一个较大的流域由许多小流域集合而成，一个小流域又由一些更小的集水单元组成。
A large valley is gathered together by a number of small valleys, a small valley is formed by some of the smaller catchment units.
一个较大的流域由许多小流域集合而成，一个小流域又由一些更小的集水单元组成。
A large valley is gathered together by a number of small valleys, a small valley is formed by some of the smaller catchment units.
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