Identifying stream networks
Stream networks can be delineated from a digital elevation model (DEM) using the output from the Flow Accumulation tool. Flow accumulation in its simplest form is the number of upslope cells that flow into each cell. By applying a threshold value to the results of the Flow Accumulation tool using either the Con or Set Null tools, a stream network can be delineated. For example, to create a raster where the value 1 represents a stream network on a background of NoData, the tool parameters could be as follows:
- With the Con tool:
Input conditional raster : flowacc
Expression : "Value > 100"
Input true raster or constant value : 1
Input false raster or constant value : ""
Output raster : stream_net
or
- With the Set Null tool:
Input conditional raster : flowacc
Expression : "Value <= 100"
Input false raster or constant value : "1"
Output raster : stream_net
In both examples, all cells with more than 100 cells flowing into them are assigned the value 1, and all other cells are assigned NoData. For future processing, it is important that the stream network, a set of raster linear features, be represented as values on a background of NoData. Once created, the stream network can be further analyzed using the Stream Order, Stream Link, and Stream to Feature tools, for ordering (ranking) the streams, assigning unique IDs to stream links, or creating a feature dataset, respectively. Determining a threshold value that represents where a permanent stream or stream channel begins is affected not only by contributing area but also by climate, slope, and soil characteristics. For more information on stream channel initiation, refer to Tarboton and Bras (1991).
Stream ordering
The Stream Order tool has two methods of assigning a numeric order to links in a stream network. While the default Strahler method is the most common, the Shreve method offers the benefit of not being as sensitive to the addition and removal of links from further analysis.
Stream links
The Stream Link tool allows you to assign unique values to each of the links in a raster linear network. This is most useful as input to the Watershed tool to quickly create watersheds based on stream junctions. It can also be useful for attaching related attribute information to individual segments of a stream.
Vectorizing a raster stream network
A raster linear network can be accurately converted to feature data with the Stream to Feature tool.
Reference
Tarboton D. G., R. L. Bras, I. Rodriguez–Iturbe. 1991. On the Extraction of Channel Networks from Digital Elevation Data. Hydrological Processes. 5: 81–100.