Kowalewski of Virginia Tech, one of the leaders in a four-university study of the price the environment has paid for reshaping the river. The study was published in last week’s issue of the journal Geology.
[D]Today, the river is, at best, a stream when it reaches the gulf that separates the Baja peninsula and mainland Mexico. The region used to be one of the most biologically productive areas on the planet, supporting billions of clams and other marine life in the nutrient rich waters of the delta.
[E]The researchers concentrated on clams, because their hard shells have survived the ravages of time, and those shells provide a biological yardstick of the marine ecosystem, according to paleontologist Karl W. Flessa of the University of Arizona, one of the principal investigators. "The clams were in a sense a proxy for the whole marine ecosystem," Flessa says. "There were so many there because of the very high productivity of marine microorganisms which thrived in the nutrient-rich waters. Thus more clams means more wildlife ranging from tiny sea creatures to fish to birds."
[F]The researchers found that clams were so abundant that a total of more than two trillion thrived in the delta over the last 1,000 years, a density of about 50 clams per square meter at any time. That number today has been reduced to about three clams per square meter, if they can be found at all.
[G]Thus the scientists estimate a drop in marine life of at least 20 fold, but Kowalewski says that figure is "quite conservative". "I think 20 times is a pretty staggering number, but in fact my hunch is that it’s probably much more than that," he says. There may have been 100, or even 200 times more marine life in the delta before the 1930s ushered in a new life for the Colorado, he adds.
[H]Some would argue, of course, that what humans took away, humans put to good use. Electricity from the Hoover Dam brought jobs and power to areas like Las Vegas, allowing that region to become one of the fastest growing metropolitan areas in the country. Without the dam, the lights of "glitter gulch" would be more like candles. Water was diverted from the river to farms and cities, turning some desert regions into bread baskets, and allowing major urban centers like Phoenix and the megalopolis of Southern California to prosper. But today, only 10 percent of the water in the Colorado even reaches the border with Mexico, depriving that desert region of an essential ingredient for agricultural and urban prosperity.
[I]In the last couple of decades, federal officials have tried to respond to demands to release more water from the Hoover to protect the downstream environment, and in wet years the river gushes anew for a while. Kowalewski and Flessa say there’s some evidence that riverside habitats north of the delta have benefited from the increased supply, but that apparently hasn’t been the case in the critical habitat where the river meets the gulf. The research shows that clams, for example, are not rebounding.
[J]The scientists believe the method they used may be even more important than their findings, because the same techniques can be used to chart the history of almost any coastal region back to the time before human intervention. Most marine species decay after death, leaving a scant record of their existence, but the hard shell of the clam persists.
[K]By dating 125 clam shells from the delta, the scientists were able to reconstruct the last 1,000 years. They found that the average clam lived about three years, so the period covers 333 generations of clams. The population remained quite stable throughout that period, Kowalewski says, with an average of about 6 billion at any time.
[L]But the significance of the findings go far beyond the lot of the clams. The huge delta — about the size of Rhode Island — is a critical habitat for many species. It is the region where fresh water mixes with salt water, providing an ideal nursery for all sorts of animals, like shrimp, fish and waterfowl.
[M]So the abundance of clams means the conditions were right for many creatures to flourish. Likewise, the decline of clams suggests that many species suffered from a deteriorating environment. That bolsters the claims of fishermen in the region who insist that the diversion of water from the Colorado has had a profound impact on all sorts of marine creatures.
[N]So, where do we go from here? Water is such a prized commodity in the southwest that it’s not likely anyone is going to give up their share to help a delta in Mexico. Wars have been fought over such issues. But Flessa points out that a little may do a lot. "The question for us right now is how much more fresh water would it take to restore some of these habitats," Flessa says. "The honest answer is we don’t really quite know."
[O]Further research may show that a little increase in water flowing down the river — maybe just 10 percent — might do wonders for the ecosystem. But even that water will most likely have to be replaced by water from somewhere else, and it’s hard to find water in the desert. Conservation might help, but the Southwest is one of the most rapidly growing regions in the nation, so every drop is precious. Without an alternative supply, Kowalewski says, the outlook is quite grim. "This is not going to improve," he says. "It is only going to get worse."
According to the research, the number of clams was pretty stable over the last 1,000 years.
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答案K
解析 根据题目中的the last 1，000 years将本题出处定位于[K]段。该段最后一句说，蛤蜊的数量在那段时间内是相当稳定的，任何时候都平均大约有60亿只，句中that period指的是the last 1，000 years。本题是对该句的同义转述。
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