high-water mark

plural high-water marks

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

   Madocks built his model town on land partly reclaimed from the sea, and was justifiably proud of the fact that it stood well below what was still the official high-water mark. Until then, the surrounding area was largely marshland.

   1980, AA Book of British Villages, Drive Publications Ltd, page 385, about Tremadog

   1 more example

   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.

   1961 January, “Talking of Trains: The Severn Bridge disaster”, in Trains Illustrated, page 5

   Antonyms: low-water mark

2. broadlyfigurativelyThe 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.

   Antonyms: low-water mark