Rivers are of very great importance as agents of change in dynamic geology, the form of valley they excavate being determined partly by the nature of the rocks, partly by the climate. In rainless or arid regions steep-walled canons are cut to a great depth across high pla teaus; in rainy regions subaerial de nudation leads to the formation of wide valleys of much gentler slopes. Bars of more durable rock crossing the course of a stream lead to the formation of water falls or rapids from the rapid erosion of the softer strata below. The river above the obstruction is reduced to what is termed the base level of erosion; the velocity of the current is checked, and wide alluvial deposits are laid down on either side. In course of time the bar of hard rock is completely cut through by a gorge, and the gradient of the stream is ultimately rendered uniform. In this way the common features of gorge and meadow are produced again and again along the course of a stream. The deposits of alluvium form terraces along the valley track of a river, and as the stream cuts its channel deeper they are left at various heights as mon uments of its erosive power. When a river is fairly established in its valley it is, geologically speaking, a more per manent feature than lakes or mountains. Upheaval, which acts very slowly, may even elevate a range of mountains across its course, yet all the while the river, cutting its way downward, remains at the same absolute level. The Uintah mountains, as they were upheaved, were divided in this way by the Green river, the chief tributary of the Colorado. In limestone regions the solvent power of river water on carbonate of lime leads to the formation of caves and under ground rivers, which, as a rule, emerge from their subterranean channels on low er ground. Sometimes they do not re appear on land, but discharge their fresh water through openings in the bed of the sea. Such submarine river entrances are not uncommon along the shores of the Adriatic, off the coast of Florida, and in other calcareous regions. When a river advances along a nearly level plain to ward the sea its carrying power falls off; gravel, sand, and finally mud are deposited on its margin, and the stream pursues a peculiar winding course. Dur ing a flood the swift and muddy stream rises, overflows its banks, and widens out on the level land. The current is at once checked and a long bar of deposit forms along each margin. These are increased in height by each successive flood, and, the river-bed being simulta neously silted up, broad muddy rivers like the Mississippi, Po, and Hoang-ho come in time to flow along the top of a gently sloping natural embankment, the sides of which are termed levees in Lou isiana. The entrances of rivers into lakes or the sea are usually marked by great banks of deposit, or by bars of gravel or sand. In some cases, however, such as the River Plate, the Thames, and Tay, the mixture of river and sea water is gradual, and the sandbanks are spread over a very large area, but not built up into a delta at any one place. In a few instances, such as the Forth, rivers enter deep arms of the sea in which neither banks nor bars are formed. The Congo sweeps directly into the ocean, throwing down great banks of deposit along the continental slope to right and left, but leaving a deep canon-like gully for the bed of the stream itself ; a similar con dition occurs where the Rhone enters the Lake of Geneva.
The ultimate source of all rivers is the condensation of water vapor from the atmosphere in the form of rain, snow, and even dew. If the land were com posed of impermeable rocks all the rain water not lost by evaporation would run off directly over the surface, and rivers would only flow during and immediately after showers. A large part of the rain fall, however, soaks into the soil, which retains it as in a sponge, especially if the land be marshy, and allows it to flow off gradually as superficial springs Some also percolates deeply into the rocks, ultimately emerging as deep-seat ed springs at a great distance. When a river flows toward a region of great evaporation and small rainfall, such as exists in the interior of each of the great continents, evaporation removes more water than is supplied by the remote tributaries, and the stream may fail to fill the hollow it enters, and therefore cannot overflow into the sea. This is the case with the Oxus entering the Aral sea, and the Volga entering the Caspian. It may be that evaporation is so far in excess of contributions from distant rain fall or snow-melting that the river dries up as it flows, and its last remnant is absorbed in the desert sand. This is the
fate of the Murghab, the Heri-rud, the Zerafshan, and many other rivers of central Asia.
