> Most people who (quite reasonably) hate corporate personhood would probably have a knee-jerk reaction that personhood for a river can/should be normalized.

Only if/because they are reading too much into the concept of legal personhood. A thing being a person doesn't mean the thing is equivalent to a human or that it has every right that every human has. It generally just means that the law attributes certain rights and obligations to that thing because that is more convenient than finding the right human(s) to attribute them to in the circumstances.