A stream is a body of water〔.〕 with a current, confined within a bed and stream banks. Depending on its location or certain characteristics, a stream may be referred to as a ''branch'', ''brook'', ''beck'', ''burn'', ''creek'', ''crick'', ''gill'' (occasionally ''ghyll''), ''kill'', ''lick'', ''mill race'', ''rill'', ''river'', ''syke'', ''bayou'', ''rivulet'', ''streamage'', ''wash'', ''run'', or ''runnel''.
Streams are important as conduits in the water cycle, instruments in groundwater recharge, and 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 corridor role in connecting fragmented habitats and thus in conserving biodiversity. The study of streams and waterways in general is known as ''surface hydrology'' and is a core element of environmental geography.〔U.S. Geological Survey (USGS). Atlanta, GA. ("What is hydrology and what do hydrologists do?" ) 2009-05-13.〕
; Brook: A stream smaller than a creek, especially one that is fed by a spring or seep. It is usually small and easily forded. A brook is characterised by its shallowness and its bed being composed primarily of rocks.
* In North America, Australia and New Zealand, a small to medium-sized natural stream. Sometimes navigable by motor craft and may be intermittent.
* In parts of New England,〔Spruce Creek Association. Kittery, ME. ("About the Spruce Creek Watershed." ) Accessed 2010-10-02.〕 the UK and India, a tidal inlet, typically in a salt marsh or mangrove swamp, or between enclosed and drained former salt marshes or swamps (e.g. Port Creek separating Portsea Island from the mainland). In these cases, the stream is the tidal stream, the course of the seawater through the creek channel at low and high tide.
; River: A large natural stream, which may be a waterway.
; Runnel: the linear channel between the parallel ridges or bars on a shoreline beach or river floodplain, or between a bar and the shore. Also called a swale.
; Tributary: A contributory stream, or a stream which does not reach the sea but joins another river (a parent river). Sometimes also called a branch or fork.
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