From Wikipedia, the free encyclopedia
"Riverine" redirects here. For a navigable river of interest to maritime geographers, see
maritime geography. For a member of the Mobile Riverine Force, see
Riverines.
A
river is a natural
watercourse,
[1] usually
freshwater, flowing toward an
ocean, a
lake, a
sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including
stream, creek,
brook, rivulet, and rill; there is no general rule that defines what can be called a river. An exception to this is the stream. In some countries or communities a
stream may be defined by its size. Many names for small rivers are specific to geographic location; one example is
Burn in Scotland and North-east England.
.^ Because of the difficulty of crossing rivers, they have sometimes become territorial boundaries.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
[3]Topography
The water in a river is usually confined to a
channel, made up of a
stream bed between
banks. In larger rivers there is also a wider floodplain shaped by
flood-waters over-topping the channel. Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and floodplain can be blurred especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry.
The term upriver refers to the direction leading to the source of the river, which is against the direction of flow. Likewise, the term downriver describes the direction towards the mouth of the river, in which the current flows.
.^ Where possible, diverted water may be returned to the river clean and near the point of diversion; specifically, some municipal water supplies treat their sewage and return fairly clean water to the river.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
^ Because flow is not constant, many rivers produce floods and droughts.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
Extensive braided rivers are now found in only a few regions worldwide, such as the
South Island of
New Zealand. They also occur on
peneplains and some of the larger river deltas. Anastamosing rivers are similar to braided rivers and are also quite rare. They have multiple sinuous channels carrying large volumes of sediment.
.^ New Orleans sits on the delta formed where the Mississippi River flows into the Gulf of Mexico.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
^ Alluvial deposits can form deltas where the river flows into a lake or ocean.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
According to Brahm's law (sometimes called Airy's law), the mass of objects that may be flown away by a river is proportional to the sixth power of the river flow speed. Thus, when the speed of flow increases two times, it can transport 64 times larger (i.e., more massive) objects.
[4] In mountainous torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the destruction of larger rocks. In U-shaped
glaciated valleys, the subsequent river valley can often easily be identified by the V-shaped channel that it has carved.
.^ New Orleans sits on the delta formed where the Mississippi River flows into the Gulf of Mexico.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
^ Alluvial deposits can form deltas where the river flows into a lake or ocean.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
.^ Alluvial deposits can form deltas where the river flows into a lake or ocean.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
.^ Farms and cities have been developed on large deltas.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
.^ Where possible, diverted water may be returned to the river clean and near the point of diversion; specifically, some municipal water supplies treat their sewage and return fairly clean water to the river.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
Throughout the course of the river, the total volume of water transported downstream will often be a combination of the free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain (called the
hyporheic zone).
.^ Because flow is not constant, many rivers produce floods and droughts.- River Systems of the World 12 January 2010 5:52 UTC www.rev.net [Source type: FILTERED WITH BAYES]
Classification
Although the following classes are a useful way to visualize rivers, there are many other factors at work.
Gradient is controlled largely by tectonics, but discharge is controlled largely by climate, and sediment load is controlled by various factors including climate, geology in the headwaters, and the stream gradient.
- Youthful river
- a river with a steep gradient that has very few tributaries and flows quickly. Its channels erode deeper rather than wider. (Examples: Brazos River, Trinity River, Ebro River)
- Mature river
- a river with a gradient that is less steep than those of youthful rivers and flows more slowly. A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. (Examples: Mississippi River, St. Lawrence River, Danube River, Ohio River, River Thames)
- Old river
- a river with a low gradient and low erosive energy. Old rivers are characterized by flood plains. (Examples: Huang He River, Ganges River, Tigris, Euphrates River, Indus River, Nile River)
- Rejuvenated river
- a river with a gradient that is raised by tectonic uplift.
The straight-line distance from the beginning to the end of most rivers is about one third their actual length.
[5][6]The way in which a river's characteristics vary between the upper course and lower course of a river is summarized by the
Bradshaw model.
An
intermittent river (or
ephemeral river) only flows occasionally and can be dry for several years at a time. These rivers are found in regions with limited or highly variable rainfall, or can occur because of geologic conditions such as having a highly permeable river bed. Some ephemeral rivers flow during the summer months but not in the winter. Such rivers are typically fed from chalk aquifers which recharge from winter rainfall. In the UK these rivers are called
Bournes and give their name to place such as
Bournemouth and
EastbourneUses
Many
riverbanks in
Japan are used as places for playing, recreation and parties
Rivers have been used as a source of water, for obtaining food, for
transport, as a defensive measure, as a source of
hydropower to drive machinery, for bathing, and as a means of disposing of waste.
In some heavily-forested regions such as
Scandinavia and
Canada,
lumberjacks use the river to float felled trees downstream to lumber camps for further processing, saving much effort and cost by transporting the huge heavy logs by natural means.
Rivers have been a source of food since pre-history.
[10] They can provide a rich source of
fish and other edible aquatic life, and are a major source of fresh water, which can be used for drinking and
irrigation. It is therefore no surprise to find most of the major
cities of the world situated on the banks of rivers. Rivers help to determine the
urban form of cities and neighbourhoods and their corridors often present opportunities for
urban renewal through the development of
foreshoreways such as
Riverwalks. Rivers also provide an easy means of disposing of waste-water and, in much of the less developed
world, other
wastes.
