Source : Webster's Revised Unabridged Dictionary (1913)

Water gall \Wa"ter gall`\
   1. A cavity made in the earth by a torrent of water; a
      washout.

   2. A watery appearance in the sky, accompanying the rainbow;
      a secondary or broken rainbow.

            These water galls, in her dim element, Foretell new
            storms to those already spent.        --Shak.

            False good news are [is] always produced by true
            good, like the water gall by the rainbow. --Walpole.