River

River

[riv-er]
Plate, River: see Plata, Río de la.
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 erosion 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 waterfalls. 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 delta, the floodplain, 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 flood; water rights; waters, territorial.

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.

Bibliography

See M. Morisawa, Rivers (1985); J. Mangelsdorf, River Morphology (1990).

Low-lying plain composed of stream-borne sediments deposited by a river at its mouth. Deltas have been important to humankind since prehistoric times. Sands, silts, and clays deposited by floodwaters were extremely productive agriculturally; and major civilizations flourished in the deltaic plains of the Nile and Tigris-Euphrates rivers. In recent years geologists have discovered that much of the world's petroleum resources are found in ancient deltaic rocks. Deltas vary widely in size, structure, composition, and origin, though many are triangular (the shape of the Greek letter delta).

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Natural stream of water that flows in a channel with more or less defined banks. Rivers are a fundamental link in the hydrologic cycle, and they play a major role in shaping the surface features of the Earth. Even apparently arid desert regions are greatly influenced by river action when periodic floodwaters surge down usually dry watercourses. River flow is sustained by the difference between water input and output. Rivers are fed by overland runoff, groundwater seepage, and meltwater released along the edges of snowfields and glaciers. Direct precipitation contributes only very small amounts of water. Losses of river water result from percolation into porous and permeable rock, gravel, or sand; evaporation; and ultimately outflow into the ocean.

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Mountain range, central Rocky Mountains, west-central Wyoming, U.S. The range extends for 100 mi (160 km) northwest-southeast to the Sweetwater River and is part of the Continental Divide. It contains many peaks above 12,000 ft (3,658 m); the highest is Gannett Peak at 13,804 ft (4,207 m). The Oregon Trail ran through the historic South Pass (7,743 ft, or 2,360 m). Parts of Bridger and Shoshone national forests and Wind River Indian Reservation are in the range. The Wind River flows from the eastern side into the Bighorn River; the Green River rises on its western slopes.

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River, Arkansas, U.S. It rises in the Boston Mountains in northwestern Arkansas, flows north into southern Missouri, then bends southeast and reenters Arkansas. It continues south to join the Arkansas River near its confluence with the Mississippi River. It is 685 mi (1,102 km) long.

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Russian Zapadnaya Dvina Latvian Daugava

River, north-central Europe. It rises in Russia's Valdai Hills and flows 632 mi (1,020 km) in a great arc south through Russia and into Belarus and then northwest across Latvia. It discharges into the Gulf of Riga on the Baltic Sea. An important water route since early times, connected in its upper reaches by easy portages to the Dnieper, Volga, and Lovat-Volkhov river systems, it constituted part of the great trade route from the Baltic region to Byzantium and to the Arabic east. Rapids and the presence of dams have restricted navigation on it.

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River, western Russia. Europe's longest river and the principal waterway of western Russia, it rises in the Valdai Hills northwest of Moscow and flows 2,193 mi (3,530 km) southeastward to empty into the Caspian Sea. It is used for power production, irrigation, flood control, and transportation. The river has played an important part in the life of the Russian people, and in Russian folklore it is characteristically named “Mother Volga.”

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Swedish Torneälv

Northernmost river of Sweden. Issuing from Torne Lake near the Norwegian border, it flows southeast and south for 354 mi (570 km) to the Gulf of Bothnia. The lower course, strewn with rapids and mostly non-navigable, forms a section of the Sweden-Finland boundary. It is known for its salmon.

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ancient Tamesis

Principal river of England. It rises in the Cotswolds in Gloucestershire and winds 205 mi (330 km) eastward across south-central England into a great estuary, through which it empties into the North Sea. It is tidal for about 65 mi (104 km). Known by the Romans and by early English chroniclers, it has been celebrated by bards throughout history. One of the world's most important commercial waterways, it is navigable by large vessels to London.

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River, north-central U.S. It rises in southeastern Wisconsin and flows across the northwestern corner of Illinois, emptying into the Mississippi River at Rock Island, Ill.; it is 300 mi (480 km) long. The bottomlands along the lower course are subject to spring floods and require levee protection.

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ancient Tamesis

Principal river of England. It rises in the Cotswolds in Gloucestershire and winds 205 mi (330 km) eastward across south-central England into a great estuary, through which it empties into the North Sea. It is tidal for about 65 mi (104 km). Known by the Romans and by early English chroniclers, it has been celebrated by bards throughout history. One of the world's most important commercial waterways, it is navigable by large vessels to London.

