Noun
a line marking the lowest level reached
an extreme state of adversity; the lowest point of anything
Source: WordNetOhio's southern border is defined by the Ohio River (with the border being at the 1792 low-water mark on the north side of the river), and much of the northern border is defined by Lake Erie. Source: Internet
The normal low-water mark is convert below datum (577.5 ft or 176.0 m). Source: Internet