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river,stream of water larger than a brook or creek. Land surfaces are never perfectly flat, and as a result the runoff after precipitation tends to flow downward by the shortest and steepest course in depressions formed by the intersection of slopes. Runoffs of sufficient volume and velocity join to form a stream that, by the erosionerosion
, general term for the processes by which the surface of the earth is constantly being worn away. The principal agents are gravity, running water, near-shore waves, ice (mostly glaciers), and wind.
..... Click the link for more information. of underlying earth and rock, deepens its bed; it becomes perennial when it cuts deeply enough to be fed by groundwater or when it has as its source an unlimited water reservoir, for example, the St. Lawrence flowing from the Great Lakes.
The lowest level to which a river can erode its bed is called base level. Sea level is the ultimate base level, but the floor of a lake or basin into which a river flows may become a local and temporary base level. Cliffs or escarpments and differences in the resistance of rocks create irregularities in the bed of a river and can thus cause rapids and waterfallswaterfall,
a sudden unsupported drop in a stream. It is formed when the stream course is interrupted as when a stream passes over a layer of harder rock—often igneous—to an area of softer and therefore more easily eroded rock; the edge of a cliff or plateau; or the
..... Click the link for more information. . A river tends to eliminate irregularities and to form a smooth gradient from its source to its base level. As it approaches base level, downward cutting is replaced by lateral cutting, and the river widens its bed and valley and develops a sinuous course that forms exaggerated loops and bends called meanders. A river may open up a new channel across the arc of a meander, thereby cutting off the arc and creating an oxbow lake.
Rivers modify topography by deposition as well as by erosion. River velocity determines quantity and size of rock fragments and sediment carried by the river. When the velocity is checked by changes of flow or of gradient, by meeting the water mass of lakes or oceans, or by the spreading of water when a stream overflows its banks, part of the load carried by the stream is deposited in the riverbed or beyond the channel. Landforms produced by deposition include the deltadelta
[from triangular shape of the Nile delta, like the Greek letter delta], a deposit of clay, silt, and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load.
..... Click the link for more information. , the floodplainfloodplain,
level land along the course of a river formed by the deposition of sediment during periodic floods. Floodplains contain such features as levees, backswamps, delta plains, and oxbow lakes.
..... Click the link for more information. , the channel bar, and the alluvial fan and cone.
The discharge, or rate of outflow, of a river depends on the width of its channel and on its velocity. Velocity is governed by the volume of water, the slope of the bed, and the shape of the channel (which determines the amount of frictional resistance). River volume is affected by duration and rate of precipitation in the drainage basin of the river. A river system may be enlarged by piracy, or the process by which one river, cutting through the divide that separates its drainage basin from that of another river, diverts the waters of the other into its own channel.
Traditionally river systems have been classified according to their stage of development as young, mature, or old. The young river is marked by a steepsided valley, steep gradients, and irregularities in the bed; the mature river by a valley with a wide floor and flaring sides, by advanced headward erosion by tributaries, and by a more smoothly graded bed; and the old river by a course graded to base level and running through a peneplain, or broad flat area. The age classification of rivers is diminishing in popularity now that quantitative studies of river behavior are more common.
See also floodflood,
inundation of land by the rise and overflow of a body of water. Floods occur most commonly when water from heavy rainfall, from melting ice and snow, or from a combination of these exceeds the carrying capacity of the river system, lake, or the like into which it runs.
..... Click the link for more information. ; water rightswater rights,
in law, the qualified privilege of a landowner to use the water adjacent to or flowing through his property. The privilege, also known as riparian rights, may be modified or even denied because of the competing needs of other private-property holders or of the
..... Click the link for more information. ; waters, territorialwaters, territorial,
all waters within the jurisdiction, recognized in international law, of a country. Certain waters by their situation are controlled by one nation; these include wholly enclosed inland seas, lakes, and rivers.
..... Click the link for more information. .
