Rivers
A river is a large natural waterway. The source of a river may be a lake, a spring, or a collection of small streams, known as headwaters. From their source, all rivers flow downhill, typically terminating in the ocean. The mouth, or lower end, of a river is known as its base level.
A river's water is confined to a channel, made up of a stream bed between banks. Most rainfall on land passes through a river on its way to the ocean. Smaller side streams that join a river are tributaries. The scientific term for any flowing natural waterway is a stream; so in technical language, the term river is just a shorthand way to refer to a large stream.
A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the meander: start to form loops and snake through the plain by eroding the river banks. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake from the cut off section. Rivers that carry large amounts of sediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form estuaries.
Over time, rivers cut away at their beds, eventually forming a more gentle gradient.
Where a river descends quickly over sloped topography, rapids with whitewater or even waterfalls occur. Rapids are often used for recreational purposes. Waterfalls are sometimes used as sources of energy, via watermills and hydroelectric plants.
Rivers begin at their source in higher ground, either rising from a spring, forming from glacial meltwater, flowing from a body of water such as a lake, or simply from damp, boggy places where the soil is waterlogged. They end at their base level where they flow into a larger body of water, the sea, a lake, or as a tributary to another (usually larger) river. In arid areas rivers sometimes end by losing water to evaporation and percolation into dry, porous material such as sand, soil, or pervious rock. The area drained by a river and its tributaries is called its watershed, catchment basin or drainage basin. (Watershed is also used however to mean a boundary between drainage basins.)
Starting at the mouth of the river and following it upstream as it branches again and again, the resulting river network forms a dendritic (tree-like) structure that is an example of a natural random fractal.
Rivers have been important historically in determining political boundaries. For example, the Danube was a longstanding border of the Roman Empire, and today forms most of the border between Bulgaria and Romania. The Mississippi in North America, and the Rhine in Europe, are major east-west boundaries in those continents. The Orange River forms the boundary between various provinces and countries along its route in Africa.
The flora and fauna of rivers are much different from those of the ocean because the water is fresh (non-salty).
Flooding is a natural part of a river's cycles. Human activity, however, has upset the natural way flooding occurs by walling off rivers and straightening their courses. Removal of bogs, swamps and other habitat. Today, many believe that the rebuilding of salmon runs is contingent upon reproducing the same environment shaped by logjams. As a result, many In wintertime, rivers can be apart, it can lead to flash flooding downstream.
The world's ten longest rivers
Well-known rivers (in alphabetic order)
Rivers Wikipedia
Amazon Losing "Flying Rivers," Ability to Curb Warming National Geographic - December 19, 2009
Britain cut off from Europe by 'super-river' as little as 30,000 years ago Telegraph.co.uk - November 30, 2009
Amazon River Dated to 11 Million Years Old Live Science - July 9, 2009
Amazon River Is 11 Million Years Old, Drilling Study Finds Science Daily - July 8, 2009
Amazon Longer Than Nile River, Scientists Say National Geographic - June 18, 2007
Santa Fe Tops 2007 List of Most Endangered Rivers National Geographic - April 18, 2007
World's Longest Underground River Discovered in Mexico, Divers Say National Geographic - March 6, 2007
Scientists Study Mysterious Stop-Start River KLS.com - November 21, 2006
In fall and winter it has a natural cycle, about 20 minutes on, 20 minutes off. The only other spring like it in the world is in France. It's a point of pride in nearby Afton, Wyoming. Al Hale, Afton, Wyo. Resident: "Well the folklore is that the Indians were the first ones to see this phenomenon." Just before it erupts, the spring emits a deep gurgling noise. A rising puddle quickly becomes a surprisingly vigorous roaring creek. Kip Solomon, University of Utah Hydrologist: "Well, everything about this spring is somewhat surprising. It's an extremely unusual occurrence."
The town of Afton built a structure to protect their water supply. It's very cold, very pure, and it tastes good. It's won national awards. Rulon Gardner, Olympic Gold Medalist: "Of course! You know, Star Valley water. It's the best in the world." Olympic gold medalist Rulon Gardner's great-great grand-dad is credited with the discovery. Rulon Gardner, Olympic Gold Medalist: "He was up there logging. He went up and found a nice little place to get some fresh water. It was intermittent. It went, and stopped. So it was pretty amazing."
In late summer, scientists collected water samples. They're exploring an old theory involving a mysterious underground chamber. Prof. Kip Solomon: "We can't think of another explanation at the moment." Here's the theory: As groundwater flows continuously into a cavern, it fills a narrow tube that leads out. As it pours over the high point of the tube, it creates a siphon effect, sucking water out of the chamber. Eventually air rushes in and breaks the siphon. Gerald Vanbrunt, Arkansas Tourist: "It's kind of like a toilet flushing. All the water goes out, it fills back up, and goes back out." The spring water's gas content has now been tested at the University of Utah. The data strongly suggests the water was exposed to air underground; strong support for the siphon theory. Prof. Kip Solomon: "Yeah, I think that we're a step closer to the answer." Someday, science may have a definitive answer. For now, we can just enjoy the natural wonder of an on-again, off-again, mystery river. The intermittent spring is at the end of a half-mile hike, in a canyon straight east of Afton, Wyoming.
Amazon river 'switched direction' BBC - October 24, 2006
Over millions of years, the Amazon has reversed its flow
The world's largest river, the Amazon, once flowed from the Atlantic Ocean to the Pacific - the opposite of its present direction, a study shows. Sedimentary rocks in the central part of South America contain ancient mineral grains that must have come from the eastern part of the continent. Geologist Russell Mapes says this must mean that about 145-65 million years ago, the Amazon flowed east to west. The age of rocks on the South American continent differs between east and west. Rocks as old as 2.5 billion years are found on the eastern side of the continent. Because of continual geological activity in the Andes, on the western side, rocks there are much younger.....
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