page_title: River ecosystem
section_title: Water flow
hierarchical_section_title: River ecosystem / Abiotic factors / Water flow
caption_attribution_description: English: Cooplacurripa River, NSW
context_page_description: River ecosystems are flowing waters that drain the landscape, and include the biotic interactions amongst plants, animals and micro-organisms, as well as abiotic physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks.
  River ecosystems are prime examples of lotic ecosystems. Lotic refers to flowing water, from the Latin lotus, meaning washed. Lotic waters range from springs only a few centimeters wide to major rivers kilometers in width. Much of this article applies to lotic ecosystems in general, including related lotic systems such as streams and springs. Lotic ecosystems can be contrasted with lentic ecosystems, which involve relatively still terrestrial waters such as lakes, ponds, and wetlands. Together, these two ecosystems form the more general study area of freshwater or aquatic ecology.
  The following unifying characteristics make the ecology of running waters unique among aquatic habitats.
  Flow is unidirectional.
  There is a state of continuous physical change.
context_section_description: Unidirectional water flow is the key factor in lotic systems influencing their ecology. Stream flow can be continuous or intermittent, though. Stream flow is the result of the summative inputs from groundwater, precipitation, and overland flow. Water flow can vary between systems, ranging from torrential rapids to slow backwaters that almost seem like lentic systems. The speed or velocity of the water flow of the water column can also vary within a system and is subject to chaotic turbulence, though water velocity tends to be highest in the middle part of the stream channel (known as the thalveg). This turbulence results in divergences of flow from the mean downslope flow vector as typified by eddy currents.  The mean flow rate vector is based on variability of friction with the bottom or sides of the channel, sinuosity, obstructions, and the incline gradient.  In addition, the amount of water input into the system from direct precipitation, snowmelt, and/or groundwater can affect flow rate. The amount of water in a stream is measured as discharge (volume per unit time). As water flows downstream, streams and rivers most often gain water volume, so at base flow (i.e., no storm input), smaller headwater streams have very low discharge, while larger rivers have much higher discharge. The "flow regime" of a river or stream includes the general patterns of discharge over annual or decadal time scales, and may capture seasonal changes in flow.
  While water flow is strongly determined by slope, flowing waters can alter the general shape or direction of the stream bed, a characteristic also known as geomorphology. The profile of the river water column is made up of three primary actions: erosion, transport, and deposition. Rivers have been described as "the gutters down which run the ruins of continents". Rivers are continuously eroding, transporting, and depositing substrate, sediment, and organic material. The continuous movement of water and entrained material creates a variety of habitats, including riffles, glides, and pools.
caption: A pensive Cooplacurripa River, NSW
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