Danube (Greek ῎Ιστρος and Δάνυβις; Latin Danubius or Danuvius): one of the largest rivers in Europe, northern frontier of the Roman Empire.
In the two sources, two little streams have their origin, today known as the Brigach and the Breg, which unite at Donaueschingen. The first real city along the river was Castra Regina (Regensburg), the base of the Third Legion Italica, a little bit beyond the place where the river became navigable for big ships. This was also the place where that part of the limes started that connected the Danube to the Rhine.
In the second and third centuries, other important legionary bases were at Lauriacum (modern Lorch; II Italica), Vindobona (X Gemina), Carnuntum (Petronel; XIV Gemina), Brigetio (Szöny; I Adiutrix), Aquincum (II Adiutrix), Singidunum (Belgrade; IIII Flavia Felix), Novae (Svishtov, I Italica), and finally, almost in the delta, Durostorum (Silistra, XI Claudia).
All in all, ten legions guarded the river. As Procopius was to declare in the sixth century:
note[Procopius, Buildings 4.5.2.]The Roman Emperors of former times, by way of preventing the crossing of the Danube by the barbarians who live on the other side, occupied the entire bank of this river with strongholds, and not the right bank of the stream alone, for in some parts of it they built towns and fortresses on its other bank.
Only two units, V Macedonica and XIII Gemina, had their base in Dacia north of the Danube. The emperor Trajan, who had conquered Dacia, also ordered his architect Apollodorus of Damascus to build a bridge across the river, the remains of which can still be seen at Turnu Severin in Romania.
In other words, the Danube was, during the High Empire, the northern frontier of the Roman world. Of course the tribes were controlled indirectly. It had not always been like that; it was only during the reign of the emperor Claudius (r.41-54) that the legions were transferred to this river, one of the measures belonging to the Claudian army reforms. To the north of the Danube, the people were no longer Roman, but Germanic or Scythian.note[Pliny, Natural History 4.80.]
Before the Romans had reached the river, it was little-known, and the Greeks had two names: the western part, about which they learned from travelers who visited the Greek towns along the Adriatic shores, they called Danube, but the eastern part, which they discovered through Thrace, was known as Ister or Hister. Herodotus may have been the first to realize that the two streams were in fact one, and he said he believed that the sources of this river was in the far west, among the Celts, near the Pyrenees.note[Herodotus, Histories 2.33.]
According to Pliny, the catfish in the Danube were so big that they had to be caught with harpoons, and brought to the land with oxen.note[Pliny the Elder, Natural History 9.45.] Herodian was convinced that the river froze over every winter.note[Herodian, Roman History 6.7.6-8.]