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Life in Freshwater

Stream Order

Rivers change from source to estuary. For years biologists have argued about the best way to classify river systems based on these changes. Ecologists have always been keen on using the names of organisms typical of an area (community) to indicate these regions, e.g. on seashores we often talk about barnacle zones. Initially a method was employed using the changes in fish species along the river: Troutbeck (in the headwaters), Grayling Zone, Barbel Zone and then Bream in the lowest zone. In recent years they have borrowed methods used by geographers and hydrologists. This is called Stream Ordering and is a process of assigning a number to each section of the river. There are different ways of doing this; two are described below. Whatever system you use make sure you quote the name of the method. Classifying rivers can be useful when making comparisons both between different rivers and along sections of the same river.

Strahler Method

Starting at the headwater the stream is assigned number one to be made 1st order. As several 1st order streams converge the resultant stream becomes 2nd order. Two 2nd order streams converging form a 3rd order, etc. However, a convergence of a lower order stream, for example a 1st order joining a 2nd order one does not change the status. It remains 2nd order. See the diagram below.

Diagram representing stream order with numbers starting in headwaters

The problem shown in the diagram is that even though the river may be a substantially large one the two examples are still 4th order by the time they reach A or B. This does not take into consideration the final discharge of the river.

Shreve Method

This alternative improves on this problem of allowing for discharge. Again, by starting at the headwaters, numbers are assigned but are always added together at the confluence of each stream. So even if there is a 1st order joining a 2nd order stream the result is a 3rd order stream. This could quickly go into larger, even double, figures than the Strahler Method.

Shreve Method of stream ordering

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