I am currently staying in Geneva, but across the Rhône from the old town and I was wondering where the Roman era provincial borders went. I understand that Geneva itself was part of the Gallia Narbonensis (Gallia Transalpina) province since 121 BCE and the territory on the other, northwestern side of the Rhône was part of the Gallia Belgica province since 22 BCE and Germania Superior province since 83 CE, but was the river always the exact border of these provinces? Is it even known that precisely? Maps like [this one](https://upload.wikimedia.org/wikipedia/commons/7/76/Gaul_in_the_Time_of_Caesar.jpg) vaguely show the border not exactly coinciding with the river.
Because I also understand that during the medieval times the area immediately across the Rhône (Saint-Gervais) was part of Geneva, but that may have been only after the construction of the Château de l’Île.