Introduction of processing logic

I build a huc12 navigation table which is saved in the geodatabase.I also have a tool that builds HUC8 to HUC8 navigation based on the HUC12 table.

My overall tool used for the NWIS project uses a loop to aggregate all HUC12s in the current HUC8, then I bring in any upstream HUC8s and dissolve. This is much faster for large basins, of course!

WBD Tools

Output table