Legend about origin of rivers

Once upon a time the serpent lived on the earth. He devoured a lot of people since there was no any other one stronger of him in the world. At that time God’s smiths Kuzma and Demyan lived. And so they decided to destroy that serpent and cast him out of this world. The serpent came to them, and they went to the forge and shut the iron door.
The serpent tells them:
- Kuzma, Demyan, God’s smiths, open the door, or I will swallow you together with your forge!
Behold they say:
- If you have power, then lick though the door. And we will then sit down on your tongue and you may swallow us.
So the dragon licks while the smiths keep heating up the iron and forging the pincers. And the serpent licked through the door and put his tongue into the licked hole. That’s when Kuzma and Demyan got serpent’s tongue with their pincers! And they began to hammer it... So they exhausted the serpent, harnessed him into a plough for twelve pairs of oxen and started to plough.

They ploughed the steppe up and down. So often the serpent asked them to let him drink or eat  - they did not respond.
- You have enough, - they say, - enough of the fat that you got from people!
- Well, - the serpent says, - if this is so than I will light the whole world with my fat before the horrible judgment!
Whether they ploughed for a long time or not – they came to the sea. A dragon dipped his tongue into the sea and began to drink. So he drank and drank and finally drank the whole sea out. Kuzma and Demyan got the serpent and dug him under a mountain. Goodness knows when in the world this was. And suddenly few years ago kerosene started pouring out of that mountain. Seems like the end of the world is near...
In town outskirts even now not everyone burns kerosene since it is impure...
Before Kuzma and Demyan exhausted the serpent they ploughed so deeply that rivers started flowing, and when they exhausted him they ploughed not so deeply - and so ravines appeared. This is how rivers and ravines appeared!