The annual inundations of the Nile are due to the monsoon rainfall on the great mountains of Abyssinia. The Ori noco is another instance of seasonal rains producing tremendous inundations, over 40,000 square miles of Llanos be ing said to be laid under water by the summer rains. The Amazon is an in stance of a river which is always more or less in flood as the various tribu taries attain their greatest height at different seasons. The Ganges overflows its banks in summer when the monsoon rainfall is reinforced by the melting of snow on the Himalayas. Where the sea sons of maximum rainfall and of snow melting are different, as ih the Missis sippi, the Tigris, and Euphrates, there are two regular floods in the year.
The danger of flooded rivers arises from the suddenness with which the wa ter rises and overflows narrow valleys or even plains. Frightful devastation fol lows the bursting of glacier obstruction lakes in mountain valleys. The most se rious floods in the Danube and Theiss have resulted from the constriction of the channel at the Iron Gates, which prevents the flood water from passing away as rapidly as it comes down; the current of the Theiss is sometimes re versed for many miles. Great rivers which have embanked their course above the level of the plain are the most dan gerous of all when flooded. The damage caused by the bursting of the levees on the lower Mississippi necessitates a great expenditure in strengthening the em bankments, and the most disastrous in undations recorded in history have fol lowed the bursting of the banks of the Hoang-ho and its consequent changes of course.
River water is spoken of as fresh, but it always contains a certain amount of solid matter in solution, varying from two grains in the gallon or less in rivers draining hard crystalline rocks to 50 grains in the gallon or more in lime stone districts.
The temperature of rivers, as a rule, follows that of the air, but is subject to variations on account of the effect of rain.
The great rivers of Europe and Asia, such as the Rhine, Danube, Volga, In dus, Ganges, Brahmaputra, Yang-tse kiang, afford access to the sea to enor mous populations. The Amazon, with its plain track extending for nearly 3,000 miles, is in many ways less like a river than a fresh inland sea ; but the Missis sippi and St. Lawrence, though less ex tensive, are of greater value for carry ing sea traffic to inland places. In their torrential-and upper valley tracks rivers are of use chiefly for transporting tim ber and driving machinery. It is inter esting to note that in Switzerland, Nor way, and Sweden, where there is no coal, there exist exceptional facilities for the use of water power on account of nu merous mountain torrents. In hot coun tries rivers are of the utmost service in irrigating agricultural land; the Zeraf shan and Murghab are entirely consumed in that service, and since the completion in 1890 of the barrage on the Nile no water escapes to the Mediterranean in the low Nile months except along irri gation canals.
The largest rivers of the world, with their length in miles, are: Amazon, 4,000; Nile, 3,766; Yangtse, 3,400; Yenisei, 3,300; Mississippi, 3,160; Missouri, 3,000; Congo, 3,000; Lena, 2,800; Niger, 2,900; Ob, 2,300; Hoang-ho, 2,600; Amur, 2,500; Volga, 2,300; Mackenzie, 2,525; La Plata, 2,300; Yukon, 2,300; St. Lawrence, 2,150; Rio del Norte, 1,800; Sao Francisco, 1,200; Danube, 1,725; Euphrates, 1,700; Indus, 1,700; Brahma putra, 1,680; Zambesi, 1,600; Ganges, 1,500; Mekong, 2,500; Amu Daria, 1,500; Ohio, 950.
The pollution of rivers has of late years, in consequence of the extension of manufactures, caused serious concern. No person has a right to poison or lute a stream, and if he do so any of the persons whose lands abut on the stream lower down may bring an action to recover damages. At common law, indeed, in every question of river tion, the real question of fact is whether there has been any material increase of pollution beyond that which is natural to the particular stream, or beyond that which has existed there for the tive period. Questions of river pollution are eminently fitted for submission to a i jury, and are generally disposed of in that way.
In the United States the common law of England was at first followed; but in some of the States it is expressly de clared that the common law is inapplic able. Mining rights have been specially determined in some districts; and the laws as to irrigation rights have been elaborately defined in Colorado and else where.