Poseidon was the god of the sea, earthquakes and horses. Although he was officially one of the supreme gods of Mount Olympus, he spent most of his time in his watery domain. Poseidon was brother to Zeus and Hades. These three gods divided up creation. Zeus became ruler of the sky, Hades got dominion of the Underworld and Poseidon was given all water, both fresh and salt.
Although there were various rivers personified as gods, these would have been technically under Poseidon's sway. Similarly, Nereus, the Old Man of the Sea, wasn't really considered on a par with Poseidon, who was known to drive his chariot through the waves in unquestioned dominance. Poseidon had married Nereus's daughter, the sea-nymph Amphitrite.