Tethys

Sister-wife of Oceanus
(Hey, why not? These are Gods we're talking about here; they don't inbreed),
she was quite prolific, bearing and rearing three thousand little Oceaniades. Tyche, Styx and Elektra were among her progeny twice removed. Possibly Poseidon. She also mothered all the great river gods, such as the Nile, and the lesser river gods, such as the Meander.
Tethys beat all when it came to inky depth proliferation.