low-water mark

plural low-water marks

1. A mark, such as a line of seaweed, showing the lowest level reached by a body of water.

   Seven miles to the north of Venice, the banks of sand, which near the city rise little above low-water mark, attain by degrees a higher level, and knit themselves at last into fields of salt morass, raised here and there into shapeless mounds, and intercepted by narrow creeks of sea.

   1853, John Ruskin, “Torcello”, in The Stones of Venice, volume II (The Sea-Stories), London: Smith, Elder, and Co., […], →OCLC, § I, page 11

   Antonyms: high-water mark

2. broadlyThe nadir or lowest point of something.

   Antonyms: high-water mark, acme