Once Amba visited the Vatsa Kingdom, and was performing penance and Goddess Ganga appeared in front of her, and heard that Amba’s penance was for the purpose of destroying Bhisma. Ganga fumed and cursed Amba saying she would flow as a crooked river just like her crooked mind, which would be dry for 8 months of a year, and would flow for 4 months. She added that the river would have crocodiles and other fierce creatures making it a difficult terrain.
After being cursed, Amba wandered in different places and visited many tirthas and finally returned to Vatsa Kingdom, where Ganga’s curse came to being. Amba due to her ascetic merit remained a human but her lower half turned into a river.
Though Amba was persuaded against her austerities (tapas), she continued to performed rigorous penance. Finally, after long, Lord Shiva appeared, and granted her a boon that she would be the reason for Bhisma’s death and in her next life she would be reborn as Shikhandi, the daughter of Drupada, the King of Panchala. After the boon was granted, Amba lit her funeral pyre and self-immolated to be reborn as Shikhandi.