[1.12.2] Custom Biome Layers

Does somebody know something about how to make own biomes with genLayers like a river? Because I want to create my own river biome and know, that the river is generating like a river because of his genLayer. My plan was to copy the river genLayer and insert my own biome instead of the river, but now I hav now idea what I must do with the layer. I think there is an easy way with worldtypes so I could create my own world type, but then there is also the problem must I change the world type for the hole world, how can I do this... If somebody can help me, I would be very thankful ;)