low-water mark

/L OW1 W AO0 T ER0 M AA0 R K/
noun
  1. 1

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

  2. 2

    (by extension) The nadir or lowest point of something.

Source: https://en.wiktionary.org/wiki/low-water%20mark