In Greek mythology, Inachus (Inachos) is the river-god and first king of Argos after whom the Argive Inachus is named — a son of Oceanus and Tethys, father of Phoroneus and (in many accounts) of Io, and the judge whose verdict for Hera moved Poseidon to dry the Argolid springs. Pausanias records both the mortal-king and pure-river traditions.
Adapted from Wikipedia, "Inachus" · CC BY-SA 4.0
I.Family
As one of the three thousand river-gods born to Oceanus and Tethys, Inachus belongs to the pre-Olympian Pelasgian landscape; iconography often shows river gods as bulls with human heads. Chief children include Phoroneus and Aegialeus (or Phegeus), and daughters Io and Philodice; mothers named include the Oceanids Melia and Argia. Other notices add Mycene, Amymone, Messeis, Hyperia, Themisto, and even Argus Panoptes.
Ancient chronographers euhemerize him as first king of Argos for some fifty years; Pausanias notes the competing claim that the father of Phoroneus was no man but the river itself.
II.Hera, Poseidon, and the dried springs
Inachus, with Cephissus and Asterion, judged a land dispute between Poseidon and Hera for Argos in Hera's favor. Poseidon in anger took away their water, so the rivers run only in rainy seasons — a drought that later sends Danaus' daughters to seek springs, and Amymone to Poseidon's gift at Lerna. Another strand has Poseidon flood the land until Hera induces him to withdraw the sea.
The country is often called the land of Inachus; he is said to have been Hera's first priest at Argos.
III.Io and drama
In Aeschylus' Prometheus Bound, oracles force Inachus to drive Io from home when Zeus desires her. Ovid shows him failing to recognize the cow-form Io until she traces her name in the dust with her hoof. Sophocles wrote an Inachos (likely a satyr play) of which only papyrus scraps survive; Virgil places Inachus on Turnus' shield in the Aeneid.
This entry is the Argive river-king only.