Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Assigning Orders to Stream Links

You can attach an order number (integer value) to each stream segment or link. Generally, streams with lower numerical values have a size that can carry smaller volumes of water, but this is not always the case, as you will see.

Two ways of determining stream order number have been devised (one by A. N. Strahler in 1957 and the other by R. L. Shreve in 1966), as discussed below. In both methods, the smallest originating streams are numbered 1, from the source of the stream continuing up to the first intersection.

In the Strahler method, when (any number of) streams of the same order merge at a point, the downhill stream takes on an order number that is the original stream order number plus 1. For example, if a stream of order 3 merges with another stream of order 3, the resulting stream is order 4.

In any other case of stream merging, the order number of the downhill stream retains the order number of the larger uphill stream. So, if a stream of order 3 is joined by a stream of order 2, the resulting stream is still of order 3. Figure 8-34 illustrates the Strahler method.

FIGURE 8-34 The Strahler method of stream ordering

When two streams merge according to the Shreve method, the order numbers of the uphill streams are added together to produce the order value of the downhill stream. So, when merged, two streams of order 3 produce a stream of order ...