WATER ON THE EARTH

Water is a chemical substance that is composed of hydrogen and oxygen and is vital for all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapour or steam.

Lake: A body of liquid on the surface of a world that is localized to the bottom of basin (another type of land form 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.

Ocean: A major body of saline water and a principal component of the hydrosphere. Approximately 71% of the Earth’s surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. 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.

Pond: A body of standing water, either natural or man-made, that is usually smaller than a lake. A wide variety of man-made bodies of water are classified as ponds, including water gardens designed for aesthetic ornamentation, fish ponds designed for commercial fish breeding, and solar ponds designed to store thermal energy.

River: 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, due to 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).

Stream: A flowing body of water with a current, confined with a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beek, burn, creek, kill, lick, rill, river, syke, bayon, rivulet, or run. In some countries or communities a stream may be defined by its size. In the United States a stream is classified as a watercourse less than 60 feet (18 metres) 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 study of steams and waterways in general is known as surface hydrology and is a core element of environmental geography.

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