Important River Systems
River valleys have been important centers of civilization; they afford travel routes, and their alluvial soils form good agricultural lands. Navigable rivers are important in commerce and have influenced the location of cities. Rivers with sufficient velocity and gradient can be used to produce hydroelectric power. Among the most important river systems of the world are the Nile, the Congo, the Niger, the Zambezi, and the Orange-Vaal in Africa; the Amazon, the Orinoco, and the Paraguay-Paraná in South America; the Mississippi-Missouri, the St. Lawrence, the Rio Grande, the Colorado, the Columbia, the Mackenzie-Peace, and the Yukon in North America; the Danube, the Rhine, the Rhône, the Seine, the Po, the Tagus, the Thames, the Loire, the Elbe, the Oder, the Don, the Volga, and the Dnieper in Europe; the Tigris, the Euphrates, the Ob-Irtysh, the Yenisei, the Lena, the Syr Darya, the Amu Darya, the Amur, the Huang He, the Chang (Yangtze), the Ganges, the Brahmaputra, the Indus, the Ayeyarwady, and the Mekong in Asia; and the Murray-Darling in Australia.
See M. Morisawa, Rivers (1985); J. Mangelsdorf, River Morphology (1990).
a stream of water flowing in a natural channel and fed by surface and underground runoff from its basin. The study of rivers, river hydrology, is one of the branches of land hydrology.
General characteristics. A river’s place of origin is called its source, and the place where it flows into a sea, lake, or another river is known as its mouth.
Rivers that flow directly into oceans, seas, or lakes or disappear in deserts and swamps are called primary rivers, and rivers that flow into primary rivers are called tributaries. A primary river and all its tributaries make up a river system, characterized by the density of its network. The land surface from which the river system takes its water is called the watershed, or drainage basin. The drainage basin and the upper layers of the earth’s crust together constitute the river basin, which includes a river system and is separated from other river systems by divides.
Rivers usually flow through long low-lying relief forms, or valleys. The lowest part of the valley is called the riverbed, and the part of the valley floor that is flooded by high water is termed the floodplain, or floodplain terrace. Deep places, called pools, and shallow segments, or shoals, alternate in riverbeds. The line of greatest depths in a riverbed constitutes the channel, or fairway, and the line of greatest current velocities is called the race. The difference in elevation between the source and mouth of a river is called its drop. The ratio of a river’s drop to its length (or the ratio of the drop of particular segments to their length) is the gradient of a river (or segment) and is expressed as a percent or per mille figure.
Rivers are subdivided into mountain and flatland rivers, depending on the relief of the terrain through which they flow, although many rivers have alternating mountain and flatland stretches. Mountain rivers usually have steep gradients and swift currents and flow in narrow valleys; erosion processes predominate. Flatland rivers typically have winding channels, or meanders, formed by channel processes. In flatland rivers, sections of channel erosion alternate with sections of sediment accumulation, causing the formation of shallows and bars and, at the mouth, deltas. Sometimes arms branching out from a river merge with other rivers; the phenomenon is called bifurcation.
Rivers are distributed very unevenly over the earth’s surface. The principal divides on each continent are the boundaries of areas of runoff that flow into different oceans. The earth’s main divide separates the surface of the continents into two primary basins: the Atlantic-Arctic basin, from which runoff enters the Atlantic and Arctic oceans, and the Pacific basin, whose runoff flows into the Pacific and Indian oceans. The runoff from the first basin is significantly greater than the runoff from the second. The density of the river network and direction of flow depend on present-day natural conditions, although they preserve some characteristics of past geological ages.
The greatest density occurs in the equatorial zone, containing the world’s largest rivers, the Amazon and Congo. In the tropical and temperate zones river networks are also very dense, especially in such mountain regions as the Alps, the Caucasus, and the Rockies. In desert regions there are intermittent rivers (intermittent streams), which become turbulent streams as a result of an influx of snowmelt or heavy rainfall. Examples include the rivers of the Kazakh plains, the wadis of the Sahara, and the creeks of Australia.
Regime. The current velocity of rivers varies greatly, ranging from a few cm/sec in flatland rivers to 6–7 m/sec in mountain rivers; it is also distributed unevenly over a cross section of a river. When the water level rises, the current velocity usually increases in deep sections and decreases in shallow parts. In addition to the current, which generally flows parallel to the channel, there are spiral circulation currents, whose nature and direction depend on the configuration of the channel in cross section and plan. In many rivers the water temperature is evenly distributed over the entire cross section because of turbulent mixing. During warm periods, the water temperature follows (with some lag) the changes in air temperature. During freeze-up, the water temperature is about 0°C.
