I may have found the issue:
I was using the GTNH version of RWG, which uses different biome IDs from the original
GTNH: Realistic-World-Gen/src/main/java/rwg/biomes/base/BaseBiomes.java at master · GTNewHorizons/Realistic-World-Gen · GitHub
Original: Realistic-World-Gen/rwg/biomes/base/BaseBiomes.java at master · Ted80-Minecraft-Mods/Realistic-World-Gen · GitHub
The biome ids from the original RWG are included in gregtech6/src/main/java/gregapi/data/CS.java at e91bbd20e004c608bd2e53589870992d8d8fab89 · GregTech6/gregtech6 · GitHub, but the ones from the GTNH version are not. Wouldn’t it be easier if GT6 river biomes were configurable instead of hardcoded anyway?