The Attack of the Serpent King (Kaliyamardhana)

The river Yamuna was polluted by the venom of the black serpent Kaliya residing in a lake within the river. The whole area was so contaminated that it emanated a poisonous vapor twenty-four hours a day. The trees and grass near the bank of the river dried up due to the effect of the poison. If a bird happened to fly over this area, it would immediately die and fall down into the water.

Once Krishna and Balarama, along with Their boyfriends, were tending Their cows near the banks of the Yamuna. The boys and cows became very thirsty and drank the contaminated water from the river. All of them fell down on the ground, apparently dead. Krishna restored their lives by His merciful glance and He decided to teach a lesson to Kaliya. He climbed up on a big kadamba tree on the bank of the river and jumped into the river. He subdued Kaliya and ordered him to leave Yamuna and go to the ocean. 

This pastime is narrated in the sixteenth chapter of the tenth canto of Srimad-Bhagavatam. It is said that anyone who recites / hears this pastime need not fear the envious activities of snakes. If one takes bath in the Kaliya lake (within the Yamuna) or if one fasts for a day and offers oblations to the forefathers from the water of this lake, he will be relieved from all sinful reactions.