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Nature's Miracles: Familiar Talks on Science, Vol. 1.

<h2>CHAPTER XIX.</h2> <h3>RIVERS AND FLOODS.</h3> Water covers such a large proportion of the earth's surface and is such an important factor in the economy of nature that it becomes a matter of interest to study the process of its distribution. Water is to our globe what blood is to our bodies. A constant circulation must be kept up or all animal and vegetable life would suffer. Here, as in every other operation of nature, the sun is the great heart and motive power that is active in the distribution of moisture over the face of the globe. The total annual rainfall on the whole surface of the earth amounts to about 28,000 cubic miles of water. Only about one-fourth of this amount ever reaches the ocean, but it is either absorbed for a time by animal and vegetable life or lifted through the process of evaporation into the air as invisible moisture, when it is carried over the region of rainfall and there condensed into water and falls back upon the earth—only to go through the same operation again. The whole surface of the earth is divided into drainage areas that lead either directly through rivulets and rivers to the ocean, or into some land-locked basin, where it either finds an outlet under ground or is kept within bounds through the process of evaporation, the same as is the case with our great oceans. In North America the amount of drainage area that has no outlet to the ocean amounts to about 3 per cent. of the whole surface. In other countries the percentage of inland drainage is much larger. The great Salt Lake in Utah is an instance where there is no outlet for the water except through the medium of evaporation. Inasmuch as all rivers and streams contain a certain proportion of salt,—especially in such strongly alkaline land regions as the Great Basin of the North American continent,—these inland lakes in time become saturated with this and other mineral substances. Salt is constantly being carried into the lake by the water of the stream that feeds it, and the water is continually being evaporated, leaving the salt behind. This process has been going on in the valley of Utah for so long a period that 17 per cent. of the contents of the lake is salt. The Humboldt River in Nevada, which empties into a small lake of the same name, and lies at the foot of the Humboldt Mountains, is said to have an underground outlet. This must be the case, because the area of the lake is very small as compared with Salt Lake, while the river that feeds the latter is very small compared with the one that flows into the former. That is to say, in the one case a very small stream empties into a large lake, while in the other case a much larger stream feeds a very small lake. Besides, Humboldt Lake, unlike the Great Salt Lake, is said to be a fresh-water lake; if it had no outlet it would become in time saturated with salt. The largest body of water in the world having no outlet to the ocean is the Caspian Sea, on the border between Asia and Russia in Europe, it being 180,000 square miles in extent. Where rivers empty into large bodies of water, such as the great chain of lakes on the northern border of the United States (and these lakes have an outlet connecting one with the other, and finally by a river to the ocean) a constant circulation is being kept up, and the water remains fresh. Owing to the fact, however, of the great evaporating surface that these lakes afford, there is a greater disproportion between the rainfall upon the drainage area tributary to these lakes, and the amount of discharge through the St. Lawrence River, than would be the case with a river that was not connected with a system of lakes. The amount of rainfall upon the area drained by the Mississippi River during one year amounts to about 614 cubic miles of water, while the discharge at the mouth of the Mississippi River is only about 154 cubic miles. The difference between the two figures has been carried up by the process of evaporation or stored in vegetation. These figures vary considerably, however, with different years. The proportion of rainfall to discharge will vary greatly in different rivers from other causes than having a large evaporating surface. This variation is due to the difference in the ability of the soil to retain water after a rainfall. In some drainage areas the ground is more or less impermeable to water, and in this case the water runs readily off, causing a sudden rise in the river; and as suddenly it reaches the low-water mark. In other drainage areas the ground is very permeable to water, so that the rain penetrates to a greater depth into the earth, where it is held, and by a slow process drains into the rivers, while much more of it is carried off by evaporation and into vegetation than is the case in the drainage district before mentioned. The courses of rivers are determined by the topography of the country through which they flow. The sinuous windings, that are found to be a characteristic of nearly all rivers, are caused by the water, through the force of gravity, seeking the lowest level, and avoiding obstructions, which they can flow around more easily than remove. Great rivers often change their courses, especially where they flow through a region of made earth, such as is the case with the lower Mississippi River, and in other great rivers of the world. The loose earth is continually shifted by the current, and where the current is not very strong it will often hold the water back to such an extent of accumulated weight that the flood will break over at some weak point on its banks and make a new course for itself. One of the great rivers of China—the Hwangho—often causes dire destruction to life and property owing to change in its bed from time to time. It is estimated that between the years of 1851-66 this river caused the loss of from 30,000,000 