It seems like you need a mask that uses multiple colors or shades to semantic segmentation. Your mask is black and white, 0 or 1, so it should be usable for two types of classification. I think it should be usable for whether or not it’s a river.
by json file like this (or opposite?):
{"0": "not river", "1": "river"}
With HF, JSON files that have been carelessly left lying around are sometimes configuration files with fixed file names…