high-water mark

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

   Hochwassermarke

   a line marking the highest level reached

2. (broadly, figuratively)The peak, apex or acme 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.

regular