Interpretation refers to the geographical location close to mountains and streams.
The source is Shi's "Anji Huaishen Province": "There are hundreds of thousands of people who cross the river for survival, and there are also hundreds of thousands of people who rely on mountains and waters."
Structural joint idioms
Used as predicate and attribute; Refers to geographical location
A synonym for leaning on mountains and rivers.
The rest of the idiom riddle:
Draw a gun when you see a fly (to use a four-word idiom)-Answer: Make a mountain out of a molehill.
Call a ghost when you see an ugly girl (a four-word idiom)-answer: make a fuss.
Flirt with each other (to use an idiom)-answer: an eye for an eye.
Sumo (to use a four-word idiom)-Answer: Go shirtless.