What is "water gall"

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.