An Etymological Dictionary of the German Language/Bach
Bach, m. (MidLG. and LG., fem.), from the equiv. MidHG. bach (plur. bęche), m. (MidG. fem.), OHG. bah, m., ‘brook.’ Comp. OSax. bęki, MidLG. beke, Du. beek; a corresponding Goth. *baki-, m., is wanting; beside which the equiv. AS. becc, and OIc. bekkr (whence E. beck), m., presuppose a Goth. *bakki-. No Aryan root bhag- with a meaning applicable here can be found; both HG. backen and Gr. πηγή), ‘source,’ are scarcely allied to it, though Sans. bhañga, ‘breach, wave’ (see Bruch) may be so.