With e1.6.0 the river_scripts entity in worldmaps has changed to provide two texture slots in the script for the river flowmap and river normalmap. Previously, these could only be overridden by naming your texture the same as Bannerlord's one. Unfortunately, while I can now load myname textures into the scripts, the scripts fail to work unless the flowmap is named worldmap_river_mask. I don't know if the name of the river normal is also still hard-coded into the scripts as that is more difficult to check. Hopefully, the hard-coding of names in the script is scheduled for correction as the texture slots are cosmetic without that.