The Celestial Emperor = ruler of heaven and earth, sometimes a bit testy
Gǔn 鲧 = his annoyingly compassionate grandson
Yǔ the Great (Dayǔ 大禹) = a legendary emperor (and/or dragon)
Yǔ the Great (Dayǔ 大禹) was the first monarch of the fabled Xià 夏 dynasty (reign 02a-1). As it happened, he was also a dragon. This is his story.
Emperor Yáo 尧 (reign 01a-8) is famous for his humility, but some of his subjects were less virtuous, and the Celestial Emperor (Tiāndì 天帝) decided to send a great flood to wipe them out. Water rose all across the world, and poisonous snakes and other dangerous animals were driven among people.
Among the gods there was one, named Gǔn 鲧, a grandson of the Celestial Emperor, who felt compassion for the humans, and pleaded with his grandfather to save them. The Emperor was unmoved.
Now the Celestial Emperor had among his treasures a substance called Self-Multiplying Dirt (xīrǎng 息壤), which could produce unlimited quantities of earth to build mountains or dikes or anything else.
Unwilling to stand by and let the humans drown, Gǔn secretly took the Self-Multiplying Dirt and headed to earth in hope of stopping the floods. The treasure worked beautifully, and the floods were indeed stopped, but the Celestial Emperor was furious and ordered the gods of fire to kill Gǔn and bring back the Self-Multiplying Dirt.
So Gǔn was killed and the Self-Multiplying Dirt returned to its place among the celestial treasures, and the floods began once again.
But for three years Gǔn’s body did not rot, for his mission was still not accomplished and his soul was not at rest. A baby dragon began to form in Gǔn’s slowly bloating gut. The dragon was none other than Yǔ, who was destined to tame the floods and found the Xià dynasty.
Hearing of this strange pregnancy, the Celestial Emperor sent an underling to slit open the bulging belly of Gǔn’s corpse. No sooner was the body opened than out flew Yǔ. He spread his wings and headed for the Celestial Palace. There he sought the Celestial Emperor and explained that he wanted to finish his father’s work to save the suffering human beings from the horrible flood.
The Emperor was impressed that Gǔn’s sincerity was so great as to have caused him to become postmortally pregnant and bear such a son, and so he agreed to the project of saving suffering humans from the flood. He even lent Yǔ the Self-Multiplying Dirt and an answering dragon (yìnglóng 应龙) to assist him. (When undertaking a great task, it is always helpful to have an answering dragon on call to help.)
Reaching the flooded earth, Yǔ recruited a large tortoise and sat on its broad back and hurled Self-Multiplying Dirt all about. Meanwhile the answering dragon went before him, drawing rivers with his tail to lead the water to the sea. (Indeed, this is why we have our modern rivers and why they lead to the sea.)
And thus did Yǔ control the flood and save humanity. And of course he fulfilled his father’s dying mission.
Persons of weaker imagination tell the story slightly differently. Yǔ, they say, was a minister under emperors Yáo 尧 (reign 01a-8) and Shùn 舜 (reign 01a-9). There were great swampy areas and much flooding, and emperor Shùn appointed Yǔ to solve the problem. He worked very hard for nine years, never even stopping to see his family when he passed his house. And in the end the flooding was controlled.
Legend credits him with the invention of dams and dikes and the best methods for irrigating fields. This version of the story has no Self-Multiplying Dirt and no answering dragon.