Nature

==Water on the Earth==
[[File:44 - Iguazu - Décembre 2007.jpg|thumb|upright=1.4|The [[Iguazu Falls]] on the border between [[Brazil]] and [[Argentina]]]]
{{Main|Water}}
'''Water''' is a [[chemical substance]] that is composed of [[hydrogen]] and [[oxygen]] (H2O) and is vital for all known forms of life.<ref>{{cite web |url=https://www.un.org/waterforlifedecade/background.html |title=Water for Life |publisher=Un.org |date=March 22, 2005 |access-date=May 14, 2011 |archive-url=https://web.archive.org/web/20110514200432/http://www.un.org/waterforlifedecade/background.html |archive-date=May 14, 2011 |url-status=live |df=mdy-all }}</ref> In typical usage, ''water'' refers only to its liquid form or [[States of matter|state]], but the substance also has a solid state, [[ice]], and a [[gas]]eous state, [[water vapor]], or [[steam]]. <!--About 1.460 [[Tonne#Multiples|petatonnes]]&nbsp;(Pt) (10<sup>21</sup>&nbsp;kilograms) of water-->Water covers 71% of the [[Earth]]'s surface.<ref>{{cite web|url=https://www.cia.gov/the-world-factbook/countries/world/|title=World|work=CIA – World Fact Book|access-date=December 20, 2008|df=mdy-all}}</ref> On Earth, it is found mostly in oceans and other large bodies of water, with 1.6% of water below ground in [[aquifer]]s and 0.001% in the [[atmosphere|air]] as [[vapor]], clouds, and [[precipitation (meteorology)|precipitation]].<ref>[https://web.archive.org/web/20070320034158/http://www.agu.org/sci_soc/mockler.html Water Vapor in the Climate System], Special Report, American Geophysical Union, December 1995.</ref><ref>[https://web.archive.org/web/20080220070111/http://www.unep.org/dewa/assessments/ecosystems/water/vitalwater/ Vital Water]. [[UNEP]].</ref> Oceans hold 97% of surface water, [[glacier]]s, and polar [[ice cap]]s 2.4%, and other land surface water such as rivers, lakes, and ponds 0.6%. Additionally, a minute amount of the Earth's water is contained within biological bodies and manufactured products.

===Oceans===
[[File:Ocean from Leblon.jpg|thumb|left|A view of the Atlantic Ocean from [[Leblon]], Rio de Janeiro]]
{{Main|Ocean}}
An ocean is a major body of [[Seawater|saline water]], and a principal component of the hydrosphere. Approximately 71% of the Earth's surface (an area of some 361 million square kilometers) is covered by ocean, a [[World Ocean|continuous body of water]] that is customarily divided into several principal oceans and smaller seas. More than half of this area is over {{convert|3000|m|ft|abbr=off|sp=us}} deep. Average oceanic [[salinity]] is around 35 [[parts per thousand]] (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the [[World Ocean]] or global ocean.<ref>"[http://www.answers.com/Ocean#Encyclopedia Ocean] {{Webarchive|url=https://web.archive.org/web/20110126035624/http://www.answers.com/topic/ocean#Encyclopedia |date=January 26, 2011 }}". ''The Columbia Encyclopedia.'' 2002. New York: Columbia University Press</ref><ref name="UNAoO">"[http://www.oceansatlas.com/unatlas/about/physicalandchemicalproperties/background/seemore1.html Distribution of land and water on the planet] {{webarchive|url=https://web.archive.org/web/20080531103749/http://www.oceansatlas.com/unatlas/about/physicalandchemicalproperties/background/seemore1.html |date=May 31, 2008 }}". [http://www.oceansatlas.com/ UN Atlas of the Oceans] {{Webarchive|url=https://web.archive.org/web/20080915105349/http://www.oceansatlas.com/ |date=September 15, 2008 }}</ref> This concept of a global ocean as a continuous body of water with relatively free interchange among its parts is of fundamental importance to [[oceanography]].<ref>{{cite journal | last1 = Spilhaus | first1 = Athelstan F | year = 1942 | title = Maps of the whole world ocean | journal = Geographical Review | volume = 32 | issue = 3| pages = 431–435 | doi=10.2307/210385| jstor = 210385 | bibcode = 1942GeoRv..32..431S }}</ref>

The major oceanic divisions are defined in part by the [[continent]]s, various [[archipelago]]s, and other criteria: these divisions are (in descending order of size) the [[Pacific Ocean]], the [[Atlantic Ocean]], the [[Indian Ocean]], the [[Southern Ocean]], and the [[Arctic Ocean]]. Smaller regions of the oceans are called seas, gulfs, [[bay]]s and other names. There are also [[salt lake]]s, which are smaller bodies of landlocked saltwater that are not interconnected with the World Ocean. Two notable examples of salt lakes are the [[Aral Sea]] and the [[Great Salt Lake]].

