The textures used by fx_Ngafalls_sml.fx are:
fx_1.bmp and fx_5.bmp
The Bear Canyon scenery by Lyons has 7 bgl's for the scenery (Bearcan5.bgl calls the falls):
Bearcan1.bgl, Bearcan2.bgl, Bearcan3.bgl, Bearcan4.bgl, Bearcan5.bgl, Bearcan6.bgl, and SailLesson.bgl
Another consideration is any add-on terrain mesh in the area. It could feasibly "bury" the effect in the ground mesh.
Also, the Lyons scenery was designed for stock fs scenery, so in addition to a conflict with add-on mesh, if you are running an altered TERRAIN_MAX_VERTEX_LEVEL (default is 19), located in the fs9.cfg, then the effect could also be obscured.
I have the freeware FSGenesis mesh and run an altered TERRAIN_MAX_VERTEX_LEVEL=20 which nearly blocks out the falls effect. At TERRAIN_MAX_VERTEX_LEVEL=19, it is much more prominent.
Hope that helps.