According to the Bhagavata Purana, a great many-headed serpent named Kaliya lived in the river Yamuna. His venom had poisoned the water so badly that the river had turned dark. Trees along the bank had withered. Even birds were afraid to fly above that stretch of the river.
The cows and calves of Gokul would sometimes drink from the Yamuna and fall sick. Krishna and his friends played near that ghat (riverbank) too. One day, their ball rolled into the river.
Krishna climbed a tall kadamba tree on the bank and dived into the water. His friends cried out — 'Kanhaiya, don't go!' But Krishna was already in the river. Kaliya rose up, hoods spread wide.
The Bhagavata Purana tells us that Krishna danced on Kaliya's hoods. He moved from one hood to the next, dancing lightly. Kaliya grew exhausted. His strength faded. Finally, he lowered his heads in submission.
Then Kaliya's wives — the she-serpents — came forward and bowed before Krishna with folded hands. 'Krishna, please spare our husband,' they said. Krishna looked at Kaliya. There was repentance in the serpent's eyes.
According to the Bhagavata Purana, Krishna did not punish Kaliya harshly. He said — 'Leave this river. Go to Ramanak Dvipa. You will have nothing to fear there.' Kaliya bowed and departed.
Slowly the Yamuna cleared. The trees on the bank turned green again. The cows and calves returned to the ghat. Krishna's friends ran to embrace him. That day, Gokul celebrated with great joy.