Who is the father of King Shantanu

What are the details of the birth of the famous Bhishma as the son of King Shantanu and Ganga?

The story of Bhishma's birth is explained in - The Mahabharata Book 1: Adi Parva: Sambhava Parva: Sections XCVI, XCVII and XCVIII. The details are as follows: -

1.Ganga is cursed by Brahma to live on earth. The 8 Vasus are also cursed by Rishi Vasishtha to live on earth. The Vasus ask Ganga to become their mother on earth. They also tell Ganga to toss her into the water immediately at birth, in order to quickly return to heaven. Ganga accepts your request.

2. As cursed by Brahma, Ganga comes to earth. A very virtuous king named Prateepa who spent many years in ascetic penance at the source of the Ganges. One day Ganga meets King Prateepa and expresses a desire to become his wife. King Prateepa politely refuses and promises to make Ganga his daughter-in-law instead. A son Shantanu (Ishvaku king, Mahabhisha, who was reborn due to a curse from Brahma) is born to King Prateepa at his old age. King Prateepa keeps his promise to make Ganga his daughter-in-law.

3. Shantanu promises not to question Ganga actions. Ganga and Shantanu get married. Eight children are born to them (the eight Vasus cursed by Rishi Vasishtha). Ganga throws seven children into the water immediately after their birth. When the eighth child (Vasu Prabhasa reborn on earth) is about to be thrown into the water, King Shantanu breaks his promise and stops Ganga.

4. Ganga's response and the following events are as follows:

Ganga replied: O you offspring, you are already the first of those who have children. I will not destroy your child.

But according to our agreement, the time of my stay with you is over.

I'm Ganga, Jahnu's daughter. I am always adored by the great sages;

I have lived with you for so long to accomplish the goals of the heavenly ones. The eight famous Vasus, endowed with great energy, had to take human forms after Vasishtha's curse. There was no one on earth but you who deserved the honor of being its producer.

There is also no woman on earth except one like me, a heavenly body of human form, to become her mother. I took a human form to bring it out. You also became the father of the eight vasus and acquired many regions of constant bliss.

It was also agreed between me and the Vasus that I should rid them of their human forms as soon as they were born. This is how I freed her from the curse of Rishi Apava. Bless you; I'm leaving you, oh king!

But raise this child of rigid vows. That I should live with you for so long was the promise I made to Vasus. And let that child be called Gangadatta. '"

5. The 8th child Gangadatta was therefore not thrown into the water. The child was named Devavrata and became Bhishma, one of the most famous figures of Mahabharata.