重庆大学网络远程教育《大学英语3》在线作业9

The ordinary raindrop is a mighty earth mover with sufficient strength to cut rock. When rainwater collects on the surface of the ground， some of it evaporates and some of it sinks into the earth. The remainder begins to flow downhill， commencing its lengthy journey form brook to stream to lake， or to a river that will carry it to the sea. As water flows along the ground， it picks up sand， pebbles， even boulders. It uses them to gnaw at the sides and bottom of its channel， gradually loosening more earth. By this process enormous amounts of mud and rock are moved from the land to the sea. Each year the Mississippi River carries 730 million tons of solid matter into the Gulf of Mexico. This constant hauling of land into the sea is lowering the United States' average height above sea level at a rate of about one foot every 9000 years. If erosion continues at the same rate， the United States will be worn completely down to sea level in about 23 million years.

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