Cross - bad 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.

Tick - good 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 exporting the data whilst retaining only the required fields. Right click on the layer in the TOC and go to data > Export Features and create a copy of your data in a file geodatabase. Set the Fields parameter such that you keep only the fields you need, the remaining 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!