|Kawarau River in Queenstown|
Rivers are bodies of flowing surface water driven by gravity. Hydrologists, scientists who study the flow of water, refer to all bodies of flowing water as streams. In common language, it is accepted to refer to rivers as larger than streams. Water flowing in rivers is only a very small portion of Earth’s fresh water.
The oceans contain about 96% of the water on Earth, and most fresh water is bound up in glacial ice near the North and South Poles. Rivers shape the landscape and are integral to the hydrologic cycle (circulation of water on and around Earth) on the continents.
Rivers shape the lands as they erode (wear away) and deposit sediment (particles of gravel, sand, and silt) along their courses. Running river water acts to level the continents. When geologic forces slowly raise (uplift) mountain ranges, rivers wear them away.
The streams that form the Ganges River of India (headwater streams), for example, are presently tearing down the Himalayas almost as quickly as they are uplifted by the movements of Earth’s crustal plates (plate tectonics).
When geologic forces create depressions or low areas on the continents, rivers act to fill them. River sediment replenishes floodplain (Flat land next to rivers that are subject to flooding) soils and coastal sands.
Earth’s major rivers, including the Nile, Amazon, Yangtze, and Mississippi, drain the waters of vast continental areas and set down (deposit) huge deposits of sediments at the ends of rivers that flow into the ocean (for example, in deltas at the end of many rivers) Rivers host vibrant communities of plants and animals, and refill groundwater reservoirs and wetlands that support biological life far beyond their banks.
Rivers are a main focus of human interaction with the natural environment. Human agriculture, industry, and biology require fresh, accessible water from rivers.
|Small boat in Nile river|
Ancient human civilizations first arose in the fertile valleys of the world’s great rivers: the Yangtze and Yellow Rivers in China, the Tigris and Euphrates Rivers in the Middle East, and the Nile River in Egypt. The distribution of Earth’s rivers and systems of rivers has influenced human population patterns, commerce, and conquest since ancient times.
Rivers flow through the great cities of the world, and the imagery of rivers is deeply embedded in our language, culture, and history. Today, billions of people depend directly and indirectly on rivers for food and water, transportation and recreation, and spiritual and religious inspiration.
Almost all major rivers are today confined by man-made dams and levees (walls along the banks) that provide people with the means to generate electricity and protection from floods. These alterations to rivers have come at an environmental cost.
When floodwaters are contained by levees or other flood-control dams, they no longer supply nutrients and sediment to floodplain soils that support agriculture. Furthermore, dams and levees that upset a river’s natural path and profile (side view) cause changes to the patterns of erosion and deposition (depositing sediments) throughout the entire river system.
Dams have contributed to beach erosion on many coastlines because dams trap sediment in reservoirs. Agricultural and urban development along riverbanks has threatened many species of plants and animals that live in riverside wetlands.
Also, the very dams and levees that prevent frequent small floods create an increased risk of infrequent, disastrous flooding. The city of New Orleans, for example, lies at a lower elevation than the bed of the Mississippi River that runs through the center of the city in an artificial channel behind massive levees. If the levees failed, a flash flood would engulf the city and potentially threaten the lives of its residents.
Earth’s largest river systems define the natural and human environment within their watersheds. A watershed is the land area that drains water into a river or other body of water. A list of the world’s major rivers is also a list of the major natural and cultural geographic regions on six continents. (The continent Antarctica is too cold for liquid water. Its fresh water is bound up in large masses of moving ice called glaciers.)
- Africa: The Nile is, by most measurements, the world’s longest river. (River lengths are difficult to measure because rivers constantly shift their courses and change length.
There is also disagreement about which branches of water (tributaries) are considered part of the main river. By some measurements, the Amazon River in South America is actually slightly longer than the Nile.) The Nile has sustained life in the inhospitable Sahara desert of eastern Africa for thousands of years.
Its headwater (uphill end) streams flow from lakes in Ethiopia and Uganda and feed two branches, the White Nile and the Blue Nile, which meet in the Sudanese city of Khartoum. From there, the Nile cuts a green-bordered lifeline through the Egyptian desert.
It flows through Cairo, the bustling capital of modern-day Egypt, past the pyramids of Giza and the ancient Egyptian capital of Thebes, to its outlet in the Mediterranean Sea. The Congo River (called the Zaire River from 1971 to 1997) makes a long loop through the equatorial rainforests and war-torn nations of central western Africa.
