Board Thread:General Mod Discussion/@comment-26453572-20160608163300/@comment-25101089-20160611051029

Normal rivers:
 * The 'River' biome still exists in the mod, though it should never generate naturally. This is for two reasons: firstly, so that the biome mouseover display in the map screen doesn't break (rivers are still drawn on the map image, and the mouseover takes the biome directly from the map image, not from the saved world) and secondly, so that in the unlikely case that the river replacement algorithm should fail somehow (if someone edits the map image and draws a river which is too wide) the mod will be able to generate the 'River' biome as a fallback, instead of breaking.
 * Therefore, any region in an old world that was a 'River' biome will remain so when loaded into new versions, completely unchanged.

Non-standard rivers like Morgulduin, Mirkwood River, Far Harad River et cetera:
 * These biomes have been removed from the mod entirely: their biome IDs are now unassigned, and on the map they have been recoloured as normal rivers.
 * Because of the way the code works, if a region is loaded from the save files with a biome ID that doesn't exist in the game, that biome ID is converted to 0 to avoid a crash. (This assumes that there is indeed a biome assigned to the ID 0.)
 * By a sheer stroke of luck, the biome assigned to ID 0 is... River. Which means that any river biomes loaded from old worlds will still be a river biome in the new update, even though that biome doesn't generate naturally anymore.