And yet here is seen only the beginning of the vast work which these forces have before them. They have built only the narrow trough of what must be developed into a wide and gently sloping valley， and have hewn out here and there a ravine in that great upland which in time they must carve into the mature forms of a thoroughly dissected country. If the region had not been so dry， the work of dissection wouldhave progressed much farther before the river had been able to sink its channel so deep. The water that falls hundreds of miles away is doing a mighty work which the meager rainfall of the region through which it passes cannot supplement. Majestic， awe-inspiring， stupendous， this gigantic trench is but a prank of the river"s boisterous youth.
Summary. -Just as the waves and ocean currents work upon the coastlines， so the rain and the streams are constantly wearing down the surface of the land. All streams come from rain or melting snow， which condenses in the air after evaporating from water surfaces. The rainfall varies from nothing at all in some places to over fifty feet a year in others， but in the United States the greatest rainfall is about eighty inches a year.
Some of the rain evaporates at once after falling； some flows away on the surface of the land； some sinks into the ground， to return as springs， wells and geysers. The water which flows along the surface has the greatest effect upon the land. It forms the little streams which remove the surface water， the huge rivers which drain the country and form great arteries of trade， and the beautiful lakereservoirs which hold back floods and offer easy transportation to mighty ships.
But most important of all is the erosion caused by flowing water.
It wears down the hills and spreads them out in fertile fields， in deep trenches and broad valleys； it fills lakes and builds great deltas. By its falls and rapids it furnishes water power for manufactures.
Rivers that have not yet widened their valleys and still have falls and rapids are called young； an old river is one whose bed has been worn smooth， and which has built for itself a broad level valley， through which it wanders， doing little if any erosive work. Rivers sometimes develop flood plains through which they wander in S-shaped meanders. Sometimes a river cuts back its divide so far that it reaches another river， thus diverting another stream through its channel.
If the region of a river becomes elevated， the river may be revived，and if it is an old river with meanders， intrenched meanders may beformed. Sometimes the elevation of the land causes a river to be laked or reversed； if it maintains its previous course in spite of the elevation， it is called antecedent.
If a river region becomes depressed， the river may be drowned andits branches may enter the sea separately as dismembered rivers. Many rivers build deltas where they empty into still bodies of water and when the slope is steep， they may form fans.
Rivers have always played a great part in history， from the time Egypt was first called the "Gift of the Nile" to the influence of the Mississippi and St. Lawrence on the settlement and development of the United States.
QUESTIONS
What conditions influence the amount of rainfall of a place？
What determines what will become of the rainfall when it falls upon the ground？
What does the water do which sinks into the ground？ Where are geysers found？ What are they？
Trace the probable journey of the water that fell near your home during the last heavy rain until it reaches its journey"s end.
What determines whether a lake is fresh or salt？ What are the great benefitsderived from lakes？
Describe some effects of running water that you have seen.
Why does not all the water that fell in your town during a heavy rain flowby your home？ Where is the "divide"？
What are some of the causes which have formed falls and rapids？ Describe the life history of a river.
What peculiarities have rivers of dry climates？
Describe some of the accidents which are liable to happen during a river"s history.
How are alluvial cones and fans formed？ Where and how do rivers build deltas？
What have been the effects of rivers upon history？ Describe the four great rivers of the United States.
【中文阅读】
149.降雨--地球表面的海水不断蒸发，升入空中，又被风吹向四方。 大部分的水蒸气被吹向陆地，它们在那里又重新凝聚，变成雨露降落大地。陆 地上不同地域受到的这个水循环的惠泽，有的地方多，有的地方少。在温暖的 海域，海水的蒸发量会更大，从那里吹来的风，遇到山脉便向上爬升，在迎风 面上因此形成大量的降雨，而背风面就没有这么滋润了。这一情形在美国北部 地区表现得很明显。就一般规律而言，各个大陆板块上的山脉都很靠近板块边 缘，这样便使得所有的内陆地区都相对比较干燥。
盛行风为寒风的地区，雨量也比较稀少，因为风从寒带吹向温带，随着温 度的升高，其保持水蒸气的能力也在不断增强。撒哈拉沙漠的气候状况就能凸 显这一点。因此，地球上不同地区的降雨量也各不相同。在东印度的卡西山南 部地区，有一年的降雨量的最高纪录到达了50英尺，而在地球上极为干旱的地 区，一年到头可能滴雨不下。
美国最大的降雨量可达80英寸以上，一般出现在西北沿海以及犹他州、亚 利桑那州和内华达州的盆地地区。不论降雨量是大是小，它对一个地域的影响 都是不容小视的。没有降雨，地球表面将会变成极度炎热而且寸草不生的茫茫 戈壁；而正因为有了降雨，才有了大自然的葱茏翠绿，才有了广阔无边的生命 之美。
150.降雨的范围--雨水降落到地面之后，一般有三种去向：可能立刻 从地表蒸发，重新进入到空气中；也可能形成涓涓细流，并很快汇入溪流于江 河，并最终流向大海；还可能浸入到土地中去。在最后一种情形中，一部分水 又可能通过毛细现象回到地面，进而又被重新蒸发；也有一部分水可能汇入泉 水；还有的则可能深深浸入到地下的土壤与岩石中。
而最终雨水会选择以上哪个过程，则完全取决于它降落的形式与土地表面 的状况。如果降落得非常迅速，则大部分会从地面流走；如果降落得很平缓， 大部分则会浸入地下。如果它们是降落在森林地区或者大面积的植被上，植 物以及它们的根茎便会阻碍雨水的流动。如果地表是硬质结构且不能渗透的 岩石，大部分雨水便会流走；但如果地表很疏松且能够被渗透，大部分雨水 便会渗入地下。不过即使在炎热干旱的沙漠地带，暴雨形成的激流也依然会 四处奔流。
151.地下水--渗入到地下的雨水，会沿着土壤的微小缝隙或者从土壤的 颗粒之间慢慢向下沉降，直到一个不能再向下继续渗透的地层，或者又会重新 汇入一股水流流向地面，当然水流在地面的流出口比雨水先前的浸入处要低一 些。它会在这里慢慢渗出，或者集中形成泉水。
如果汇入泉水的部分雨水渗透到地表以下足够深，以致太阳热量无法影响 到它的话，便会显得非常寒凉，当它再次涌出地面时，就形成了寒泉。但若在 泉水涌出的地方，地表岩石很热，乃至于让泉水在找到裂缝到达地面之前，所 处的底层岩石也被加热的话，就会形成温泉。
由于地球外壳在许多地方都是由岩石构成，雨水也常常降落在褶皱多孔的 岩石层上，而在这个岩层之下，雨水便不能再继续渗透了，这样雨水就会在多 孔岩石层中不断聚集。如果这一岩层在其他地域被不能渗透的岩层覆盖，先前 岩层中的水分便会慢慢被涵持在这些岩层中，形成自身的水压。若这时在岩层 上开一个孔的话，水便会自然涌出，如果水压足够大，还可能直接喷出来。
这种形式开孔后便会形成自流井，它在一些很难找到地表水源的地方非常 重要。在美国西部一些省份，人们主要便用井水来进行广泛的农作物灌溉。尽 管这些水含有一定矿物质，但由于它们远离地面污染，因此一般都很健康的水 源，可直接饮用。