to 40,000,000 lives through drowning and famine by the destruction of crops. Floods in rivers are occasioned from various causes. Of course the primary cause is the same in all cases, that is, from precipitation of moisture in the form of rain or snow. Some rivers are so related to the area of rainfall and to the permeability of the soil that there is but little variation in the amount of discharge throughout the year. The great river of South America, the Amazon, is an instance of a river of this class. A certain number of the smaller rivers that feed it lie in the area of rainfall during the whole of the year; for instance, the streams of the upper Amazon are being fed by rains at one season of the year, when those feeding the river lower down are at the lowest stage. When the rainy season prevails in the upper section of the river the dry season prevails farther down, while at another season of the year these conditions are reversed. Therefore, though the Amazon has a larger drainage basin than any other river in the world, and in some parts the yearly rainfall is 280 inches, there is no very great fluctuation in the stages of water. The Orinoco River, which flows through Venezuela, and whose drainage area is largely covered with mountains, has a greater fluctuation than any other river, the difference between high and low water amounting to seventy feet. The River Nile has an annual rise of from fourteen to twenty-six feet. This river is the sole dependence of the inhabitants of lower Egypt, and their sustenance depends upon the height to which the river rises; if it does not rise high enough the agricultural lands are not sufficiently irrigated, and if it rises too high their crops are destroyed by the floods. In this section they depend entirely upon the overflow of the Nile for irrigation, and not upon the rainfall. There is scarcely ever a rainfall in lower Egypt except about once a year on the coast of the Mediterranean. After ascending the river for a short distance we come into an area of no rain for a distance of 1500 miles along the river. Egypt has a superficial area of about 115,200 square miles, and only about one-twelfth of this area is in a position to be cultivated. As there is no rainfall in this region, the sole dependence for agricultural purposes is from the River Nile when it rises to a sufficient height to admit of irrigation. The river brings down quantities of rich earth which during the overflow is deposited, and thus the agricultural regions are refertilized annually. The River Nile is what is called a tropical river and is fed by the rains in upper Egypt caused by the monsoon winds that prevail in that section of Africa during the summer season, as they do in India. As has been explained in a former chapter, the monsoon winds blow steadily for about six months from off the southern ocean. These winds are highly charged with moisture, which is not precipitated till it strikes the mountainous regions of the interior. Here the high mountains, which are often snow-capped, cause a profuse precipitation, which runs off into the various feeders of the Nile, causing a gradual rise in the river that reaches the highest point about September of each year. If the Nile should dry up, or if the annual floods should materially change in height, it would make a desert region of all that portion of Egypt now so productive. The great rivers of China, the Yang-tse-Kiang and the Hwangho, are also tropical rivers and have an annual flood. Sometimes the rise is as much as fifty-six feet. These annual floods are also caused by the monsoon winds that carry moisture from the ocean, which is condensed and precipitated in the mountains of central Asia. The conditions are substantially the same as those which prevail at the sources of the Nile in Africa. Rivers are produced from all sorts of causes, some of them flowing only during the rainy season, while others are fed by melting snow from the higher mountains, and as the snow is rarely melted away entirely during the summer, in the high mountains, there is a continual flow from this source. The snow forms a system of storage, so that the water is held back and is gradually given up as it melts. If this were not true mountainous regions would be subjected to disastrous floods. If the precipitation were always in the form of rain it would immediately run off instead of being distributed over a whole season. The Platte is an instance of a river largely fed by the melting snows—of the Rocky Mountains. In the region of glaciers in the mountains of Alaska and Switzerland rivers are fed by the melting ice. These rivers are usually of a milky color occasioned by the pulverization of rock caused by the grinding of the great glaciers as they flow down the gulches in the mountain side. In some regions these glacial rivers have a diurnal variation. This is caused by the fact that the glacier is so situated that it freezes at night, which checks the flow, and thaws in the daytime, which increases it. Rivers are to the globe what the veins are to the animal organization. They pick up the surplus moisture not needed in the growth of vegetation and for the sustenance of animal life, and carry it on, together with the débris that it gathers in its course, to the great reservoirs, the seas and oceans, where it is redistilled and purified by the action of the sun's rays. From here it is carried back in the form of invisible moisture and again precipitated in the purified state, to help carry on the great operations of growth—animal and vegetable. The vaporized moisture that is carried back by the winds and redistributed corresponds to the blood, after it has been purified and is carried back through the arteries to the extremities and capillary vessels which feed and nourish the bodily organs. <hr style="width: 65%;" /> <div style="break-after:column;"></div>

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