===Lakes===
[[File:Lake mapourika NZ.jpeg|thumb|[[Lake Mapourika]], New Zealand]]
{{Main|Lake}}
A lake (from Latin word ''lacus'') is a [[terrain feature]] (or [[physical feature]]), a body of liquid on the surface of a world that is localized to the bottom of [[Depression (geology)|basin]] (another type of landform or terrain feature; that is, it is not global) and moves slowly if it moves at all. On Earth, a body of water is considered a lake when it is inland, not part of the ocean, is larger and deeper than a pond, and is fed by a river.<ref>
{{cite web
|url=https://www.britannica.com/EBchecked/topic/328083/lake
|author=Britannica Online
|access-date=June 25, 2008
|title=Lake (physical feature)
|quote=[a Lake is] any relatively large body of slowly moving or standing water that occupies an inland basin of appreciable size. Definitions that precisely distinguish lakes, ponds, swamps, and even rivers and other bodies of nonoceanic water are not well established. It may be said, however, that rivers and streams are relatively fast moving; marshes and swamps contain relatively large quantities of grasses, trees, or shrubs; and ponds are relatively small in comparison to lakes. Geologically defined, lakes are temporary bodies of water.
|archive-url=https://web.archive.org/web/20080611194244/https://www.britannica.com/EBchecked/topic/328083/lake
|archive-date=June 11, 2008
|url-status=live
|df=mdy-all
}}
</ref><ref>{{cite web|title=Lake Definition|url=http://www.dictionary.com/browse/lake|website=Dictionary.com|access-date=September 6, 2016|archive-url=https://web.archive.org/web/20160905184816/http://www.dictionary.com/browse/lake|archive-date=September 5, 2016|url-status=live|df=mdy-all}}</ref> The only world other than Earth known to harbor lakes is [[Titan (moon)|Titan]], Saturn's largest moon, which has lakes of [[ethane]], most likely mixed with [[methane]]. It is not known if Titan's lakes are fed by rivers, though Titan's surface is carved by numerous river beds. Natural lakes on Earth are generally found in mountainous areas, [[rift zone]]s, and areas with ongoing or recent [[glacier|glaciation]]. Other lakes are found in [[endorheic basin]]s or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the [[Last Glacial Period|last ice age]]. All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.

====Ponds====
[[File:Mill Pond Sunset.jpg|thumb|The Westborough Reservoir (Mill Pond) in [[Westborough, Massachusetts]]]]
{{Main|Pond}}
A '''pond''' is a [[body of water|body]] of [[Water stagnation|standing water]], either natural or human-made, that is usually smaller than a lake. A wide variety of human-made bodies of water are classified as ponds, including [[water garden]]s designed for aesthetic ornamentation, [[fish pond]]s designed for commercial fish breeding, and [[solar pond]]s designed to store thermal energy. Ponds and lakes are distinguished from streams via [[current (stream)|current]] speed. While currents in streams are easily observed, ponds and lakes possess thermally driven micro-currents and moderate wind driven currents. These features distinguish a pond from many other aquatic terrain features, such as [[stream pool]]s and [[tide pool]]s.

===Rivers===
[[File:View from Cairo Tower 31march2007.jpg|thumb|left|The [[Nile]] river in [[Cairo]], Egypt's capital city]]
{{Main|River}}
A river is a natural [[watercourse]],<ref>[http://www.merriam-webster.com/dictionary/river River {definition}] {{Webarchive|url=https://web.archive.org/web/20100221200545/http://www.merriam-webster.com/dictionary/river |date=February 21, 2010 }} from Merriam-Webster. Accessed February 2010.</ref> usually [[freshwater]], flowing towards 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, due to vagueness in the language.<ref>[http://geonames.usgs.gov/domestic/faqs.htm USGS – U.S. Geological Survey – FAQs] {{Webarchive|url=https://web.archive.org/web/20150701155523/http://geonames.usgs.gov/domestic/faqs.htm |date=July 1, 2015 }}, No. 17 What is the difference between mountain, hill, and [[Summit (topography)|peak]]; lake and pond; or river and [[stream|creek]]?</ref> A river is part of the [[hydrological cycle]]. Water within a river is generally collected from [[Precipitation (meteorology)|precipitation]] through [[surface runoff]], [[groundwater]] recharge, [[Spring (hydrology)|springs]], and the release of stored water in natural ice and snowpacks (i.e., from [[glacier]]s).

===Streams===
[[File:Hawaii Creek.jpg|thumb|A rocky stream in [[Hawaii]]]]
{{Main|Stream}}
A stream is a flowing body of water with a [[Current (stream)|current]], confined within a [[stream bed|bed]] and [[stream bank]]s. In the United States, a stream is classified as a watercourse less than {{convert|60|ft|m|abbr=off}} wide. Streams are important as conduits in the [[water cycle]], instruments in [[groundwater recharge]], and they serve as corridors for fish and [[wildlife]] migration. The biological [[habitat]] in the immediate vicinity of a stream is called a [[riparian zone]]. Given the status of the ongoing [[Holocene extinction]], streams play an important [[wildlife corridor|corridor]] role in connecting [[Habitat fragmentation|fragmented habitats]] and thus in conserving [[biodiversity]]. The study of streams and waterways in general involves many branches of inter-disciplinary natural science and engineering, including [[hydrology]], [[geomorphology|fluvial geomorphology]], [[aquatic ecology]], [[Freshwater fish|fish biology]], [[riparian|riparian ecology]], and others.