low-water mark
noun [ C usually singular ] uk/ˌləʊˈwɔː.tə ˌmɑːk/ us/ˌloʊˈwɑː.t̬ɚ ˌmɑːrk/ (also low-water line)
最低水位标记，低潮线
the lowest point on a beach that is reached by the sea at low tide
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