The Yadhavas whose chief was Nandagopal were cowherds by profession. They had huge herds of cattle, and they made a living out of the milk that the cows yielded. Being cowherds, the river Yamuna was a life source for the Yadhavas.
One day new creature came to make the river his home. This was Kalinga, the giant many-headed serpent. He was very poisonous. So powerful was his venom, that just a breath from him could kill any creature, large or small.
As soon as the evil Kaliya entered the river to make it his home, he poisoned the water to such an extent that all the creatures living inside the river died.
The next day, Krishna as usual went towards the river Yamuna with his friends to graze the cows. They spent a lot of time playing and laughing. Being extremely thirsty the boys got down to the lake to drink some water.
Krishna looked at the lake. The blue waters of the river Yamuna had turned black because of the poison of the snake. Krishna decided to purify the river by getting rid of the black serpent. The children who were watching from the banks of the river ran home to inform Yashoda.
Hearing the news all the Gopas were frightened and ran to the spot. The wives of Kaliya were looking helplessly at Krishna.
Krishna: Well. I shall not kill any of you. But do not stay any longer in this lake. Men and cattle come here to drink water. It is not right for you to poison the waters and kill them. So all of you proceed to the sea and make it your adobe.
Yashoda and all others were happy to see Krishna back on the bank of the lake unhurt. He was taken in a procession surrounded by cattle, with his friends singing and dancing.