How to understand "when the river runs out, a book will be published"?

Luo Shu refers to the origin of He Tu Luo Shu, also known as He Tu Luo Shu, which is a famous legend about ancient civilization in China. Six or seven thousand years ago, the dragon horse jumped out of the Yellow River and had a river map; The tortoise came out of the puddle with a letter on his back. Fuxi drew eight diagrams according to the book of He Tuluo. Since then, Dayu has managed water, Hebo presented a river map, and Fu Fei presented Luo Shu, so that Dayu finally defeated the flood.