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Calculating Stream Order
The volume of water flowing through a stream is a function of many things, including the stream’s width (number of cells) and its depth. For that and other reasons, the number of “width cells” that depict a stream is not a good indicator of its size or volume. As previously discussed, one way to get an idea of stream size is to assign a stream order number, which indicates the relative volume of water in a stream segment. In this step, you will use the Strahler ordering.
____ 17. Use the Stream Order tool on the StrmChannels layer (the input stream raster) and the FlowDir2 layer (the input flow direction raster). Make this dataset active with the name to StrmOrder_StM. Use the STRAHLER method. How many cells are in the channels of 1st order streams? _________. How many in 4th order streams? _______
The Strahler method is more conservative than the Shreve method—that is, the numbers tend to be smaller. In Shreve, every time one stream joins another, the order number goes up. Not so with Strahler. Only when two streams of the same magnitude join does the order number increase, and then only to identify the downstream by a number greater by 1 than those above. Notice that there are only four categories of this Strahler layer, even though you had a large number of stream segments. With Shreve, the largest order is almost 40, as you can prove to yourself if you choose to by making StrmOrder_ShM.

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