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or Great Ouse

River, central and eastern England. It rises in Northamptonshire and flows 156 mi (251 km) past Buckingham, Bedford, Huntington, and St. Ives to Earith, then to the North Sea. Locks make the river navigable upstream to Bedford.

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or Lower Avon

River, southwestern England. Rising in Gloucestershire, it flows 75 mi (121 km) southwest through Bristol and into the Bristol Channel at Avonmouth, Bristol's port. Below Bristol it has cut through a limestone ridge to form Clifton Gorge, noted for its suspension bridge.

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River, south-central U.S. It rises in the high plains of eastern New Mexico and flows southeast across Texas and Louisiana to a point northwest of Baton Rouge, where it enters the Atchafalaya River. It is 1,290 mi (2,080 km) long and forms a part of Texas's borders with Oklahoma and Arkansas. In Texas it was the site of the Red River Indian War (1874).

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ancient Padus.

River, northern Italy. The country's longest river, it is about 405 mi (652 km) long. It rises in the Cottian Alps on the western frontier and flows northeast to Turin, then east across Piedmont and Lombardy into the Adriatic Sea. Its delta is one of the most complex of any European river, with at least 14 mouths. It is navigable from its mouth to Pavia. Industrial cities in its valley include Milan, Padua, and Verona. It has suffered devastating autumn floods, including those in 589, 1438, 1951, and 1966.

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River, central Mississippi, U.S. It flows southwest through Jackson, then into Louisiana and the Gulf of Mexico. About 410 mi (660 km) long, it forms the boundary between Mississippi and Louisiana. Honey Island Swamp, lying in the mid-delta area southwest of Picayune, Miss., is noted for its wildlife and fishing.

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or Great Ouse

River, central and eastern England. It rises in Northamptonshire and flows 156 mi (251 km) past Buckingham, Bedford, Huntington, and St. Ives to Earith, then to the North Sea. Locks make the river navigable upstream to Bedford.

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River, southern Africa. It rises in the Lesotho Highlands as the Sinqu River and flows west as the Orange across South Africa. It passes the southern edge of the Kalahari Desert and winds through the Namib Desert before draining into the Atlantic Ocean in South Africa. It forms the border between South Africa and Namibia. It is about 1,300 mi (2,100 km) long. There are some irrigated sections along the river and many dams but no large towns.

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River, southwestern Virginia and southern West Virginia, U.S. It is formed in North Carolina and flows north across Virginia into West Virginia, where it joins the Gauley River to form the Kanawha River. It is about 255 mi (410 km) long and is spanned by one of the longest steel-arch bridges in the world, the New River Gorge bridge near Fayetteville, W.Va., whose main span is 1,700 ft (518 m).

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ancient Halys River

River, central and north-central Turkey. The longest river wholly within Turkey and the largest in Anatolia, it rises in north-central Turkey and flows southwestward. It turns north and empties into the Black Sea after a total course of 734 mi (1,182 km). Unsuitable for navigation, it is a source of irrigation and hydroelectricity for the region.

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U.S. landscape painters of several generations, active circa 1825–70. The first of them were inspired by the natural beauty of New York's Hudson River valley and Catskill Mountains. The leading figures were Thomas Cole, Asher B. Durand, and Thomas Doughty (1793–1856). Others, such as Frederic Edwin Church and George Inness, had studied in Europe and found inspiration in the grandiose landscapes of J.M.W. Turner. By mid century they were widely admired for their depictions of a common theme, the splendour of the untamed U.S. landscape. The name Hudson River school, applied retrospectively, is extended to artists of the same vision who painted imposing scenes of the Rocky Mountains, Grand Canyon, and Yosemite Valley. The first native school of painting in the U.S., it remained the dominant school of landscape painting throughout the 19th century.

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River, western U.S. It flows from western Wyoming south into Utah, where it turns east to make a loop through the northwestern corner of Colorado. Turning south in Utah, it enters the Colorado River in Canyonlands National Park after a course of 730 mi (1,175 km). Originally known as the Spanish River, it was renamed in 1824, probably for its colour, derived in places from green soapstone banks along its course.

