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题目内容
题目材料：
In 1996, scientists caused an experimental flood of the Colorado River by releasing water from Glen Canyon Dam above the Grand Canyon. Because an unintentional flood in 1983 had reduced the river`s introduced population of nonnative trout, biologists were concerned that the experimental flood would wash many fish, native and nonnative, downstream. To find out, biologists placed nets in the river. The nets captured a few more trout than they would have without the flood but did not show substantial flushing of native fish, whose ancestors had, after all, survived many larger natural floods. The biologists surmised that the native species(and most of the trout) must have quickly retreated to protected areas along the riverbank.
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