high-water line

variants: or high-water mark

Definition of high-water line

1a : the line of the shore of the sea or of a lake or river to which the waters usually reach at high water:
(1) : the line that marks the limit of the rise of the medium tides of the sea between the spring and neap tides
(2) : the line that marks the limit of the soil so affected by the water of a lake or river as to have a nature and vegetation distinct from that of the banks
b : a mark showing the highest level reached by a body of water
2 : the highest point : acme the high-water mark of a girl's social career— Hamilton Basso

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First Known Use of high-water line

1691, in the meaning defined at sense 1a

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The first known use of high-water line was in 1691

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