Thanks Hai-Etlik. Bridges are actually part of the equation.
The point is that this is a huge and very long river (15% longer than the Amazon river, roughly), to the point that it cannot be bridged for a very long span of its course. This city is in a place where the river has some islands that splits it and make it bridgeable, that are themselves the result of a rocky bottom in that area (which I figure could be brought to surface by soil erosion, and the islands would simply be random clumps of slightly harder, thus less eroded, rock). The narrowing of a large river, plus the kind of bottom it would have in that tract, would make (I guess, but I don't really know) for a more turbulent flow. Thus the city has to be there, for otherwise no bridge, but it would use a system of canals and artificial port to avoid the problem of the rougher flow. Possibly, or at least that was my idea, the system could be used to completely sidestep the area with the rocky islands (which is comparably small).
I hope I was able to make my idea clear.