Miscellaneous Hydrology Tools for ArcGIS - Find Longest Stream Path

Here is part four of the blog post on the Miscellaneous Hydrology Tools for ArcGIS. This post covers a tool called the Find Longest Stream Path tool. You can read the original blog post by clicking HERE or download the tool be clicking HERE. It is the fourth and final in this series about the Miscellaneous Hydrology Tools.

Surprisingly, there are no conventional tools in ArcMap that identify the longest stream in a watershed. However, many geomorphic metrics, such as relief ratio and watershed shape, are based on knowing what the longest stream is. In order rectify this situation I built a small model that calculates the longest stream. Unlike the other two tools in the Miscellaneous Hydrology Toolbox it does not provide an absolute answer.  For reasons unknown to me occasionally some watersheds get left out.  Nonetheless I feel that this tool is a useful and helpful addition that some people will enjoy having in their toolbox.

On the left is a sample of watersheds from central Nevada with the longest stream from each watershed shown in bold blue.  All streams are shown in light blue.  Watershed boundaries are in black. Pour points (those places where the stream exits the basin) are shown in green. Channel heads for the longest stream are shown in yellow.

There are four parameters for running this tool.  A input flow stream flow direction tool is required for understanding flow routing.  A standard flow direction raster can be clipped to the stream network to achieve this.  Input watershed polygons are required.  A temporary folder is required for storing outputs.  Finally, a dissolve field (ID for example) is required.  This should coincide with a field with the same name in the watersheds file.

On the right is a picture of the model.  Let me step you through how it works. For each stream flow direction cell the model calculates an upstream and downstream flow length. Using those flow lengths the model generates channel heads and pour points in each watershed. Using cost distance the model identifies the channel head with the greatest cost distance from the pour point. This becomes the end point. The combination of the end point and the pour point is used to create a least-cost path. Finally, grid cells are converted into flow lines.