Why is the Thames called "Old Father Thames"

E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Father Thames,

or Old Father Thames. The Thames, so far as it belongs to London. 1
“Say, Father Thames, for thou hast seen
Full many a sprightly race
Disporting on thy margent green,
The paths of pleasure trace.”


Gray: Distant Prospect of Eton College.

The epithet is not uncommonly applied to other great rivers, especially those on which cities are built. The river is the father of the city, or the reason why the site was selected by the first settlers there. 2
“O Tiber, Father Tiber,
To whom the Romans pray.”


Macaulay: Lay of Horatius.