Shantanu in his previous birth was Mahabhisha. The story of his previous birth too is associated with Goddess Ganga and this is an important turning point in the Mahabharata. Shantanu is the father of Bhishma Pitamaha and he later married Satyavati. Pandavas and Kauravas belong to the lineage of Shantanu and Satyavati.
Mahabhisha was a pious man and due to his merits, he was able to go to heaven in his physical form.
Here he enjoyed all the luxury including the company of heavenly maidens and Sura, the wine of gods.
One day Goddess Ganga visited heaven.
A slight breeze ruffled her garments and her upper body was exposed.
All Devas, demigods, looked down out of respect.
But Mahabhisha was spellbound and he kept staring at Ganga.
Ganga too enjoyed the attention of Mahabhisha.
Indra, the king of Devas who is always insecure and jealous, was angry at the blatant display of lust and cursed both of them to return to earth. He told Ganga that she would get the entry in heaven only after breaking the heart of Mahabhisha.
Mahabhisha was born on earth as Shantanu.
Shantanu and Ganga got married and later Ganga abandoned him.