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This tool, located on Tools Menu > Show Map Editing Tools > Advanced Tools > Watersheds, works with a digital elevation model (DEM) layer on the map to determine the size of the watershed that feeds a point on a stream.
Prerequisites
One or more layers on the map must be setup as DEM layers. See the Image Properties topic for more about that.
Step 1
Users will first be prompted to draw a single line segment perpendicular to the flow direction marking the down stream termination point of the watershed. That is the point through which all water in the shed drains to. Draw this line sufficiently long enough to catch all flow paths. For example, users might draw it from bank to bank plus about 10% on each side to be safe.
Step 2
The result of this analysis will be a newly created watershed feature marking the boundary. Select the existing layer to add this feature to and specify the name of the feature to be created.
Step 3
Use the box prompt to indicate the estimated size of the watershed. Position the box with the mouse and size it using the scroll wheel or PgUp/PgDn keys. Think GIS uses this area to set up its analysis of the DEM data. This may involve some trial and error. If the resulting watershed extends outside of the anticipated area, then run it again with a larger area. if the resulting watershed covers only a tiny portion of the anticipated area then users might want to run it again with a smaller box in order to increase the resolution of the analysis due to the closer zoom level.
Analysis
Finally the analysis is performed and the resulting watershed is displayed as a boundary on the map.
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