high-water mark 
noun
1The level reached by the sea at high tide, or by a lake or river in time of flood.We have to protect that strip of land that is from the high-water mark to the low-water mark....
- South Africa claims that the border is at the high-water mark on the northern side of the river while Namibia insists it is in the middle of the river.
- So, in our submission, the mean high-water mark is the average point to which the sea reaches between those times.
1.1A maximum recorded level or value: unemployment and crime both stand at a high-water mark...
- I think he'll be a fine major leaguer, but it's not a sure thing and this could very well be the high-water mark for his value.
- That's far too long for a high-water mark to stand.
- Board of Education stands as a high-water mark in the modern civil rights movement.
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