Ancient geographers discussed this great forest at length, esp. Strabo in his geography of the Celts (7.1.3-5, 2.2, 3.1; cf. 4.6.9), cf. Plutarch (Mar. 11.9.4), Diodorus Siculus (5.21, 32), Dionysius of Halicarnassus (14.1), Eustathius (Comm. ad Dionys. Per. 298.16-45), and the Emperor Julian (Epist. 25b.1,6, cf. chi 473; Misopogon 30.5), often depending on Hecataeus and Posidonius. It was of huge extent and generally agreed to harbour the sources of the Danube, today contained within the Black Forest, which must cover some portion of the ancient forest.
The noun here is from δρῦς , often translated 'oak' but in origin the IE for 'tree'; hence the forests are not necessarily of oak (cf. delta 1541, delta 1544).
The Danube was formerly navigable from the Black Sea only as far as the cataracts (kappa 745) in the narrow gorges carved through the Carpathians and known as the Iron Gates and the Cauldron (Kazan). Modern dams below these points have caused the cataracts to disappear – to the anger of those who loved them. The river is, of course, navigable in its upper reaches.
The adjective Hercynian is unexplained. Hesychius mentions a festival of Demeter with this name (epsilon5931). Hera shouts from a Hercynian cliff in Apollonius Rhodius (4.640, cf. scholiast ad loc.). We might speculate that it is a borrowing from Celtic or Thracian for the very ancient mother-goddess whose icons are so ancient in this area. There is also mention of a Hercynian mountain in Italy.
[1] The Greeks named the river Istros, Roman Ister, probably its Celtic name. The many Thracians (Dacians, Getae, etc.) along its banks and the delta mouth called it Matoas (Greek ἄσιος , ‘muddy’), while the Scythians gave it the name Danube or Danousis, “because of a disaster crossing it.” So Eustathius, ad D.P. 298.34f., an account apparently misunderstood in Steph. Lex. 217.22-218.5; cf. R.R. Dyer, "Matoas, the Thraco-Phrygian name for the Danube, and the IE root *madu" (Glotta 52, 1974, 91-95). The element dan occurs in many river names from the territories controlled by the Scythians (modern Don, Dnieper, Dniester) and is sometimes seen as the final element in the name of Poseidon.
[2] Chi 473.