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A stream is a flowing body of water with a detectable current. Synonyms include creek, river, rio, waterway, run, branch, and fork.

In the United States, an intermittent stream is one that only flows for part of the year and is marked on topographic maps with a line of blue dashes and dots. A blue-line stream is one which flows for most or all of the year and is marked on topographic maps with a solid blue line.

Streams in geographic terms are awarded order designations. A stream of the first order is a blue-line stream which does not have any other blue-line stream feeding into it. A stream of the second order is one which is formed by the joining of two or more blue-line streams. A third-order stream is one below the confluence of two or more second-order streams; a fourth-order stream is formed by the confluence of at least two third-order streams, and so forth.

Typically, streams are said to have a particular profile, beginning with steep gradients, no flood plain, and little shifting of channels, eventually evolving into streams with low gradients, wide flood plains, and extensive meanders. The initial stage is sometimes termed a "young" stream, and the later state a "mature" or "old" stream. However, a stream may meander for some distance before falling into a "young" stream condition.

The gradient of a stream is a critical factor in determining its character, and is entirely determined by its base level of erosion. The base level of erosion is the point at which the stream either enters the ocean, a lake or pond, or enters a stretch in which it has a much lower gradient, and may be specifically applied to any particular stretch of a stream. In geologic terms, the stream will erode down through its bed to achieve the base level of erosion throughout its course. If this base level is low, then the stream will rapidly cut through underlying strata and have a steep gradient, and if the base level is relatively high, then the stream will form a flood plain and meanders.

When a stream flows over an especially resistant stratum and forms a waterfall or cascade[?], or the same results because for some reason the base level of erosion suddenly drops, perhaps as a result of a fault, the resulting sudden change in stream elevation is called a nickpoint. The stream, of course, expends kinetic energy in "trying" to eliminate the nickpoint.

Meanders are looping changes of direction of a stream. These may be somewhat sine-wave in form. Typically, over time, the meanders don't disappear but gradually migrate downstream. However, if some resistant material slows or stops the downstream movement of a meander, a stream may erode through the neck between two legs of a meander to become temporarily straighter, leaving behind an arc-shaped body of water termed an oxbow lake or bayou. A flood may also result in a meander being cut through in this way.

The study of streams and waterways in general is known as surface hydrology and is important in environmental geography or environmental geology.

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