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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Determining the Stream Channels
You can use the Flow Accumulation raster to identify a drainage network. We will identify those cells that have high accumulated flow values. The cells that contain the most water are the stream channels. We can create a drainage network of any detail by choosing those cells that have more than a certain minimum threshold value. In this case, say that a stream exists at a given cell if the flow accumulation in that cell is equal to or greater than 140—the equivalent of 1 unit of rain over at least 140 other cells that drain to that cell. Let’s say that a cell either is or is not in the drainage network, so we will make a raster consisting of binary values: 0 or 1. We can do this with the Raster Calculator.
____ 16. In the Raster Calculator, build and evaluate this expression:
“FlowAccu” >= 140
Call the resulting raster StrmChannels. Turn off all layers except StrmChannels. The cells that constitute the stream channels have values of 1; the rest of the cells have values of 0.

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