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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 normally confined to a channel, made up of a stream bed between banks.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. Topography: A river conducts water by constantly flowing perpendicular to the elevation curve of its bed, thereby converting the potential energy of the water into kinetic energy. Where a river flows over relatively flat areas, the river will 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 lakesediment develop conspicuous deltas at their mouths. Rivers whose mouths are in saline tidal waters may form estuaries. from the cut off section. Rivers that carry large amounts of water over time, rivers cut away at their beds, eventually forming a more gentle gradient. - Youthful river - a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider.
- Mature river - a river with a gradient that is less steep than those of youthful rivers and flows more slowly than youthful rivers. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper.
- Old river - a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains.
- Rejuvenated river - a river with a gradient that is raised by the earth's movement.
Where a river descends quickly over sloped topography, rapids with whitewaterwaterfalls occur. Rapids are often used for recreational purposes (see Whitewater kayaking). 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 levelsea, 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. Geopolitical boundaries: Rivers have been important historically in determining political boundaries.The Orange River discharges into the Atlantic ocean and forms the International boundary between various provinces and countries along its route. The source of the Orange River is in the Lesotho mountains. The Lesotho Highlands Water Project was conceived to divert water from Lesotho into the Vaal River system, itself a tributary of the Orange River, to supply the water needs of the GautengSouth Africa. Water from the Delivery tunnelAsh River. The spot has become a popular Kayaking is discharged into the venue. The Danube was a long standing 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. Rating systems: - International Scale of River Difficulty - The scale is used to rate rivers -- particularly those with rapids -- for navigation.
- Class I is the easiest and Class VI is the hardest.
- Strahler Stream Order - The Strahler Stream Order is a simple algorithm to rank rivers based on the heirarchy and strength of their tributaries.
- Headwaters are class one while the Amazon River is class twelve. 80 percent of the rivers and streams on the planet are class one and two.
Biology: The flora and fauna of rivers are much different from those of the ocean because the water is fresh (non-salty). Living things in a river must be adapted to the current of the moving water.
Water quality: Human pollution of rivers is common, and few rivers in the world are free of man-made substances. The most common point source of pollutant is probably sewage piped into rivers, but chemical pollution is also common, and industrial accidents (and/or negligence) account for significant destruction of riparian biomes. Heated water discharged into rivers by power plants and factoriesagriculture, urbanisation, logging, construction activity and slash-and-burn also affects river biota. Non-point source pollutants such as diffuse pollution from techniques are all recognised as sources of sediment and other pollutants. Techniques have been developed to analyze the dispersal of water pollutants in river systems. For example the U.S. Environmental Protection Agency funded research to develope the DSSAM Model to assess the transport of soluble nitrogen, reactive phosphorus, total dissolved solids and dissolved oxygenTruckee River, which flows through both California and Nevada. Dams: In places where the elevation changes of a river are great, dams for hydroelectric plants and other purposes are often built. This disrupts the natural flow of the river, and creates a lake behind the dam. Often the building of dams affects the whole of the river, even the part above the dam, as migrating fish are hindered , waterflow is no longer bounded by seasonal changes and sediment flow is blocked. Dams are useful in many ways, such as: providing hydro-electric power; acting as regulator of river flow so as to regulate the occurrence of flooding, which is especially important to wet-rice agriculture; and also to improve navigation and transport on the river. Often, dams such as Hoover Dam along Colorado River become famous tourist attractions. Critics of dams, especially 'green' advocates, argue that dams remove upper-river biodiversity such as through deforestation and forced migration of rural villages and indigenous tribes. Furthermore, trapping of river sediments behind the dams lead to salination and loss of nutrients for down-water fish. It also raises concern of earthquakes due to instablity of incompetent dams which have to support thousands of tonnes of sediments behind them. One very famous and problematic dam is the Aswan High Dam in the Nile. Flooding: 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 wetlands in order to produce farmland has reduced the absorption zones for excess water and made floods into sudden disasters rather than gradual increases in water flow. In ancient Egypt, life was made possible through the floods of the Nile and the accompanying siltsediment which enriched the fields with fresh nutrients. Nowadays, since people have built on these floodplains, floods are disasters, causing untold property loss each year.Human interference in the form of deforestation can also worsen conditions. The removal of vegetation leads to a reduction in interception (vegetation stopping precipitation) and the 'weakening' of soil since plant roots no longer hold it together. As a result there is a reduced Infiltration capacity (how much water the soil can hold) and greater infiltration (precipitation going into the ground). This leads to faster soil saturation and therefore greater overland flow (also known as surface runoff) and therefore, there are flash floods as the lag time decrease. Logjams: Logjams are barriers within rivers, created by dead and uprooted trees. Over time, the obstruction prevents further logs to bypass, resulting in the creation of new network channels. According to author David R. Montgomery in his book, King of Fish, a logjam also causes water to buildup within a small space, forming peaceful pools within the main channel which provide shelter for young salmon. The existence of these deep pools along with the complex web of channels creates an ideal salmon habitat. Today, many believe that the rebuilding of salmon runs is contingent upon reproducing the same environment shaped by logjams. As a result, many scientists have attempted to recreate artificial logjams. Marc Duboiski and Mike Ramsey of the Salmon Recovery Funding board staff, George Pess of the National Marine Fisheries Service, and Kevin Bauersfeld of Washington Department of Fish and Wildlife have prepared the Report to the Salmon Recovery Funding Board On the Engineered Log Jam (ELJ) Workshop, with the hope of mimicking natural logjams. Management: In its natural state a river may be inconvenient to man in a variety of ways. Rivers in inhabited areas have therefore been managed or controlled to make them more useful and less disruptive to human activity. - The river channel may be dredged to make it deeper for navigation or to prevent flooding.
- Dams or weirs may be built to control the flow, store water, or extract energy.
- Levees may be built to prevent flooding.
- Sluice gates provide a means of controlling flow and adjusting river levels.
- floodways may be added to draw off excess river water in times of flood.
- Canals connect rivers to one another for water transfer or navigation.
- River courses may be modified to improve navigation, or straightened to increase the flow rate.
River management is an ongoing activity as rivers tend to 'undo' the modifications made by man. Dredged channels silt up, sluice mechanisms deteriorate with age, levees and dams may suffer seepage or catastrophic failure.
PADI 5 Star National Geographic Instructor Development Center. 198/12 Rat-U-Thit Road, Patong, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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