Ice phenomena are observed on rivers over roughly one-fourth of the earth’s land area; they occur almost entirely in the northern hemisphere. In the USSR the freeze-up comes first to the rivers of Northeastern Siberia (in late September) and last to the rivers in the southwestern European USSR and in Middle Asia (late December and early January). The thickest ice layer, averaging 1.5 m to 2 m, forms on the Eastern Siberian rivers and lasts nine or ten months. The swift current of mountain rivers prevents the formation of an ice sheet, but during the winter many such rivers carry large amounts of slush ice.
Fluctuations in water level are related to changes in the discharge, or rate of flow, expressed in cu m per sec. Water levels and discharges and the fluctuations they undergo—these are the primary characteristics of a river’s regime. Fluctuations in discharge vary, depending primarily on long-term seasonal cycles and the seasonal periodicity of the water volume.
Rivers are an important part of the earth’s hydrologic cycle. They distribute fresh water over land areas and return water to the ocean. River water has a high rate of water exchange. The total volume of water in the channels of the world’s rivers is roughly 1,200 cu km, and this water is replaced about 33 times a year, or every 11 days.
The sources of river water are liquid precipitation, the snow cover, alpine snows and glaciers, and underground water. Rivers almost never have just one type of source. They are usually fed by mixed sources, with one source predominating. The Rioni River in the USSR, for example, is fed by rain in its lower reaches and snowmelt and glacier water in its upper reaches. The chief phases of the streamflow regime of a river, governed by the source of water, are high water, freshet, and low water.
The rivers of the equatorial zone are deep throughout the year, with a certain tendency toward increased flow in the autumn; the surface runoff is derived exclusively from rain. In tropical savanna, rain is the predominant source of water, and the water volume is proportional to the length of the wet and dry seasons. In wet savanna, high water lasts from six to nine months, and in dry savanna, it decreases to three months. There is considerable summer runoff. In the Mediterranean-type sub-tropics, most rivers have a medium or low water volume, with winter runoff predominating. In the eastern oceanic sectors of this belt (Florida, the lower reaches of the Yangtze River) and in the vast areas of Southeast Asia adjoining the oceanic sectors, the regime of rivers is affected by the monsoons. The maximum water volume occurs in the summer and the minimum in the winter.
In the temperate zone of the northern hemisphere, water typically rises in the spring, owing chiefly to rain in the south and to snowmelt in the central and northern areas, where more or less stable low water occurs in summer and winter. An extreme type of temperate-zone river regime, which develops under markedly continental conditions, is a brief spring high water, after which the river dries up for most of the year. Examples include the rivers of the region north of the Caspian Sea and the Kazakh plains. Summer high water caused by rain occurs on the rivers of the Far East because of the monsoons.
In permafrost regions, rivers typically dry up in winter, a phenomenon that is sometimes inaccurately called freezing solid. Aufeis forms on some Eastern Siberian and Ural rivers during the freeze-up. In the subarctic the snow cover thaws late in the year, and therefore the spring high water carries over into summer. On the polar ice caps of Antarctica and Greenland, ablation processes in narrow peripheral areas result in the formation of unique rivers in ice channels. These rivers are fed entirely by glacial waters during the short summers.
Mountain rivers have a distinct streamflow regime. Their feeding and water volume are governed by altitude zonation, which varies depending on the exposure of the slopes. The regimes of mountain rivers generally follow the pattern mentioned above for latitudinal zonation. They range from snow and glacier feeding in the alpine zone to the type of regime characteristic of the particular latitudinal zone at the foot of the mountains. With the exception of the rivers in the mountains of Central Asia and the mountains of the Atacama Desert, mountain rivers are usually deep. This is especially true of the southwestern slope of the Scandinavian Mountains, southern Alaska, and the South Island of New Zealand, where the annual depth of runoff of some rivers reaches 10,000–12,000 mm. (See Table 1 for data on the dimensions and discharge of the world’s largest rivers.)
Rivers are responsible for a great deal of erosion—both linear erosion by streams in channels and on floodplains and soil erosion by surface (slope) runoff in the drainage basins. (See Table 2 for data on the amount of sediment [solid load] and dissolved material transported by rivers.) The present-day mechanical and chemical work of rivers, expressed as the annual depth of the solid and dissolved load, is 0.077 mm a year for the world’s land area. The load ranges from a maximum of 0.16 mm in Asia to a minimum of 0.014 mm in Africa.
