Note that RiverCAD can extract cross sections at pre-defined distances for any length of river (up to 800 cross sections). All you need to do is select the river alignment (at the downstream end), and specify some additional parameters regarding spacing, section numbering, what to do for cross section overlapping, etc.
For
RiverCAD XP, select :
Input | Ground Geometry | Automated Centerline Cut
For
RiverCAD Pro, select :
Input | Design | Automated Centerline Cut
Selecting this command will display the Automated Centerline Cut dialog, as I have shown in the attached image.