An Etymological Dictionary of the German Language/Annotated/-a, -ach

by Friedrich Kluge

-a, -ach, a frequent suffix in the formation of the names of brooks and rivers (or rather the places named after them); on the whole, -ach (Urach, Steinach, Salzach, Rotach, Schwarzach) is more Upper German, -a more Middle German, and Low German (Fulda, Berra, Schwarza); from Old High German aha, ‘running water,’ Gothic ahwa, ‘river' (for details see Au), whence also the names of the rivers Aa (Westphalia), Ohe (Hesse).