英语听力：自然百科 秋日黄石公园 Autumn—8
时间：2014-08-27 08:16:05
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（单词翻译）
They may be a little safer here. But the food in the forest is far less nutritious1 than on the riverbanks. If they want to eat well and avoid wolves this winter, they'll need to keep on moving. As the elk2 move gradually downwards3, they follow the rivers out of Yellowstone’s central plateau.
The rivers in turn follow the path of
glaciers4 that flowed from this great bowl in the last Ice Age, and carved their way right through the barrier of surrounding mountains. Today, these valleys are escape routes for animals from the returning ice of winter.
Lower down, the valleys broaden, the rivers slow, and a richer variety of trees grows in the
alluvial5 soils, the perfect home for Yellowstone’s most
industrious6 creature.
A
beaver7 can fell a cottonwood tree in just a few hours, hundreds in a year. The beaver doesn't chew through the whole trunk, just enough to make the tree
unstable8. It then retreats, and lets the wind do the rest.
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