In the past, the course of the Yellow River was not fixed and would often change its course (this happened countless times in history). A certain place was originally on the east side of the river. A few years later, due to the change of flow of the Yellow River, this place will be on the west side of the river. This sentence is a metaphor for the ups and downs of personnel, which are constantly changing and sometimes turn to the opposite direction, which is unpredictable.
Thirty years in Hedong, thirty years in Hexi, don’t deceive young people into poverty
It means: Thirty years in Hedong, thirty years in Hexi, the wind and water are changing, the world is changing, so Don't bully a young man now just because he is poor.
Synonyms: this time, that time.