This below sentence is extractet from Commentarri de Bello Gallico - in the section of Conspiracy of Orgetorix:
... ūnā ex parte flūmine Rhēno lātissimō atque altissimō, QUI agrum Helvētium ā Germānīs divīdit
I have referred to a translation from website
... on one side by the Rhine, a very broad and deep river, which separates the Helvetian territory from the Germans
That seems that the demonstrative Pronoun QUI refers to flumine, ...