River runoff is a highly important source of fresh water. In terms of the annual volume of runoff, the Soviet Union is second only to Brazil, accounting for one-eighth of the runoff of the world’s rivers. However, in terms of runoff per unit of area, expressed in depth of runoff, the USSR is among the water-poor countries, primarily because its average precipitation
|Table 1. Principal rivers of the world1|
|Length (km)||Drainage basin (sq km)||Discharge (cu m per sec)||Annual depth of runoff (mm)|
|1 All measurements of river lengths and basin areas are based on maps. The resulting discrepancies may be explained by the different degrees of measurement precision depending on the methodology and scale of the maps used, by the different degrees of map precision, by the arbitrary nature of the concepts of the beginning and end points of a river and of its main channels (if it branches), and by the actual change in the length of a river because of natural or artificial channel straightening (as much as dozens and even hundreds of kilometers). 2The Mississippi proper is 3,950 km long|
|Enisei (with the Angara and Selenga).........||5,075||2,580,000||19,800||219|
|Mississippi-Missouri (North America).........||6.4202||3,268,000||19,000||163|
|Amazon-Marañón (South America)..........||6,400||7,180,000||175,000||770|
|Paraná (South America).................||4,380||4,250,000||15,000||170|
|Mackenzie-Peace (North America)...........||4,250||1,804,000||14,000||247|
|Yukon (North America)..................||3,700||855,000||6,300||255|
|St. Lawrence (North America)..............||3,350||1,269,000||9,800||300|
|Orinoco (South America).................||2,730||1,086,000||29,000||850|
(about 500 mm annually) is considerably less than that of other parts of the world. The total runoff is divided into two parts, which differ by origin and economic importance: underground runoff and surface runoff (see Table 2). The former is regulated naturally and therefore can be used throughout the year. The latter generally becomes available for use through regulation by means of reservoirs.
The average annual volume of regulated runoff is estimated at 1,855 cu km, which increases the stable world runoff by 15 percent; the corresponding figures for the USSR are 280 cu km and 28 percent. Changes in runoff occur under the influence of various measures taken to increase the productivity of agriculture and forestry. These activities promote an increase in the infiltration capacity of soil, the accumulation of water in soil, and an increase in the expenditure of soil moisture for evaporation. As a result of these processes, runoff decreases. For example, in the Federal Republic of Germany, agricultural and forestry practices caused runoff to diminish by about 15 percent from 1931 to 1960, compared to the period between 1891 and 1930.
Organic world. The flora and fauna of rivers consist of benthos, plankton, and nekton. Rivers are inhabited by various benthic animals, whose composition depends on the nature of the bottom; they are especially varied on compact ground. Thickets of higher water plants (phytobenthos) are found primarily along stretches with slow currents. These thickets, as well as stones overgrown by algae and sometimes also mosses, serve as the habitat and food for many small animals. Freely floating organisms suspended in the water (plankton) are represented by semimicroscopic and microscopic animals, or zoo-plankton (copepods and rotifers) and algae (phytoplankton). Fish belong to the nekton—free-swimming organisms capable of moving against the current. Invertebrate animals and certain plants, both floating and bottom-rooted, serve as food for the fish. The lower reaches and deltas of rivers are richest in fish.
The biological productivity of rivers varies considerably under the influence of the planned or spontaneous effect of human economic activity. The building of reservoirs increased the water surface of many rivers, changed their regimes and food supply (not only the quantity of food organisms but also the biomass), and increased the amount of plankton, which is usually little developed in riverbeds. Thus, new fishing areas have been created, permitting a larger output. But the building of
|Table 2. Runoff and solid and dissolved loads of rivers, by continent and in the USSR|
|Annual river runoff (cu km)||Solid load (million tons)||Dissolved load (million tons)||Runoff regulated by reservoirs (cu km)||Stable runoff (cu km)||Runoff per capita (cu m per year)|
|1Excluding the Canadian Arctic Archipelago and including Central America and the West Indies including Tasmania, New Guinea, and New Zealand excluding Greenland, the Canadian Arctic Archipelago, and Antarctica; the total runoff ice and water from these areas is estimated at roughly 2,200 sq km4Including runoff regulated by lakes, excluding about 300 cu km of transit runoff|
|North America1 .......||5,960||1,740||4,220||2,030||410||500||2,400||7,640||19,100|
|All land area3.........||38,830||11,885||26,945||22,355||2,480||1,855||14,0254||3,955||10,963|
dams and hydroengineering complexes hinders the migration and reproduction of the most valuable migratory fish, especially salmon and sturgeon. Pollution by industrial and domestic sewage, waste from timber rafting, and fertilizer and toxic chemicals washed from farm fields have a negative effect on biological productivity. To compensate for the loss, migratory and freshwater fish are bred artificially on a large scale; experiments are being conducted to acclimatize certain species; and fish-farming is expanding.
