I also thought it was coastal and it took me ages to identify the location on the map of Faerûn that I found; not really helped by previously having zero experience of the place; and that the map significantly pre-dated BG3. While I haven't personally seen sandy beaches along rivers (just swamps; in fact Jarrow, the town where I was born, is thought to be named as such) I'll defer to other people's experiences, and only after locating it on the map did I zoom in to see that it is indeed a river, and a remarkably impressive one at that. Then again I have seen some IRL that are similarly impressive even far inland, such as the Severn.
I just defer to National Geographic which I'm fairly confident on betting knows a lot more than you're average poster in this forum. According to them, Rivers do not have white sandy beaches, they're silty and muddy. It's not a big deal, just caused confusion. The sound of waves crashing on the rocks in the Secluded Cove after the Harpy fight don't help convey the feeling of a river either. All it needed was a chippy and a few Seaguls to complete the effect. Seriously though it just causes confusion but it's not a big deal. I just couldn't find it on the map because I was searching for a coastal area. There's nothing about it that says River.
Oh and to the person who mentioned fresh water, yes the game says that. There are fresh water seas.