tideline

noun

1. A line of floating debris, seaweed etc. that marks the boundary between two surface currents.

   “And last year, at 5:26 a.m. on August 10, a whole mountainside toppled into the tideline of the shrunken Sawyer Glacier, not far from the Alaskan capital, Juneau. The impact jetted a tsunami a staggering 500 meters up the walls of the fjord.”