River.
Publié le 11/05/2013
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IV RIVER PATTERNS
River patterns, or general shapes, depend on the geologic zone and the climate of the location.
There are four river patterns: meandering, braided, anastomosing, andstraight.
A meandering pattern follows a winding, turning course.
A braided pattern has connected channels that resemble a hair braid.
An anastomosing river patterncombines features of the meandering and braided patterns.
Some river patterns are simply straight channels.
Meandering and braided are the most common patterns.Braided and straight patterns are usually located in the mountains or hills below the headwater zone of rivers, while meandering and anastomosing patterns are locatedin the middle and mouth zones of most rivers.
The Mississippi River is a classic example of a meandering river that has looping bends of different sizes along its valley.
Each bend is the result of sediment depositingon the inside of the bend.
As sediment deposits gradually build up, a point bar forms on the inside of the bend.
The point bar pushes the river flow against the outsidebank of the bend, eroding the bank opposite the point bar.
Eventually the bend becomes so sharp that the river bypasses it, cutting a straighter path.
The arc of thebend is left behind as the river moves past.
The arc may form an oxbow lake (also called a billabong), a pool of water enclosed by the arc and riverbank.
A meanderingriver’s bed is usually covered with sand, while the floodplain is filled with silt and clay.
Braided rivers look completely different from meandering rivers.
They have many channels that are constantly changing position because of frequent changes in flowrate and sediment supply.
The channels of a braided river—such as portions of the Platte River that flow through Nebraska—change course frequently, so the river’swater may cover the entire floodplain on a regular basis.
The sediments of braided rivers are usually gravel and cobbles.
Sometimes a meandering river may changeinto a braided river in the middle zone if the supply of sediment increases as a result of farming or grazing activities in the watershed.
Anastomosing rivers combine the bends of meandering rivers with the multiple channels of braided rivers.
The sediments are typically sand, silt, and clay.
Oxbow lakesmay be rare.
The Amazon River is an example of an anastomosing river.
Straight rivers are not common.
They are typically located in canyons in mountainous areas or exist as the result of engineering structures that force a river into astraight course.
Portions of both the Columbia River (between Washington and Oregon) and the Colorado River (in the southwestern United States) flow straightthrough canyons.
V MEASURING RIVERS
Rivers come in many different sizes.
Scientists and geographers rank rivers according to their length, flow rate, or sediment supply.
Scientists have traditionallyconsidered the Nile River to be the longest river in the world, although in the 1990s some debate arose as to whether the Amazon River is longer, as new satellite mapsrevealed a small tributary in the Andes Mountains.
The flow rate of a river is the volume of water that passes a section of the river in a unit of time.
Scientists calculate flow rate by multiplying the depth of the river by itswidth and the speed of the flowing water.
Flow rate is usually expressed in cubic meters per second or cubic feet per second (cfs).
Flow rate is an important measurement when examining a river’s size.
The average flow rate of the Amazon River is about 200,000 cu m/second (7,100,000 cfs).However, during flood levels the discharge of the Amazon increases to nearly 300,000 cu m/second (10,000,000 cfs).
For comparison, the average flow rate of theMississippi River is 16,800 cu m/second (593,000 cfs), and its flood discharge at St.
Louis, Missouri, during the floods in the summer of 1993 was 30,000 cu m/second(1,000,000 cfs).
Scientists measure the amount of sediment in a river in two ways: from a boat or by means of a satellite.
Using a boat, they collect a water sample and filter thesediment out of the water.
Higher concentrations of sediment cause the water to become more turbid, or cloudy.
Using images of rivers collected by satellites, scientistscan analyze the color of the water for patterns of sediment concentration in the channel and on the floodplain.
During floods, small mountain rivers may have sedimentconcentrations 1,000 times higher than those of large rivers because the small rivers are still in the mountain zone, where the stream is steep and the sediment supplyfrom hillsides is rapid.
For example, during the El Niño storms of 1998, the Santa Clara River in California had sediment concentrations of 60 g/liter (0.5 lb/gallon).
Incontrast, the Amazon River rarely carries more than 0.3 g/liter (0.003 lb/gallon).
VI IMPORTANCE
Rivers are important to humans because they supply fresh drinking water, serve as home for important fisheries, provide transportation routes, and are the source forirrigation water and hydroelectric power.
Humans have used rivers since the beginning of civilization.
In Asia, people have revered the life-giving importance of rivers forthousands of years.
Many ancient temples are located near streams and rivers that needed protection to ensure high-quality water for society.
The Chinese writtencharacters for the word politics express the sense of responsibility for waterways—the literal interpretation of the characters includes the meaning of “protection of water.” Many of the ancient, legendary leaders in China were respected because of their ability to control water so that fields could be irrigated and floods prevented.The first great African civilization began along the banks of the Nile around 5000 BC.
The agricultural wealth along the valley of the Nile River gave the pharaohs in ancient Egypt their power.
Many pyramids and shrines stood along the banks of the Nile.
Other important aspects of rivers are the ecological characteristics of river channels and floodplains.
These areas provide a zone between land and water environments.Floodplains and channels are diverse habitats that support the world’s largest wetlands, which are home to innumerable species of plants and animals.
Most of the fishthat live in rivers use the channel and floodplain, and in some rivers, the deltas and estuaries, during their life cycle.
VII CONSERVATION AND PRESERVATION
There is increasing uncertainty regarding the possible effects of global climate change on worldwide patterns of rainfall and snowfall.
Hence, the conservation andpreservation of rivers and their corridors have become even more important.
Surveys show that the supply of potable, or drinkable, water is poorly distributed aroundthe globe and that the largest unpolluted rivers are far from the centers of densest population.
Human use, especially damming and agricultural use, has affected over77 percent of the annual discharge of the large rivers in the northern third of the world.
Many studies show that there are approximately 36,000 dams over 15 m (45ft) high that, when full, contain 20 percent of the annual runoff—rainfall not absorbed by soil—for the globe.
While offering some benefit to humans, these dams havereduced the ability of rivers to transport water and sediment to the ocean.
This change affects the ecology of rivers as well as the biology of the oceans receiving theriver water.
Some of the oldest dams have stopped functioning because their reservoirs have filled with a huge amount of sediment.
Dams also block the passage of fishupstream to spawning grounds.
Some of these dams are now being removed and their river corridors restored for fisheries and wetlands, but at a tremendous cost.
The Clean Water Act of 1972 passed by the United States Congress and similar laws in other countries have changed the way that pollution is allowed to enter riversystems.
In the 1960s some rivers were so badly polluted that they actually caught fire and burned, including, in 1969, the Cuyahoga River in Cleveland, Ohio.
Todaythose once-polluted rivers have new parks on their banks.
Conservation of rivers is also important in other parts of the world.
People in the countries that share theRhine River watershed in Europe are working together to help salmon return to the river..
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