Comparing maps between AD&D (2e) and 3e D&D brings up numerous unexplained changes, mountains, coastlines, positions of towns,...continuity just wasn't a very big concern for the new owners. The 2e hex maps were very abstract, so when you saw a patch of trees next to a line of hills, you could assume that the hills were wooded on at least one side, even if there was a stretch of lime green standard background showing in between. There often were descriptions in the books that explained how these blank areas look. The Sword Coast hinterlands are described as jagged in the area we're looking at, so we can safely assume that the rocky wooded terrain depicted in BG3 matches the original concept.
The Baldur's Gate and Neverwinter areas more or less disappeared from tabletop sources during the late 1990s, so there is very little material outside of the Forgotten Realms Campaign Setting when it comes to the Chionthar Valley between Elturel and Baldur's Gate, but as a DM - or game dev, you can of course always decide how things are in your campaign which is why these maps leave many blank spaces to be filled out in your games. You can always invent new towns and say there's a river bend or tributary, even if they don't show. Those maps have a huge scale and intentionally omit some details. In my 2e AD&D home campaign which is set about 120 years before BG3 and before the onset of the Shadow Curse, Reithwin is located on the southern bank of the Chionthar, north of the Wood of Sharp Teeth or rather on the edge or a prong of said forest. This would also place the Wilderness somewhere along the edges further east. If the Tieflings wanted to leave the jurisdiction of Elturgard as soon as possible, going south of the Chionthar would have made perfect sense. The liberty Larian took in this scenario was placing a mountain chain between Baldur's Gate and Elturel called the Trielta Crags. On the old maps, Triel is north east of Elturel and the Trielta Hills named after it described as pastoral rolling hills, not at all like the tall peaks we see in the game.
In Act 3, we're coming from Reithwin and have to cross the Rivington bridge from south, so I assume my other observations are correct.
Rivington was the southern-most district of Baldur's Gate. It was the only region of the Outer City located on the southern shore of the River Chionthar, on the far side of Wyrm's Crossing.
