River.

River. Water falling on the land in the form of rain, or resulting from melting snow, or rising to the surface in springs, flows over the surface to a lower level. Where two slopes of land dip together the surface drainage collects to form a stream, and when evaporation is not very rapid several such streams ultimately unite and the volume of water they carry flows to the sea or to a salt lake. Small streams are termed runnels, rivulets, rills, brooks, becks, or burns; large streams are termed rivers, but the word has no precise reference to the magnitude of the stream to which it is applied. Dr Johnson gives as definitions: 'Brook, a running water less than a river;' and 'River, a land current of water bigger than a brook,' and this fairly illustrates the use of the words as popularly applied.

The beginning of a stream—whether brook or river—is called its source, and may be a spring issuing from underground, a lake or marsh in which rainfall accumulates, melting snow, or simply the gathering tricklings from falling rain. The path of a stream is its course, and is the line of lowest level from the source to the end, which if occurring in a lake or the sea is termed its mouth. The connected streams which unite in one river form a river-system. The series of convergent slopes down which a river-system flows—the land which it drains—forms its basin or catchment area, and the name watershed is also sometimes erroneously applied to it. The names watershed, water-parting, and divide are used to designate the boundary line separating adjacent basins. A watershed is always the meeting-place of the highest part of divergent slopes, and from the characteristic form of continents the main watershed of a continent is almost always the crest of a range of mountains. In many cases, however, the diverging slopes meet in a low plain the summit of which may be occupied by a great marsh whence rivers creep away in opposite directions. The basins of all the rivers draining into the same ocean are called collectively the drainage area of that ocean. The main river to which the others are said to be tributary gives its name to the whole river-system. It is often difficult to decide which of several converging streams is entitled to carry the name of the main river to its source. Some geographers give this distinction to the longest, others to that with the highest source, and others to that with the most direct course. This diversity of opinion is increased when the name of a river leaving a large lake is given to one of several nearly equal streams which enter it. Hence it is that different computers disagree as to the length of rivers. The course of a typical river has been divided into three parts, although these are not represented in all cases. The torrential or mountain track is the steepest, its gradient usually exceeding 50 feet in a mile, and the velocity of its current being very great. The valley or middle track has a gradient which is rarely greater than 10 feet and often less than 2 feet in a mile. The plain track nearest the mouth of a river has a gradient of only a few inches in a mile. Rivers such as the Amazon, Mississippi, Ganges, Volga, and the long rivers of Siberia, in which the plain track is of very great length, are the most valuable for navigation, the limit of easy navigability being a gradient of about 1 foot in a mile.

The velocity of a river is proportional to the slope of the bed, but it also bears a relation to the depth of the channel and the volume of water flowing in it. On account of friction on the bottom and sides of the channel retarding the stream, the water flows fastest on the surface and in the middle. The carrying power of a river for suspended solid particles and for stones and gravel pushed along the bed depends on the velocity alone. The following table shows how rapidly the carrying power falls off as the velocity diminishes.

Rivers in flood, even in the plain track, sometimes attain a velocity of over 5 miles an hour, and torrents may even flow as fast as 20 miles an hour. The course of a river is gradually carved out and shaped by the flow of the water. The sediment and stones carried along are powerful erosive agents in the torrential and valley tracts, and the character of the valleys or gorges produced depends largely on the geological structure of the region. The course of a river is frequently determined by lines of faults, but perhaps more often it appears to be independent of the nature of the strata. Some great rivers, notably the Volga, press against the right bank, cutting it into a steep cliff, while the left bank is left as a very gentle slope. This is explained by the directive influence of the earth's rotation (see EARTH, Vol. IV, p. 165).

