thalweg (TAHL-veg, -vek) noun

   1. A line, as drawn on a map, connecting the lowest points of a valley.

   2. The middle of the main navigable channel of a waterway that serves as
      a boundary line between states.

[From German, equivalent to Thal, now obsolete spelling of tal, valley + weg,
way.]

   "Five others and I have been detailed to Los Trancos Creek, a major
   tributary of the San Francisquito, to prepare a Thalweg profile."
   Edwin Kiester, Jr., A creek defies the odds, Smithsonian, Dec 1997.

This week's theme: words that make one say, I didn't know there was a word
for that.

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Pronunciation:
http://wordsmith.org/words/thalweg.wav
http://wordsmith.org/words/thalweg.ram