A canal built by Marius (Caesar’s uncle) has been found in Gaul in the Rhône River delta. It is dated to 104-102 BCE, and is thought to have been constructed to help Rome in the Cimbrian Wars (113-101 BCE). The Cimbrian Wars were between the Roman Republic and the tribes of the Cimbri and Teutons, among others.
https://www.livescience.com/archaeology/romans/archaeologists-may-have-finally-discovered-famous-lost-canal-built-by-julius-caesars-uncle