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 Cañons (q.v.) are cut to a great depth across high plateaus; in rainy regions subaerial denudation 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 Waterfalls (q.v.) 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 monuments of its erosive power. When a river is fairly established in its valley it is, geologically speaking, a more permanent feature than lakes or mountains. Upland, 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 (q.v.) and underground rivers, which as a rule emerge from their subterranean channels on lower ground. Sometimes they do not reappear 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 toward 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. During 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 simultaneously 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 Louisiana. Professor Lapparent, calculating from Dr Murray's data regarding the amount of sediment carried down by rivers, finds that they would suffice to wear the entire surface of the land down to sea-level in four million years. The entrances of rivers into lakes or the sea are usually marked by great banks of deposit (see DELTA), 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. Professor Osborne Reynolds has shown, by a remarkable series of experiments, that the form of the sandbanks is due to the outline of the coasts of the estuary and to the tides. 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 cañon-like gully for the bed of the stream itself; a similar condition occurs where the Rhone enters the Lake of Geneva.

The ultimate source of all rivers is the condensation of water-vapour from the atmosphere in the form of rain, snow, and even dew. If the land were composed 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 rainfall, 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-seated springs at a great distance. The indirect and permanent supply of water to rivers by springs and by the outflow of lakes is independent of local rainfall at the time, and serves to maintain the volume of the river at a certain minimum during the dry seasons. 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 rainfall 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.

Contrasted with these cases are those in which the periodical or occasional increments of direct inflow increase the volume so much as to cause a great rise of level or even extensive inundations. The annual inundations of the Nile are due to the monsoon rainfall on the great mountains of Abyssinia, which increases the discharge at Assouan to fifteen times the amount of the river at its lowest. The Orinoco is another instance of seasonal rains producing tremendous inundations, over 40,000 square miles of the Llanos being said to be laid under water by the summer rains. The Amazon is an instance of a river which is always more or less in flood as the various tributaries attain their greatest height at different seasons. In June, when the highest level occurs in the main river, 20 or 30 miles of forest on each side of its banks are laid under water for hundreds of miles. The Ganges overflows its banks in summer when the monsoon rainfall is reinforced by the melting of snow on the Himalayas. Where the seasons of maximum rainfall and of snow-melting are different, as in the Mississippi, the Tigris, and Euphrates, there are two regular floods in the year.

The danger of flooded rivers arises from the suddenness with which the water rises and overflows narrow valleys or even plains. Frightful devastation follows the bursting of glacier obstruction lakes in mountain-valleys (see LAKE). The great rivers of Siberia remain frozen at their mouths long after the ice and snow have been melted in the interior, and broad strips on their margins are necessarily laid under water by the natural outflow being stopped. The most serious 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 reversed for many miles. The widening of the channel has been repeatedly attempted as a remedy by increasing the outlet; and an elaborate system for regulating the river here, to be completed in 1895, was begun in 1890. In other cases, such as the tributaries of the Loire, and the southern rivers of the Argentine Republic, the melting snow swells the torrential track, and, on account of the abrupt change of level and the flatness of the plain, the lower part of the rivers cannot carry away the immense volume of water rapidly enough, and floods result. In some instances torrential rivers have been successfully diverted into lakes, which regulate their outflow, preventing either dangerously high or extremely low water. Great rivers which have embanked their course above the level of the plain are the most dangerous of all when flooded. The damage caused by the bursting of the levees on the lower Mississippi necessitates a great expenditure in strengthening the embankments, and the most disastrous inundations recorded in history have followed the bursting of the banks of the Hoang-ho (q.v.) 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 fifty grains in the gallon or more in limestone districts. The nature of the salts dissolved naturally differs according to the geological character of the country traversed, but all samples of river-water differ from sea-water in containing a much smaller proportion of chlorides, and a very much larger proportion of carbonates and of silica.

The temperature of rivers as a rule follows that of the air, but is subject to variations on account of the effect of rain. During sudden floods in summer the temperature of the water may fall many degrees in a few hours as the melted snow or hail precipitated on the lofty mountains is carried toward the sea.

The great rivers of Europe and Asia, such as the Rhine, Danube, Volga, Indus, Ganges, Brahmaputra, Yang-tse-kiang, afford access to the sea to enormous populations. The Amazon, with its plain track extending for nearly 3000 miles, is in many ways less like a river than a fresh inland sea; but the Mississippi and St Lawrence, although less extensive, are of greater value for carrying sea traffic to inland places. In their torrential and upper valley tracks rivers are of use chiefly for transporting timber and driving machinery. It is interesting to note that in Switzerland, Norway, and Sweden, where there is no coal, there exist exceptional facilities for the use of water-power on account of numerous mountain-torrents. In hot countries rivers are of the utmost service in irrigating agricultural land; the Zerafshan 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 irrigation canals.

