high-water mark

high-water mark (plural high-water marks)

1. (literal) A mark, such as a line of seaweed, showing the highest level reached by a body of water.
   • 1961 January, “Talking of Trains: The Severn Bridge disaster”, in Trains Illustrated, page 5:

     This imposing structure, which not only carries the single track branch line but also the gas main supplying the Forest of Dean area, cost £200,000 when it was built in 1879, and at its highest point is 70ft above the high-water mark.

2. (by extension, figurative) The peak or apex of something; the maximum level; the furthest or highest point.

   The Gettysburg campaign was the high-water mark of the Confederacy's invasions of the North.