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formerly Glåma River

River, eastern Norway. The longest river in Scandinavia, it rises in a series of small streams near the Swedish-Norwegian border. It flows south, then west into Lake Øyeren. From there it continues south to Sarpsborg and enters Oslo Fjord at Fredrikstad, after a course of 372 mi (599 km). It is a major source of hydroelectric power. It is navigable up to Sarpsfoss, the waterfall at Sarpsborg.

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Hindi Ganga

River, northern India and Bangladesh. Held sacred by followers of Hinduism, it is formed from five headstreams rising in Uttaranchal state. On its 1,560-mi (2,510-km) course, it flows southeast through the Indian states of Uttar Pradesh, Bihar, and West Bengal. In central Bangladesh it is joined by the Brahmaputra and Meghna rivers. Their combined waters (called the Padma River) empty into the Bay of Bengal and form a delta 220 mi (354 km) wide, which is shared by India and Bangladesh. Its plain is one of the most fertile and densely populated regions in the world. Millions of Hindus bathe in the river annually at special holy places (tirthas). Many cast the ashes of their dead into its waters, and cremation temples are found along its banks in numerous places.

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River, western North Carolina, U.S. Rising in the Blue Ridge and flowing 210 mi (340 km) north through the Great Smoky Mountains into Tennessee, it then turns west to join the Holston River near Knoxville, forming the Tennessee River. Douglas Dam, part of the Tennessee Valley Authority, is on the river near the junction.

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Navigable tidal strait in the U.S. linking Upper New York Bay with Long Island Sound in New York City. It separates Manhattan and the Bronx from Brooklyn and Queens. About 16 mi (26 km) long and 600–4,000 ft (200–1200 m) wide, it connects with the Hudson River via the Harlem River and Spuyten Duyvil Creek at the northern end of Manhattan Island. Roosevelt (formerly Welfare), Wards, Randalls, and Rikers islands are in the East River, which has numerous port facilities.

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ancient Druentia.

River, southeastern France. The principal river draining the French side of the Alps toward the Mediterranean, its origin is in the Montgenèvre region. To its confluence with the Rhône River below Avignon, it is 189 mi (304 km) long. It is the site of extensive hydroelectric projects that were established after World War II.

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or Donnai River

River, southern Vietnam. Rising in the central highlands, it flows southwest for about 300 mi (480 km), joining the Saigon River northeast of Ho Chi Minh City and combining with it and other streams to form an estuary north of the Mekong delta.

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or Maenam River

River, Thailand. Flowing south from the highlands on the country's northern border to the head of the Gulf of Thailand near Bangkok, it is some 225 mi (365 km) long and is Thailand's principal river. It is important for the transport of the country's exports. It also forms a highly productive agricultural valley. The name strictly applies only to the river's lower course, which begins at the confluence of the Nan and Ping rivers and is 140 mi (225 km) long.

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River, central and southeastern North Carolina, U.S. Formed by the confluence of the Deep and Haw rivers, it flows southeast about 200 mi (320 km) to enter the Atlantic Ocean near Southport at Cape Fear. The southern estuary forms part of the Atlantic Intracoastal Waterway. A series of locks and dams makes the river navigable from Wilmington to Fayetteville.

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River, Central and South Asia. From its headsprings in the Tibet Autonomous Region of China (as the Yarlung River), it flows across southern Tibet to break through the Himalayas in great gorges (where it is known as the Dihang). It flows southwest through the Assam Valley and south through Bangladesh (where it is known as the Jamuna). There it merges with the Ganges (Ganga) to form a vast delta. About 1,800 mi (2,900 km) long, the river is an important source for irrigation and transportation. Its upper course was long unknown, and its identity with the Zangbo was only established by exploration in 1884–86.

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ancient Chrysorrhoas

River, western Syria. It flows about 50 mi (80 km) from the Anti-Lebanon Mountains past Damascus. Throughout history, channels have been cut at different levels parallel to the main branch of the river to divert its flow. The channels (of Nabataean, Aramaean, and, especially, Roman origin) fan out as they reach Damascus, irrigating a large area. This system created Al-Ghūtsubdotah, a fertile belt near Damascus.

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or Lower Avon

River, southwestern England. Rising in Gloucestershire, it flows 75 mi (121 km) southwest through Bristol and into the Bristol Channel at Avonmouth, Bristol's port. Below Bristol it has cut through a limestone ridge to form Clifton Gorge, noted for its suspension bridge.

