RivEX warns of number of fields is greater than 255
After the build process RivEX displays the message: WARNING: The river network has more than 255 fields! This is very unusual and some tools may fail, consider reducing number of fields in dataset.
Your river network has more than 255 fields. This is an unusually large number of fields and may cause RivEX to fail. It is recommended that you reduce the number of fields in your dataset. You can do this easily by using the layer properties to turn off all fields visibility, then turn back on the OBJECTID, SHAPE and your unique PolylineID field (if you have allowed RivEX to build that field it would be called RivID). Then right click on the layer in the TOC and go to data > export and create a copy of your data in a file geodatabase. Fields with their visibility turned OFF will be dropped during the export. By keeping your original unique polyline ID field you can join data back to the network if you so wished.
Make sure you have no polylines selected before you do the export!