This is used by the "World Chunk Manager" option "CUSTOM_DC" for generating the biome map.

• Type:
  • Map<Integer, Integer>
• Usage:
  • Key:
    • Type:
      • Integer
    • Usage:
      • This key is the id of the biome you are setting the river biome for.
  • Value:
    • Type:
      • Integer
    • Usage:
      • This key is the id of the biome you are setting as the river biome in the Key biome. Setting this to -1 will use the Key biome.
• Default:
  • If not set, this will use frozen river in ice plains, mushroom island shore in mushroom island and mushroom island shore, nothing in ocean and deep ocean, and normal river in all other biomes.

Example:

Approximation of Vanilla:

#!Json
    "Biome Rivers": {
        "0": 0,
        "24": 24,
        "12": 11,
        "14": 15,
        "15": 15
    }