THE LARGEST RIVER-SYSTEMS.

The statistics of this table, in which account is taken of rainfall and discharge, are taken from Dr John Murray's paper in Scot. Geog. Mag. iii. (1887) p. 65. The lengths in all cases and the areas of basins in those for which no rainfall statistics are available are according to the statistical tables in Justus Perthes' Taschen-Atlas.

Rivers in Law.—A distinction is made between public navigable rivers and private fresh-water rivers. Where the tide ebbs and flows, the ownership of the bed is in the crown for behoof of the public, and, consequently, the crown is entitled to deepen the channel or perform any other operation on the alveus that may improve the navigation. The banks, however, beyond the foreshore are the private property of the riparian owner. It is settled in England—and an opinion to the same effect has been delivered in Scotland—that the public have no common law right to set up even a towing-path along the bank of a navigable river; but, of course, such a privilege of roadway along a public waterway may be established by prescriptive possession. Above the flow and reflux of the tide all rivers and streams are primâ facie private, although, either by immemorial uses or by act of parliament, many have become subject to public rights of navigation. In the case of private rivers the alveus belongs to the proprietor through whose ground the river runs; or, if the river separates the lands of two owners, each is owner of the soil of the bed to the middle of the stream. The waters of a stream passing through or between the lands of different proprietors may be subject to two kinds of rights, natural and acquired. Natural or proprietary rights are those possessed by every riparian proprietor; they consist principally of a right to a reasonable use of the water, while it is flowing past his land, and a right to have the water flow in its accustomed manner, without sensible disturbance or diminution by the superior or inferior riparian proprietors. Thus, although each proprietor may employ the water while it is within his own grounds, he must allow it to pass onwards to the inferior proprietors in its original channel, and cannot alter its level, either where it enters or leaves his property. The riparian proprietor, either in a public or private river, may protect his side of the stream by embankments; but such embankment must be constructed only for defence, and not in such a manner as to throw the force of the current upon the opposite bank. Acquired rights, on the other hand, are those easements which entitle a riparian proprietor to interfere with a natural stream of water to an extent not justified by his natural or proprietary rights—by diminishing or obstructing the flow of water, by polluting it, &c. Such acquired rights in respect of water may exist in the inhabitants of a district by virtue of immemorial custom, and, both as to kind and extent, are regulated wholly by prescriptive use.

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 pollute 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. While, however, this right to object to an existing nuisance may be excluded by acquiescence or by prescription, it is so excluded only to the extent of the actual use or possession, and any material increase of the pollution or annoyance may be challenged and interdicted by the injured parties. At common law, indeed, in every question of river-pollution, 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 prescriptive period. Questions of river-pollution are eminently fitted for submission to a jury, and are generally disposed of in that way. The whole circumstances must be considered; for example, the size and character of the stream, the uses to which it can be and is applied, the nature and importance of the use claimed and exercised by one party, as well as the inconvenience or injury to the other party. In England, where the pollution of a stream amounts to a public nuisance, the party causing it may be prosecuted by indictment, or proceeded against by information at the suit of the Attorney-general. All the chief modern sanitary acts have provisions regarding the pollution of water; but most of them are local or deal with the pollution of water used for special purposes. In 1868 a Royal Commission was appointed to consider the question of river-pollution, and its recommendations were followed in 1876 by the Rivers Pollution Act (39 and 40 Vict. chap. 75), which is applicable to both Scotland and England. See Higgins, On the Obstruction and Pollution of Water-courses (1877).—For fishing rights, see SALMON, and TROUT.

In the United States the common law of England was at first followed; but in some of the states it is expressly declared that the common law is inapplicable. Mining rights have been specially determined in some districts; and the laws as to irrigation rights have been elaborately defined in Colorado and elsewhere.