Fast flowing rivers and waterfalls are widely used as sources of energy, via
watermills and
hydroelectric plants. Evidence of watermills shows them in use for many hundreds of years such as in the
Orkneys at Dounby click mill. Prior to the invention of steam power, water-mills for grinding
cereals and for processing
wool and other textiles were common across
Europe. In the 1890s the first machines to generate power from river water were established at places such as
Cragside in
Northumberland and in recent decades there has been a significant increase in the development of large scale power generation from water, especially in wet mountainous regions such as
NorwayThe coarse
sediments,
gravel and
sand, generated and moved by rivers are extensively used in construction. In parts of the world this can generate extensive new lake habitats as gravel pits re-fill with water. In other circumstances it can destabilise the river bed and the course of the river and cause severe damage to spawning fish populations which rely on stable gravel formations for egg laying.
Rivers have been important in determining political boundaries and defending countries. For example, the
Danube was a long-standing border of the
Roman Empire, and today it 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 and
Limpopo Rivers in southern
Africa form the boundaries between provinces and countries along their routes.
Ecosystem
The
flora and
fauna of rivers use the aquatic habitats available, from torrential
waterfalls through to lowland mires. Although many organisms are restricted to the fresh water in rivers, some, such as
salmon and
hilsa, have adapted to be able to survive both in rivers and in the sea. The organisms in the
riparian zone respond to changes in river channel location and patterns of flow. For example, in rapidly
migrating streams,
ecological successions develop in accordance with the prevailing patterns of erosion and deposition.
Chemistry
The chemistry of rivers is complex and depends on inputs from the atmosphere, the geology through which it travels and the inputs from man's activities. The chemistry of the water has a large impact on the ecology of that water for both
plants and
animals and it also affects the uses that may be made of the river water. Understanding and characterising river water chemistry requires a well designed and managed programme of sampling and analysis
Like many other
aquatic ecosystems, rivers too are under increasing threat of pollution. According to a study of the
WWF's Global Freshwater Programme, the 10 most polluted rivers are:
Ganges,
Indus,
Yangtze,
Salween-Nu,
Mekong-
Lancang,
Rio Grande/Rio Bravo,
La Plata,
Danube,
Nile-
Lake Victoria, and the
Murray-Darling.
[11]Brackish water
Nile River delta, as seen from Earth orbit. The Nile is an example of a wave-dominated delta that has the classic Greek delta (Δ) shape after which River deltas were named. Photo courtesy of
NASA.
Some rivers generate brackish water by having their river mouth in the ocean. This, in effect creates a unique environment in which certain species are found.
Flooding
Flooding is a natural part of a river's cycle. The majority of the erosion of river channels and the erosion and deposition on the associated
floodplains occur during
flood stage. In many developed areas, human activity has changed river channel form, altering different magnitudes and frequencies of flooding. Some examples of this are the building of levees, the straightening channels, and the draining of natural
wetlands. In many cases human activities in rivers and floodplains have dramatically increased the risk of flooding. Straightening rivers allows water to flow more rapidly downstream increasing the risk of flooding places further downstream. Building on flood plains removes flood storage which again exacerbates downstream flooding. The building levees may only protect the area behind the levees and not those further downstream. Levees and flood-banks can also increase flooding upstream because of back-water pressure as the upstream water has to squeeze between the levees.
Flow
Direction
Rivers flow downhill from
river source to
river mouth, but they do not necessarily take the shortest path. For
alluvial streams, straight and
braided rivers have very low sinuosity and flow directly down hill, while
meandering rivers flow from side to side across a valley. Bedrock rivers typically flow in either a
fractal pattern, or a pattern that is determined by weaknesses in the bedrock, such as
faults,
fractures, or more erodible layers.
Rate
Volumetric flow rate can be thought of as the mean velocity of the flow through a given cross-section, times that cross-sectional area. Mean velocity can be approximated through the use of the
Law of the Wall. In general, velocity increases with the depth (or
hydraulic radius) and slope of the river channel, while the cross-sectional area scales with the depth and the width: the double-counting of depth shows the importance of this variable in determining the discharge through the channel.
Management
Rivers are often managed or controlled to make them more useful, or less disruptive, to human activity.
- Dams or weirs may be built to control the flow, store water, or extract energy.
- Levees, known as dikes in Europe, may be built to prevent river water from flowing on floodplains or floodways.
- 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 a continuous activity as rivers tend to 'undo' the modifications made by people. Dredged channels silt up, sluice mechanisms deteriorate with age, levees and dams may suffer seepage or catastrophic failure. The benefits sought through managing rivers may often be offset by the social and economic costs of mitigating the bad effects of such management. As an example, in parts of the developed world, rivers have been confined within channels to free up flat flood-plain land for development. Floods can inundate such development at high financial cost and often with loss of life.
Rating systems
- International Scale of River Difficulty – The scale is used to rate the challenges of navigation—particularly those with rapids. Class I is the easiest and Class VI is the hardest.
- Strahler Stream Order – The Strahler Stream Order ranks rivers based on the connectivity and hierarchy of contributing tributaries. Headwaters are first order while the Amazon River is twelfth order. Approximately 80% of the rivers and streams in the world are of the first and second order.
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