Who was the mother of Bhishma?

Ganga was the mother of Bhishma in the Mahabharata.

Bhishma was born to King Shantanu of the Kuru dynasty and Ganga, who is portrayed as the divine personification of the river. At birth he was named Devavrata. His connection to Ganga is preserved in epithets such as Gangaputra, meaning “son of Ganga,” and Gangeya.

The birth story explains why Ganga left Shantanu. She had agreed to marry him on the condition that he never question her actions. After each of their first seven sons was born, she cast the child into the river. Shantanu remained silent until the eighth birth, when he intervened. Ganga then revealed that the children were the cursed Vasus and that drowning them released them quickly from mortal life.

The eighth son survived and became Devavrata. Years later, he renounced his claim to the throne and vowed lifelong celibacy so Shantanu could marry Satyavati. That terrible vow earned him the name Bhishma. He later served as elder, adviser, and commander of the Kaurava army, despite his affection for both Kauravas and Pandavas.