Caves can generate on a river floor, creating long rivers, as shown below on the 1st image.
I have also found a big river that is actually a mix of the plains, sunflower plains and dark forest biome (don't know if that one is a bug)
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HARADA
violine1101
Thank you for your report!
We're tracking this issue in MC-214836, so this ticket is being resolved and linked as a duplicate.
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these are new caves, not a bug.