Economic importance. Rivers are an extremely important part of the natural environment and are closely related to other components of the environment. Since ancient times, rivers have attracted man as a source of drinking and industrial water, as a natural waterway and, during the winter, a sleigh route, as a perpetually self-replenishing source of waterpower, and as a receiver of water drained from adjacent marshy areas. They are valuable sources of fish. Floodplains generally have fertile soils and very rich meadows, which are often used for crops. Main railroad lines and highways usually follow river valleys, and most cities and towns are located along rivers.
River water is a major water resource. The annual world water intake from rivers, partly from subterranean horizons, reached almost 3,600 cu km in the early 1970’s, with more than 75 percent of this amount being used for irrigation farming. Of the 600 cu km of water taken from water resources for all types of nonagricultural water supply, lost water (water included in products and water losses to evaporation) totals 150 cu km, or less than 1 percent of the stable river runoff. Moreover, 450 cu km of waste water is produced and, after preliminary decontamination or without it, poured into rivers or lakes, where it pollutes about 5,000–6,000 cu km of river runoff, or almost 15 percent of the total river runoff. As a result of these unfavorable phenomena, the waters of many rivers are so polluted, especially between floods, that it is impossible to use them for drinking and domestic purposes without labor-intensive purification. For this reason, groundwater is used for water supply in regions with significant pollution.
The problem of the pollution of river water is especially serious in Europe and North America, especially in the eastern part of the USA, and in certain regions of Asia. A number of legislative, technical, and sanitary measures are being taken to combat river pollution. They should lead to a gradual cessation of the dumping of waste water into rivers and lakes—to a separation of polluted water from sources of water. Such measures include developing techniques for waterless and waste-free industrial production, reusing specially processed waste water in industry and agriculture, reducing water use per unit of industrial and agricultural output, and carefully decontaminating waste water, along with developing methods to fully purify it. Also important is the regulation of river flow by surface and, especially, subterranean reservoirs and the diversion of river water from regions with an excess of water.
REFERENCESLopatin, G. V. Nanosy rek SSSR. Moscow. 1952.
Davydov, L. K. Gidrografiia SSSR, parts 1–2. Leningrad, 1953–55.
Zhadin, V. I., and S. V. Gerd. Reki, ozera i vodokhranilishcha SSSR, ikh fauna i flora. Moscow, 1961.
Voskresenskii, K. P. Norma i izmenchivost’ godovogo stoka rek Sovetskogo Soiuza, Leningrad, 1962.
Apollov, B. A. Uchenie o rekakh, 2nd ed. Moscow, 1963.
Alekin, O. A., and L. V. Brazhnikova. Stok rastvorennykh veshchestv s territorii SSSR. Moscow, 1964.
Velikanov, M. A. Gidrologiia sushi, 5th ed. Leningrad, 1964.
Glushkov, V. G. Voprosy teorii i metody gidrologkheskikh issledovanii. Moscow, 1961.
Sokolov, A. A. Gidrografiia SSSR. Leningrad, 1964.
Avakian, A. B., and V. A. Sharapov. Vodokhranilishcha gidroelektrostantsii SSSR, 2nd ed. Moscow, 1968.
Kalinin, G. P. Problemy global’noi gidrologii. Leningrad, 1968.
Sokolovskii, D. L. Rechnoi stok. Leningrad, 1968.
Shnitnikov, A. V. Vnutrivekovaia izmenchivost’ komponentov obshchei uvlazhnennosti. Leningrad, 1969.
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M. I. L’VOVICH
What does it mean when you dream about a river?
Rivers, like other bodies of water, may represent the dreamer’s emotional state. Watching a river roll by may indicate that one is allowing his or her life to float on down the river without any particular direction, perhaps indicating that one should take a more decisive hand in directing one’s life. With too little control in one’s life, the river may have raging waters that run up over its banks. If the water is peaceful and tranquil, then a restful break to regenerate one’s energies might be in order. In some mythologies rivers are symbols of death, which could also be interpreted as the passing from one state of consciousness to another.