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 evaporates① and some of it sinks into the earth. The remainder begins to flow downhill， commencing② its lengthy journey from brook③ to stream to lake， or to a river which will carry it to the sea.
As water flows along the ground， it picks up sand， pebbles④， even boulders⑤. Rainwater uses them to gnaw⑥ at the sides and bottom of the channel， gradually loosening more earth.
Through this process， enormous amounts of mud and rock are moved from the land to the sea. Each year the Mississippi River carries 730 million of solid matter into the Gulf of Mexico.
This constant hauling⑦ of land into the sea is lowering the United States average height above sea level at a rate of about one foot every 9，000 years. If erosion continues at the same rate， the United States will be worn completely down to sea level in about 23 million years.
① evaporatev. 使蒸发，使挥发
② commencev. 开始，着手［+v-ing］［+to-v］
③ brookn. 小河，小溪
④ pebblen. 小卵石［C］
⑤ bouldern. 卵石，大圆石，巨砾
⑥ gnawv. 啃，啮［(+at/into/on)］
⑦ haulv. 拖，拉，曳，拖运，搬运
2300万年后的美国
普通的雨滴能移地，其力之大足以切割岩石。当雨水在地面集结时，一部分会蒸发掉，一部分就会渗入地下，剩下的部分则会向低处流动，由小溪注入小川，再注入湖泊或江河，然后由江河送入大海，开始了其漫长的旅程。
雨水沿着地面流动时，会夹带泥沙、卵石，甚至大圆石等东西。雨水也正是利用它们啮噬着水道的两边和底部，逐渐使更多的土壤松动。
通过这一过程，巨量的泥沙和石块便从陆地移入海洋。每年，密西西比河都要把7.3亿吨的固体物质送入墨西哥湾。
这种不断的移地填海，正在使美国海拔以上的平均高度以每9，000年减少1英尺的速度下降。如果土壤流失以同样的速度持续下去，那么，在大约2，300万年以后，美国就会完全被磨蚀而下降到与海平面一般的高度了。
