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 from 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 Rivercarries 730 million 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 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．

                 2，300万年后的美国

普通的雨珠力能移地，其力之大足以切割岩石。当雨水在地面集结时，一部分蒸发掉，一部分渗入地下，剩下的部分则向低处流动，开始其漫长的旅程，由小溪注入小川，再注入湖泊或江河，然后由江河送入大海。

雨水沿着地面流动时，夹带了泥沙、卵石，甚至大圆石。雨水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，巨量的泥沙和石块从陆地移入海洋。每年，密西西比河要把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海平面一般高了。