Minecraft wiki entry for Before_1.18, associated text: A layer stack for river noise generation was used as a random number generator to generate areas of hills and mutated biomes, which was scaled twice before applied to the biome stage of biome generation at scale 1 to 64. Since Update Aquatic, modified biomes could conform to an entire biome or border a river. A separate layer stack to generate rivers throughout was scaled 4 times, before it was merged with the rest of the generation at scale 1 to 4.