Caesar makes the Rhine out to be both a major barrier to movement and as a natural ethnic division - Gauls on one side, Germans on the other. An example:

Ea quae secuta est hieme, qui fuit annus Gnaeo Pompeio, Marco Crasso consulibus, Usipetes Germani et item Tencteri magna cum multitudine hominum flumen Rhenum transierunt, non longe a mari, quo Rhenus influit. Causa transeundi fuit quod ab Suebis complures annos exagitati bello premebantur et agri cultura prohibebantur. Sueborum gens est longe maxima et bellicosissima Germanorum omnium.

On the other hand, we know that rivers were much easer to use for bulky cargoes than the poor state of roads (wagons often travelling beside roads rather than on them, as they lacked suspension) and a boat travelling along a river can as easily land on one bank as the other.

Archaeology seems to indicate that the Rhine was not such a barrier, and that there was more differentiation north-south than there was east-west:

Before Caesar's arrival in Gaul in 58 bc, the abundant archaeological evidence indicates very similar patterns west and east of the Rhine river. From the archaeology, we would never suppose that the Rhine formed an ethnic boundary orany other kind of division between peoples. The oppida are similar on both sdides of the river. Pottery, iron tools, bronze and glass ornaments, coins, and other materials are very much alike in the two regions .... Much of caesar's characterisation of the Germans could fit what we know about groups characterised by Jastorf material culture. If Caesar had said that east of the lowerRhine the lands were inhabited by Germans then this description could correspond to the archaeological situation. But Caesar does not make this stipulation ...
Wells, Peter "The Barbarians speak: how the conquered peoples shaped roman europe", Princeton University Press, 199, pp113-114.