The Congo is the main trade and travel route into the African interior, and it is the setting for Joseph Conrad’s famous novel Heart of Darkness. The Limpopo, Okavango, Ubangi, and Zambezi are other major African rivers.
- Asia: Huge rivers drain water from the massive Asian continent into the Pacific, Indian, and Arctic oceans. In China, the Yangtze (Chang Jiang), Yellow (Huang He) and Pearl Rivers carry flowing waters (runoff) from the northern slope of the Himalayan Mountains and western China to the East China Sea.
Hundreds of millions of Chinese people depend on these rivers for their electricity, food, and livelihoods. Water moving south from the Himalayas flows into the rivers of India and South Asia, including the Ganges-Bramaputra system and the Mekong River. The Ganges River of northern India is sacred in the Hindu religion.
Hindus travel to its banks to meditate and wash away their sins. Upon death, cremated remains are placed into the Ganges in hopes of improving the deceased’s fortunes in the afterlife. The Ob, Ikysh, Amur and Lena Rivers run across the northern forests and wind-swept tundra (treeless arctic plains) of Siberia (the Asian portion of Russia) into the icy Arctic Ocean.
In the Middle East, rivers play an important role in the history and mythology of western civilization. The ancient civilizations of Sumeria and Mesopotamia arose in the “fertile crescent” between the Tigris and Euphrates Rivers (Shat-al-Arab) in what is today Iraq. Along with the Jordan River, they play major roles in Jewish, Christian, and Islamic history.
- Australia: The island continent of Australia has only a few major rivers, and its central desert, the outback, is extremely dry. The Murray River and its major tributary (major branch), the Darling, make up Australia’s largest river system. The Murray drains water from the southeastern states of Victoria, New South Wales and southern Queensland and its floodplains are Australia’s most productive farmlands.
- Europe: Rivers are intertwined in the history, culture, and geography of Europe. The capital cities of Europe are synonymous with their rivers (London and Thames, Paris and Seine, Vienna, Budapest and Danube). By their very names, the Rhone (France), Rhine (Germany), Volga (Russia), Oder and Elbe (Germany, Poland, Czech Republic), Po and Tiber (Italy), and Ebro (Spain) conjure images of great art and fine wine, desperate battles and bloody conquests, grand castles and ancient hamlets.
- North America: The Mississippi and its major tributaries, the Missouri, Ohio, and Arkansas Rivers, collect water from a huge drainage basin that spans the central plains of North America between the Rocky Mountains and the Appalachians.
Canada’s Mackenzie and Churchill Rivers empty into the Arctic Ocean, and the St. Lawrence River empties the Great Lakes into the Atlantic Ocean. The mighty Yukon River of northern Canada and Alaska carried prospectors to mines and mills during the Alaskan gold rush (1898–99).
Many of the great ports of the Atlantic seaboard and Gulf of Mexico lie near river mouths (the end of a river where the river empties into a larger body of water): New York (Hudson), Philadelphia and Washington, D.C. (Potomac, Susquahana), Norfolk (Delaware), New Orleans (Mississippi), and Houston (Brazos). Rivers, including the Mississippi, Missouri, Colorado, Rio Grande, and Columbia, played central roles in European exploration and settlement of the American West.
Today, the rivers that carried explorers Meriwether Lewis, William Clark, John Wesley Powell, and other legendary frontiersmen across the continent are used for agricultural irrigation, drinking water, recreation, and power generation. Their water is a valuable and heavily-sought resource.
- South America: The Amazon is the largest river in the world. It flows from the Andes Mountains of Peru, across the Brazil and empties into Atlantic Ocean on the northeast coast of Brazil.
The Amazon has more than 1,100 tributaries, 17 of which are longer than 1,000 miles (1,609 kilometers) long. The main river runs from west to east just a few degrees south of the equator, and its massive watershed lies entirely within the warm, wet tropical zone.The central Amazon contains Earth’s lushest, wettest, most biologically diverse rainforest. The Orinoco (Venezuela), Sao Francisco (Brazil), Parana (Argentina, Paraguay) and Uruguay (Uruguay, Brazil) rivers are other major waterways of South America.
|Amazon river, aerial view|