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COVER STORY: The raging, roaring river
Rivers carry excess water from the land areas that they drain to larger bodies of water or directly to the oceans. Collectively, they represent the world’s water resources, as they carry virtually all the water that is available for human use.
The four great civilizations of early human history developed in close dependence on rivers — the Sumerians on the Tigris and Euphrates rivers in Iraq, the Harrapans on the Indus in Pakistan, the Chinese on the Huang He (Hwang Ho) (Yellow River) and the Chang Jiang (Yangtze) River in central China, and the Egyptians on the Nile River in Egypt. Rivers and their valleys have continued to play important roles in the course of history; the exploration of much of North America was through river routes, and most of its major settlements were adjacent to rivers.
The importance of rivers
In addition to providing direct sources of water for domestic, agricultural, and industrial uses, rivers produce energy directly through hydropower generation (see hydroelectric power); they also provide cooling of water for many fossil and nuclear fuelled power plants. They serve as transportation routes, as carriers and natural “treatment plants” for human wastes, and as the habitats of ecologically and economically valuable fish and wildlife. The continually changing nature of rivers and their often spectacular scenic features make them a source of inspiration for many writers and artists, as well as for casual observers.
Rivers, streams, brooks
Terms such as river, stream, and brook have been applied historically; the hydrologic distinction between them is negligible.
Although a river is commonly considered a linear feature, in reality it is a tree-like branching network. The smallest branches, which do not have tributaries, are designated as first order streams. Where two first order streams join, they form a second order stream; the junction of two second order streams forms a third order stream, and so forth.
River channels
In general, a river shapes its own channel. Most of the time, however, stream flow is considerably less than required to fill the channel, and it only occasionally exceeds the channel’s capacity. This general disparity between stream flow and channel capacity arises because the channel size is determined by moderately large flows, capable of significant erosional work, that occur relatively frequently (every 2 to 3 years); very large floods do much erosional work but are too infrequent to have a long-term effect on the channel size, whereas the small flows that are present most of the time do little work. Because the channel capacity is determined by flows that occur every few years, most rivers naturally overflow out of their banks every two to three years.
Solid concentrations
Rivers carry dissolved and suspended solids as well as water. In the United States the average concentration of dissolved solids in natural (unpolluted) river water ranges from about 50 mg/l (0.06 oz/gal) in the humid western mountains and the Appalachians up to 1,000 mg/l (1.3 oz/gal) in the arid non mountainous regions of the West.
The average concentration of suspended solids is 100 mg/l (0.13 oz/gal) or less in humid forests but up to 100,000 mg/l (133 oz/gal) in desert areas.
Some famous rivers
River Tigris
The Tigris River flows 1,900 km through southeastern Turkey and Iraq, draining an area of 373,000 sq km.
Navigable only between Baghdad and al-Qurnah, the Tigris begins at Lake Golcuk in east central Turkey and flows southeast past the Turkish agricultural centre of Diyarbakir and through petroleum fields to Mosul in Iraq. The Tigris provides irrigation waters for the cultivation of wheat, barley, millet, and rice.
River Euphrates
The Euphrates, with the Tigris River, was the cradle of civilization in ancient Mesopotamia. The river begins at the confluence of the Kara Su (or Western Euphrates) and the Murat (or Eastern Euphrates), at Keban Dam in east central Turkey. Above Basra, in southern Iraq, it joins the Tigris to form the Shatt-al-Arab. In the first third of its total length of 2,735 km the Euphrates drops steeply through the foothills of the Taurus Mountains to Syria, where it is joined by the Balikh and the al-Khabur rivers. The river’s course narrows between limestone cliffs in Iraq until it reaches the town of Hit, where it enters a silt covered alluvial flood plain.
River Indus
The Indus River, one of South Asia’s longest rivers, flows for 2,900 km from its source in Tibet through northern India and then for most of its course through Pakistan. Its drainage basin covers 1,165,500 sq km. Its waters irrigate the arid Punjab plain, making it Pakistan’s most productive agricultural region, where wheat, corn, rice, millet, dates, and other fruits are grown. Large dams have been constructed by both Pakistan and India, providing water for irrigation and hydroelectric power. Tarbela Dam, when completed in 1974, was the world’s largest earth fill dam. The river’s source is the melt water and streams of the Himalayas and the Karakoram Range. The Indus flows northwestward through the mountainous Indian state of Jammu and Kashmir as well.
River Huang He
The Huang He (Hwang Ho), or Yellow River, is the second longest river in China after the Chang Jiang (Yangtze). Flowing generally east from the Tibetan Highlands to the Yellow Sea in north China, it has a length of 4,830 km. Its drainage area is more than 750,000 sq km, encompassing 49.5 million acres of farmland and a population of more than a 100 million. The river received its name because its unusually high silt content (mainly soil) gives it a yellowish appearance. The river is known as the “sorrow of China” because of its frequent, and often catastrophic, flooding. The volume of traffic is very small because the river is shallow and parts of its lower course are choked with ice during December and January.
River Yangtze
The Yangtze River, in central China, is the longest river in Asia and the third longest river in the world. Rising in western Qinghai (Tsinghai) province, the river flows 5,990 km to the East China Sea. The drainage basin of the river covers more than 1,827,000 sq km. The river’s mouth discharges 34,000 m(3) (1,200,000 ft (3)) water per second. Ancient texts call the Yangtze the “Great River,” and it is commonly referred to in China as Chang Jiang (Ch’ang Chiang, or “Long River”). The name Yangtze, although applied to the whole river in the West, is only a local name for the river in Jiangsu (Kiangsu) province.)
River Nile
The Nile River of northeastern Africa, the longest river in the world, flows north for 6,650 km from the most distant head-waters of the White Nile in Burundi to the Mediterranean Sea, crossing Rwanda, Uganda, Sudan, and Egypt. The word Nile is probably derived from the Semitic root nahal, meaning “river valley,” which later took the forms Neilos in Greek and Nilus in Latin. Although references to the Nile appear in the earliest records from the eastern Mediterranean region, the Nile’s tributaries remained unknown in Europe and the Muslim world until the modern era. In 1613, Pedro Paez, a Spanish Jesuit, visited the headwaters of the Blue Nile; in 1860, the English explorer John Speke identified the Kagera River in present-day Burundi as the southernmost source of the White Nile.
River Amazon
The Amazon, flowing southward across Brazil in the broad equatorial part of South America, has the world’s largest drainage basin, more than 7 million sq km, or nearly 5 per cent of the world’s total land area. It carries nearly 20 per cent of the Earth’s total water discharge to the ocean in a flow so powerful that it perceptibly dilutes the ocean water of the Atlantic 160 km beyond the coastline.
The Amazon has long been considered the world’s second longest river, after the Nile, with a length of about 6,450 km. In 1994, however, a Peruvian research team declared that the river’s true source was the Ucayli River, not the Maranon, which may make the Amazon the longest river in the world.
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