Stream Order

A Classification of the Rank of Streams and Rivers

View of complex river system from the air


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One of the most important aspects of physical geography is the study of the world's natural environment and resources - one of which is water. Because this area is so important, geographers, geologists, and hydrologists alike use stream order to study and measure the size of the world's waterways.

A stream is classified as a body of water that flows across the Earth's surface via a current and is contained within a narrow channel and banks.

Based on stream order and local languages, the smallest of these waterways are also sometimes called brooks and/or creeks. Large waterways (at the highest level the stream order) are called rivers and exist as a combination of many tributary streams. Streams can also have local names such as bayou or burn.

The Stream Order

Geological Society of America Bulletin

When using stream order to classify a stream, the sizes range from a first-order stream all the way to the largest, a 12th order stream. A first-order stream is the smallest of the world's streams and consists of small tributaries. These are the streams that flow into and "feed" larger streams but do not normally have any water flowing into them. In addition, first and second order streams generally form on steep slopes and flow quickly until they slow down and meet the next order waterway.

First through third order streams are also called headwater streams and constitute any waterways in the upper reaches of the watershed. It is estimated that over 80% of the world’s waterways are these first through third order or headwater streams.

Going up in size and strength, streams that are classified as fourth through sixth order are medium streams while anything larger (up to 12th order) is considered a river. For example, to compare the relative size of these different streams, the Ohio River in the United States is an eighth order stream while the Mississippi River is a tenth order stream. The world’s largest river, the Amazon in South America, is considered a 12th order stream.

Unlike the smaller order streams, these medium and large rivers are usually less steep and flow slower. They do however tend to have larger volumes of runoff and debris as it collects in them from the smaller waterways flowing into them.

Going Up in Order

If however, two streams of different order join neither increases in order. For example, if a second order stream joins a third order stream, the second order stream simply ends by flowing its contents into the third order stream, which then maintains its place in the hierarchy.

The Importance of Stream Order

Stream order also helps people like biogeographers and biologists in determining what types of life might be present in the waterway. This is the idea behind the River Continuum Concept, a model used to determine the number and types of organisms present in a stream of a given size. More different types of plants, for example, can live in sediment-filled, slower flowing rivers like the lower Mississippi than can live in a fast-flowing tributary of the same river.

More recently, stream order has also been used in geographic information systems (GIS) in an effort to map river networks. The algorithm, developed in 2004, uses vectors (lines) to represent the various streams and connects them using nodes (the place on the map where the two vectors meet). By using the different options available in ArcGIS, users can then change the line width or color to show the different stream orders. The result is a topologically correct depiction of the stream network that has a wide variety of applications.

Whether it is used by a GIS, a biogeographer, or a hydrologist, stream order is an effective way to classify the world’s waterways and is a crucial step in understanding and managing the many differences between streams of different sizes.