I got to know about a tale, of Ganga's curse to Amba to become a river without water or something similar to that because of her attempt to kill Bhishma. What was that exact curse and in which parva of the Mahabharata it's mentioned or is it from some other Hindu scripture or folktale?
The story is told in this chapter of the Udyoga Parva of the Mahabharata. As I discuss in this answer, when Bhishma refused to marry Amba, Amba sought the help of Vishnu's incarnation Parashurama, who was Bhishma's guru, to force Bhishma to marry her. Bhishma and Parashurama fought an eighteen-day war in Kurukshetra, mirroring the Mahabharata war that would happen generations later in the same place, but ultimately Parashurama was convinced to stop fighting. Parashurama tried to reason with Amba, but Amba wasn't interested in anything but revenge at this point. So she started engaging in Tapasya (deep meditation) with the goal of killing Bhishma.
Then Ganga, goddess of the Ganga river and mother of Bhishma, appeared before her and asked why she was engaging in Tapasya. Amba said she was trying to kill Bhishma. Here's how Ganga responded:
Hearing these words of hers, the Ocean-going (river Ganga) replied unto her, saying, "O lady, thou art acting crookedly! O weak girl, this wish of thine thou shalt not be able to achieve, O faultless one? if, O princess of Kasi, thou observest these vows for destruction of Bhishma, and if thou takest leave of thy body while observing them, thou shalt (in thy next birth) become a river, crooked in her course and of water only during the rains! All the bathing places along thy course will be difficult of approach, and filled only during the rains, thou shalt be dry for eight months (during the year)! Full of terrible alligators, and creatures of frightful mien thou shalt inspire fear in all creatures!" Addressing her thus, O king, my mother, that highly-blessed lady, in seeming smiles, dismissed the princess of Kasi.
That highly fair damsel then once more began to practise vows, foregoing all food, aye, even water, sometimes for eight months and sometimes for ten months! And the daughter of the king of Kasi, wandering hither and thither for her passionate desire of tirthas, once more came back, O Kauravya, to Vatsabhumi. And it is there, O Bharata, that she is known to have become a river, filled only during the rainy seasons, abounding with crocodiles, crooked in her course, and destitute of easy access to her water. And, O king, in consequence of her ascetic merit only half her body became such a river in Vatsabhumi, while with the other half, she remained a maiden as before!
So half of Amba remained in human form, and she continued to engage in Tapasya. Finally Shiva appeared before her, and Amba asked that she be reborn as a man so that she could kill Bhishma. Shiva granted her the boon, and she immediately jumped into a fire. She was reborn as Draupadi's brother Shikhandi, who was instrumental in Bhishma's death, because Bhishma was unwilling to fight someone who had been a woman in a past life.