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Stream order is a measure of the relative size of streams. The smallest tributaries are referred to as first-order streams, while the largest river in the world, the Amazon, is a twelfth-order waterway. First- through third-order streams are usually called headwater streams. Streams classified as fourth- through sixth-order are considered medium streams. A stream that is seventh-order or larger constitutes a river.

When diagramming stream order, scientists begin by identifying the first-order streams in a watershed. First-order streams are perennial streams--streams that carry water throughout the year--that have no permanently flowing tributaries. This means no other streams "feed" them.


Stream Types

There are three types of streams:

Perennial streams
Water flows in the stream at least 90 percent of the time in a well defined channel.

Intermittent streams
Flow generally occurs only during the wet season (50 percent of the time or less).

Ephemeral streams
Flow generally occurs for a short time after extreme storms. The channel is usually not well defined.

For more information do a search on stream order and stream type on a search engine.

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