The Iron Gates are a physical landmark in the Lower Danube which divides the river. Its waterfalls and rapids made ship navigation during antiquity problematic, and it was not until 1831 that a successful channel was dug. See the photograph below.
The Roman solution to this Iron Gates problem was to deploy two navies. Classis Moesica on the eastern side and Classis Pannonica on the western side. These fleets were responsible for supporting the volatile Balkan frontier. The upper Danube, running from the Alps through modern Hungary was a more stable boundary which did not require a naval force during the first two centuries AD. A third naval force, the Classis Germanica was based near modern Bonn and was responsible for the Rhine region.