Does anyone know how this is determined in the CK? Are various grass meshes / textures linked to the various ground textures?

I have a mod that is adding grass to areas where grass should not be i.e. river beds. If anyone could point me in the right direction on this one iw ould be greatly appreciated.

grass is part of the ground texture. to get rid of it, replace the affected area with a non-grass one (or overpaint it with one) in landscape editor.