Where was the River Styx situated?

In Greek mythology, Styx (Ancient Greek: Στύξ) is both a deity and a river that forms the boundary between Earth and Hades, the Underworld. Hades is also the name of its ruler. Styx is also a goddess with prehistoric roots in Greek mythology as a daughter of Tethys, after whom the river is named and because of whom it had miraculous powers. Charon is the ferryman who transferred the dead to the Underworld.

The river Styx converges with other rivers at the center of the underworld on a great marsh, which sometimes is also called the Styx.

So, we have a river and a deity with the same name and we have an underworld and its ruler with the same name. What do these names mean?

The word 'Styx' is cognate with Greek stygos 'hatred', stygnos 'gloomy', and derives from stygein 'to hate', 'abominate'. Both the words 'Hades' and 'Charon' are reputedly of unknown origin, which always makes me suspicious. Every linguist seems to try to find an etymology by comparing a Greek word to the languages of its neighbours, but always tend to forget its most influential and powerful neighbour: Egypt.

The question is therefore: can we find an etymology in Egyptian that mirrors the Greek version. The answer is: yes, we can.

In Egyptian, stkn (remember they didn't use vowels in Egyptian) is a causative of tkn 'approach' with the specific sense of 'to induct, bring on doom'. Hades received his/its name from ḥdi as a verb with the meaning 'to be destroyed' and as a noun 'damage', 'destruction'. Diodoros believed that the name Charon (Kharon) was Egyptian:

The other myths about Hades, current among the Greeks, also agree with the customs which are practised even now in Egypt. For the boat which receives the bodies is called baris and the passenger's fee is given to the boatman, who in the Egyptian tongue is called charon. And near these regions, they say, are also the 'Shades'.

But the baris was also the ship of Osiris and dead pharaohs were also transported on a baris to their final resting places. The journey from Osiris to the Underworld was therefore reenacted time and time again.

We can identify the Egyptian god of the Underworld Anubis in this description of Hades. 'Anubis' was  the Greek rendering of this god's Egyptian name. In the Old Kingdom (ca 2686 BC–ca 2181 BC), the standard way of writing his name in hieroglyphs was composed of the sound ı͗npw followed by a 'jackal' over a ḥtp sign.

The problem is that there does not seem to be a plausible Egyptian root of the word 'Charon', but it is possible that it has an etymology from the West-Semitic deity Ḥrn, vocalized as Horon in the Bible and known as the 'Lord of Hell'. Still, the region was frequently conquered by Egypt and we might assume that Hades ultimately has Egyptian roots.

[Ancient branches of the Nile]

Thus with a river with several tributaries that end in a great march. Which other river can that be other than the river Nile? But what of all these words that signify death and doom? These are very reminiscent of the spells from the 'Book of the Dead', the ancient Egyptian group of magical and religious texts. The spells are meant to help the dead progress through the many challenges in the underworld (the Duat) to the afterlife. Pharaohs who had died were transferred in boats via the river Nile to their lavishly decorated tombs in the Valley of the Death. These tombs were protected by powerful spells that would ensure that they were not violated.

[Book of the Dead]

Based on the evidence it seems very probable that the concept of the River Styx was borrowed from Egypt and 'transplanted' to Ancient Greece. My conclusion is that the River Styx is non other than the River Nile.