ˌhigh ˈwater mark BrE AmE noun [singular]
1 . the mark that shows the highest level that the sea or a river reaches
2 . the time when someone or something is most successful
high water mark of
the high water mark of Herrera’s presidency
HIGH WATER MARK
Meaning of HIGH WATER MARK in English
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012