A
river is a natural
watercourse, 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.
Many names for small rivers are specific to geographic location;
one example is
Burn in Scotland and North-east England.
Sometimes a river is said to be larger than a creek, but this is
not always the case, because of vagueness in the language.
A river is part of the
hydrological
cycle. Water within a river is generally collected from
precipitation through
surface runoff,
groundwater recharge,
springs, and the release of stored
water in natural ice and snowpacks (i.e., from
glaciers).
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 is referred 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.
The river channel typically contains a single stream of water, but
some rivers flow as several interconnecting streams of water,
producing a
braided river.
Extensive
braided rivers are now found in only a few regions worldwide, such
as in southmost Alabama
and 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.
A river flowing in its channel is a source of energy which acts on
the river channel to change its shape and form. 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. 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. In the middle reaches where the river may flow over flatter
land,
meanders may form through erosion of
the river banks and deposition on the inside of bends. Sometimes
the river will cut off a loop, shortening the channel and forming
an
oxbow lake or
billabong. Rivers that carry large amounts of
sediment may develop conspicuous
deltas at their mouths, if conditions permit.
Rivers whose mouths are in saline
tidal waters
may form
estuaries.
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). For many rivers in large valleys, this unseen component
of flow may greatly exceed the visible flow.
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.
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.
Most rivers flow on the surface; however
subterranean rivers flow underground in
caves or caverns. Such rivers are frequently
found in regions with
limestone geologic formations.
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 Eastbourne
Uses
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.
Rivers have been used for navigation for thousands of years.
The
earliest evidence of navigation is found in the Indus Valley Civilization, which
existed in northwestern Pakistan
around 3300 BC. Riverine navigation
provides a cheap means of transport, and is still used extensively
on most major rivers of the world like the Amazon, the Ganges, the
Nile, the Mississippi, and the Indus
. Since river boats are often not regulated,
they contribute a large amount to global
greenhouse_gas emissions, and to local cancer due to
inhaling of
particulates emitted by the transports.
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. 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 Norway
The 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.
The
beauty of rivers and their surroundings contributes to tourist
income in many parts of the world from Shakespeare's Avon to
the wilds of Alaska
's glacier streams.
In upland rivers,
rapids with
whitewater or even
waterfalls occur. Rapids are often used for
recreation, such as
whitewater
kayaking.
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.
Brackish water
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
On a large scale, rivers always flow down hill from
river source to
river
mouth. On small scales, rivers almost always flow downhill,
with a few exceptions being small-scale flow over obstacles during
floods where the pressure of the water overcomes the force of
gravity. Not all rivers move in the shortest path down hill,
however. 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, also
called discharge, volume flow rate, and rate of water flow, is the
volume of water which passes through a given cross-section of the
river channel per unit time. It is typically measured in
cubic meters per second (cumec) or
cubic feet per second (cfs),
where 1 m³/s = 35.51 ft³/s; it is sometimes also measured in
litres or
gallons per
second.
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.
Rivers are increasingly managed for
habitat conservation, as they are
critical for many
aquatic and
riparian plants,
resident and
migratory fish,
waterfowl,
birds of
prey,
migrating bird, and many
mammals.
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.
Gallery
Image:Río
Peralonso 2.jpg| Río Peralonso - El Zulia
(Norte de
Santander), Colombia
.Image:River_gambia_Niokolokoba_National_Park.gif|
River
Gambia
flowing through Niokolo-Koba
National Park
, Senegal
.Image:Howrah Bridge 01.jpg|
Bridges are a common way of crossing rivers.
(Hooghly River
, Kolkata
, India
)Image:Victoria5.jpg| Zambezi
and Victoria Falls
in Zambia
and
Zimbabwe
Image:Hooghly River 02.jpg| Hooghly River, Kolkata, India.Image:Chosen5.jpg|
Heathcote National Park,
Australia.
Image:Chosen4.JPG|This river flows from
Woronora
Dam
, Sydney
, Australia.Image:PolandSzczecinPanorama.JPG|Oder River in Szczecin
, Poland
.Image:Freshwater river redirection.png|A
schematic showing how a
freshwater river
can be disconnected from a
seawater
mouth.
See also
Crossings
Transport
References
- Hans-Henrik Stølum: "River Meandering as a Self-Organization
Process", Science 271 (5256), 1710
- Fermat's last theorem, Simon Singh, 1997
- Panda.org
- DOI.org
- DOI.org
- NMP.org
- Top 10 most polluted rivers
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