[XaspR8d]

> Just as you can't actually say inriver and pretend it means something in the river in English

I think "in-river" is pretty acceptable in speech, and mildly so in writing. If you said there was an "in-river pool", I wouldn't bat an eyelash.

That said, the relation between the adjectivizing(?) morphemes "in-" and "at-" (e.g. "at-home bar") and their corresponding prepositions is weird and makes my brain hurt.