RIVERS (Fr., rim ilre; Span., ribera; It., rivicra; Lat., riparius, from ripa, hank, of or belonging to a bank), bodies of water of con siderable size flowing with perceptible currents in definite channels, and usually without cessa tion during the year. Some watercourses are designated as rivers, although their beds are dry a part of the year. Rivers, obeying the laws of gravity, seek the lowest level and eventually reach the ocean. The regions having rivers which do not flow into the ocean either di rectly or indirectly are parts of the earth's sur face where the evaporation is so great that no overflow of water results from precipitation. Usually the overflow of a river discharges into another river, and perhaps a second or even a third discharge is made before the waters reach the sea. The Republican River of Nebraska and Kansas flows into the Kansas River in the State of Kansas, and the Kansas River flows into the Missouri. the Missouri into the Mis sissippi and the Mississippi into the Gulf of Mexico, which last discharges its waters into the Atlantic Ocean through Florida Strait and Yucatan Channel.
Origin.— Rivers are usually formed by springs or the gradual melting of the ice and snow which perpetually cover the summits of all the most elevated ranges of mountains upon the globe. Springs and ice are themselves due to the precipitation of water from the at mosphere in the form of snow, rain, mist or dew. These fill the springs, lakes or other reservoirs from which rivers flow by the natural gravitation of water to a low level. The union of various springs, or of these meltings, forms rivulets; these last follow the declivity of the ground and commonly fall at different stages into one great channel called a river, which at last discharges its waters into the sea or some great inland lake. A new-made river at first usually flows over the surface until it has worn itself a channel; or it may follow some depression or deformity in a mountainous re gion. Rivers are swollen during their course by the rain which falls on the surface. In temperate climates the source of supply tends to greater equality in the volume of the river than in torrid regions, where the evaporation is great. In the former the rains are not only more equally distributed in point of time, but also from a more protracted source of supply; part of the rainfall sinks into the ground and forms springs, part forms rills which flow di rectly into the river and part feeds those rills with the drainage of the saturated ground long after the rain has ceased. As mountainous re gions abound in springs, most rivers begin from a chain of mountains; each side of a chain :144s has its springs, and the rivers which orig inate on one side flow in the opposite direction to those which rise on the other. As it is the property of water to follow the most precipitous descent that comes in its way the courses of - streams point out the various declivities of the earth's surface, and the line from which large rivers flow in contrary directions generally marks the highest parts of the earth. This line need not, however, be of any great height; in European Russia, for instance, where the rivers are very extensive, the line which sepa rates their sources is very little above the level of the Baltic or of the Black Sea.
Changes in Form and Size.—As soon as the stream begins to flow, its channeling and the deepening of its, trough commences. If no ob structions prevented, the water would go on enlarging the channel, by widening and deepen ing. The nature of the soil, the softness or hardness of the rocks along its course, the amount of sediment it carries in its waters and where this sediment is deposited all contribute to change of form and size. The erosive action of rivers on their beds is continually exercised, especially in the early part of their course; where the channel broadens as they approach the sea it may almost cease. Remarkable in stances of erosive action are, almost everywhere to be observed; but in no case is such action more striking than in the cafions or river gorges in the Colorado region of the United States. There the rivers have hollowed out for
themselves channels that present almost perpen dicular rock walls on either side rising to the height of 6,000 feet for hundreds of miles. Such action is also seen where there is a water fall, in which case the water gradually wean away the rock at the place where the fall occurs and thus causes the latter to recede, as Niagara -Falls. In perhaps every case a river has had much to do with the formation of the valley in which it flows Rivers are very permanent -features in the earth's history, and in some cases have hollowed out a channel through a mountain range gradually elevated across their source: • The size of a river depen upon two main circumstances,— the octmi and character of its drainage area, and the degree of humidity pos sessed by the climate of the region from which it draws its supplies,— the latter being often dependent upon winds blowing from the ocean. The peculiar position of the Andes dionntains with respect to the rest of South America, the fact that by very far the largest proportion of its running waters are drained off in one general direction, toward the At lantic, and the humidity of the climate, all con tribute to the immense size of die. rivers. The Andes being placed so near. the coast of the Pacific, the riven which flow into the Pacific Ocean are small, while those which flow on the other side, having such an immense space to traverse, are increased into a vast volume be fore they reach the Atlantic. The physical con formations of some continents are unfavorable to the accumulation of such vast bodies of water as the riven of South America. Europe is not of sufficient extent; Africa has a climate which tauses rapid evaporation and abounds in sandy deserts. A large part of Asia has not the humidity of the Amazon region, and its vast interior lakes serve as ireeeivers for some of the large Streams. The arrangements of its mountains conduce to long and somewhat nar rote drainage areas. As the river goes on from year to year, in Most cases it cuts a trench be low the: original constructional channel, and in time forms what is called a consequent valley. By means of the new constructional troughs many of the constructional lakes and the rivers become continuous streams, that increase the length as well as the depth of the channel. Where the waters pass from a hard to a soft rock, or from a resistant to a weak part of the material through which it is channeling, a greater slope is formed making rapids (q.v.) or falls, or cascades (q.v.). The angle of the slope depends upon the difference in the de gree of resistance of the hard rock in the old channel as compared with the soft rock in the formation channel. Gradually the channel is cut down close to the sea-level or base-level, and its course is over a gentle, gradual slope. The depth of the channel depends upon the dryness of the climate, softness of the rock, height of the land and the volume of the stream. The dry climate really lessens the volume of the stream, and consequently it can not force its way with the rapidity of a stream with greater volume even if the widths of the channels are equal. The river flowing through a region of hard rock will cut a deeper valley in a given time than the river of the same size and volume flowing through a region of soft rock; the latter will have a mass of detritus, an accumulation of waste from the weak rock, and it will need a vast volume of water or a steep grade to do the same amount of channeling as the first-mentioned river. Rivers flowing over comparatively low land, with drainage areas not much above the base-level, have shallow chan nels, as in the southeastern part of the United States, especially in Florida. A river with a large volume of water has more power to carry off sediment, and even where the slope is gentle its momentum is greater than in that of the stream with a small volume of water. There are two causes for the shallow channels of the long rivers of the plains east of the Rocky Mountains — the dryness of the climate and the soft rock material through which they pass.