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Longest river in Greece. It rises in the Pindus Mountains and flows south 140 mi (220 km) to the Ionian Sea. Hydroelectric dams harness its waters at Kastraki and Kremasta. In ancient mythology, Achelous was a protean river god, who was vanquished by Hercules.

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The Clark Fork is a river in the U.S. states of Montana and Idaho, approximately 360 mi (579 km) long. The largest river by volume in Montana, it drains an extensive region of the Rocky Mountains in western Montana and northern Idaho in the watershed of the Columbia River, flowing northwest through a long mountain valley and emptying into Lake Pend Oreille in northern Idaho. The Pend Oreille River, which drains the lake to the Columbia, is sometimes included as part of the Clark Fork, giving it a total length of 479 mi (771 km), with a drainage area of 25,820 sq mi (66,870 km²). In its upper 20 mi (32 km) in Montana near Butte, it is known as Silver Bow Creek. Interstate 90 follows much of the upper course of the river from Butte to northwest of Missoula.

The Clark Fork should not be confused with the Clarks Fork of the Yellowstone River, which is located in Montana and Wyoming.

Description

It rises as Silver Bow Creek in southwestern Montana, less than 5 mi (8 km) from the continental divide near downtown Butte, from the confluence of Basin and Blacktail creeks. It flows northwest and north through a valley in the mountains, passing east of Anaconda, where it changes its name to the Clark Fork, then northwest to Deer Lodge. From Deer Lodge it flows generally northwest across western Montana, passing south of the Garnet Range toward Missoula. Five miles east of Missoula, the river receives the Blackfoot River.

Northwest of Missoula, the river continues through a long valley along the northeast flank of the Bitterroot Range, through the Lolo National Forest. It receives the Bitterroot River from the south-southwest approximately 5 1/2mi west of downtown Missoula, and receives the Flathead River from the north near Paradise. It receives the Thompson River from the west near Thompson Falls in southern Sanders County.

At Noxon, Montana, along the north end of the Bitterroots near the Idaho border, the river is impounded by the Noxon Rapids Dam to form a 20 mi (32 km) long reservoir. It crosses into western Bonner County in northern Idaho near the town of Cabinet, Idaho. Approximately 5 mi (8 km) west of the Idaho-Montana state line, the river enters the eastern end of Lake Pend Oreille, near the town of Clark Fork.

History

During the last ice age, from approximately 20,000 years ago, the Clark Fork Valley lay along the southern edge of the Cordilleran ice sheet covering western North America. The encroachment of the ice sheet formed an ice dam on the river, creating Glacial Lake Missoula which stretched through the Clark Fork Valley across central Montana. The periodic rupturing and rebuilding of the ice dam released the Missoula Floods, a series of catastrophic floods down the Clark Fork and Pend Oreille into the Columbia which sculptured many of the geographic features of eastern Washington and the Willamette Valley of Oregon.

In the 19th century the Clark Fork Valley was inhabited by the Flathead tribe of Native Americans. It was explored by Meriwether Lewis of the Lewis and Clark Expedition during the 1806 return trip from the Pacific. The river is named for William Clark. A middle segment of the river in Montana was formerly known as the Missoula River.

In 1809 David Thompson of the North West Company explored the region and founded several fur trading posts, including Kullyspell House at the mouth of the Clark Fork, and Saleesh House on the river near the present-day site of Thompson Falls, Montana. Thompson used the name Saleesh River for the entire Flathead-Clark Fork-Pend Oreille river system. For most of the first half of the 19th century the Clark Fork river and surrounding region was controlled by the British-Canadian North West Company and Hudson's Bay Company.

Since the late 19th century many areas in the watershed of the river have been extensively mined for minerals, resulting in an ongoing stream pollution problem. Most pollution has come from the copper mines in Butte and the smelter in Anaconda. Many of the most polluted areas have been designated as Superfund sites. Nevertheless the river and its tributaries are among the most popular destinations for fly fishing in the United States.

Today, the Clark Fork watershed encompasses the largest Superfund site in America. As a mega-site, it includes three major sites: Butte, Anaconda, and Milltown Dam/Clark Fork River. Each of these major sites is split up into numerouse sub-sites known as Operable Units. Remediation and/or restoration of these sites is ongoing.

See also

References

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