Hi Sarah, Sorry for the delay in getting back to you. There are indeed freshwater jellyfish. In fact, I wrote an article about them years ago on a previous website (curiousnature.info/A1-Jellyfish.htm). But I don’t think they’re usually found in rivers, at least not if there’s any current. And everything I know says they aren’t capable of leaving the marks you got.
The one thing I know of that is in the water that might give you bites like that is the small bugs called backswimmers. I’ve never been bitten by them, but apparently, they are capable of biting and it stings. Seems odd that you would have gotten bitten so many times, but do you remember seeing them in the river? Tom