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

Assigning Identities to Streams

The most basic hydrologic unit (outside of the individual cell) is the stream segment. Generally, streams segments (also called links) run between intersections in the linear network. A segment consists of all the cells between the junctions of two or more streams or between junctions and the pour points. (The cell that is the junction is considered to belong to one of the streams.) Figure 8-32 illustrates this numbering. The integer number is only a nominal, identifying value.

FIGURE 8-32 Numbered stream links

On a raster, ArcGIS places the same unique identifying number in all the cells of a given stream segment. In the discussion of the Flow Direction and Flow Accumulation calculations, every cell was considered a contributor to the creeks, streams, and rivers that developed (“Into each cell some rain must fall.”). But you do not want to define all the cells in the study area as part of the water network. Instead, the software will delineate specific stream channels, running from intersection to intersection. In other words, all of the study area contributes to the total amount of water to be dealt with, but only a small part of the study area carries most of that water. That area is known variously as the water network or the stream channels. This area is defined by including only those cells with flow accumulations greater than a chosen value; ...