Definitions

# Strahler Stream Order

The Strahler Stream Order is a simple hydrology algorithm used to define stream size based on a hierarchy of tributaries.

The streams range from one (1) to the most powerful which is the Amazon River at "12." The Ohio River is an "8" and the Mississippi River is a "10." 80 percent of the streams and rivers on the planet are first or second order.

To qualify as a stream it must be either recurring or perennial. Recurring streams have water in the channel for at least part of the year. When two first-order streams come together, they form a second-order stream. When two second-order streams come together, they form a third-order stream. Streams of lower order joining a higher order stream do not change the order of the higher stream. Thus, if a first-order stream joins a second-order stream, it remains a second-order stream. It is not until a second-order stream combines with another second-order stream that it becomes a third-order stream.

Arthur Newell Strahler first proposed the hierarchy in 1952 in an article “Hypsometric (area altitude) analysis of erosional topology.” in the Geological Society of America Bulletin. It is often referenced in professional descriptions of rivers as Strahler 1952.

## GIS algorithms

Gleyzer et al. (2004) developed a recursive algorithm which would process vector river networks for Strahler stream order values in a GIS application. This algorithm is implemented by RivEX, an ESRI ArcGIS 9.1 tool. The algorithm requires the vector network to be topologically correct to successfully process. The network must be a centre-lined network where each arc (sometimes referred to as an edge) must be joined at their node (sometime referred to as a junction). No left and right banks or lake side shores should be present. The image below demonstrates a map representation of a river network, an invalid network where lake and river bank sides have been digitally captured and a valid, topologically correct, centre-lined river network which the algorithm can process.

If the network is "broken" (arcs not connecting) then the output will be incorrect. The algorithm would treat the disconnected catchment as a separate river system, so it is important to check the connectivity of your river network before attempting to compute Strahler order values.

## References

1. Gleyzer, A. , Denisyuk, M. ,Rimmer, A. and Salingar, Y. (2004). A Fast Recursive GIS Algorithm for Computing Strahler Stream Order in Braided and Nonbraided Networks. Journal of the American Water Resources Association, 40(4), 937 - 946.
2. Lanfear, K. J. (1990). A fast algorithm for automatically computing Strahler stream order. Water Resources Bulletin, 26(6), 977 - 981.
3. Strahler, A. N. (1952). Hypsometric (area altitude) analysis of erosional topology. Geological Society of America Bulletin, 63, 1117 - 1142.

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