1.
The upper, middle, or lower ~ of a river are parts of a river. The upper ~ are nearer to the river’s source and the lower ~ are nearer to the sea into which it flows.
This year water levels in the middle and lower ~ of the Yangtze are unusually high.
N-PLURAL: usu the adj N of n
2.
You can refer to the distant or outer parts of a place or area as the far, farthest, or outer ~. (FORMAL)
...the outer ~ of the solar system.
N-PLURAL: usu the adj N of n
3.
You can refer to the higher or lower levels of an organization as its upper or lower ~. (FORMAL)
...the upper ~ of the legal profession.
= echelons
N-PLURAL: usu the adj N of n