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I have a gauging station point with elevation. Also have a watershed generated for the same area. I just want to know how to calculate the distance along the flowpath from the station to the outlet(point)

The green point being the gauging station with an elevation of 100m and the red one being the outlet.

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Would you like plan distance, 3D distance, or distance along the flowpath (as computed, say, by FlowLength)? – whuber Apr 9 '13 at 19:13
distance along flowpath – SNT Apr 9 '13 at 19:14
up vote 3 down vote accepted

You can calculate Flow Length from a Flow Direction raster, but you need your original DEM to do that. Your watersheds are just "areas that drain into a point" and don't retain the necessary flow direction information.

Since your stream gauge is in the middle of the larger watershed, calculate the downstream flow length and then use the identify tool (or similar method) to find the raster value at your gauge location.

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