The Qinling Mountains are the watershed between northern and southern China.
Tips
usage
Both the literal (geography) and figurative ('turning point') senses are everyday. 分水岭 as metaphor parallels English 'watershed moment', so the translation usually lines up one-to-one.
memory
Break it apart: 分 (divide) + 水 (water) + 岭 (ridge) = a ridge that divides water. The water on each side flows to a different river system, just like a 'watershed moment' sends events flowing in different directions.
