hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...

, a current in a stream or other

water body A body of water or waterbody (often spelled water body) is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ...

is the flow of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

influenced by

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...

as the water moves

downhill Downhill may refer to: Arts and entertainment * ''Downhill'' (1927 film), a British film by Alfred Hitchcock * ''Downhill'' (2014 film), a British comedy directed by James Rouse * ''Downhill'' (2016 film), a Chilean thriller directed by Patrici ...

to reduce its

potential energy In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors. Common types of potential energy include the gravitational potentia ...

. The current varies spatially as well as temporally within the stream, dependent upon the flow

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...

of water,

stream gradient Stream gradient (or stream slope) is the grade (slope), grade (or slope) of a stream measured by the ratio of drop in elevation per unit horizontal distance, usually expressed as metre, meters per kilometre, kilometer or Foot (length), feet per mile ...

, and channel geometry. In tidal zones, the current and streams may reverse on the flood tide before resuming on the

ebb tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...

. The term ''upstream'' (or ''upriver'') refers to the direction towards the source of the stream (or

river A river is a natural flowing watercourse, usually freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the ...

), i.e. against the direction of flow. Likewise, the term ''downstream'' or ''downriver'' describes the direction towards the mouth of the stream or river, in which the current flows. The term "

left bank In geography, a bank is the land alongside a body of water. Different structures are referred to as ''banks'' in different fields of geography, as follows. In limnology (the study of inland waters), a stream bank or river bank is the terrai ...

" and "

right bank In geography, a bank is the land alongside a body of water. Different structures are referred to as ''banks'' in different fields of geography, as follows. In limnology (the study of inland waters), a stream bank or river bank is the terrai ...

" refers to

banks A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. Because ...

as seen from the direction of flow.

See also