The Collaborative International Dictionary of English v.0.48:

Water gall \Wa"ter gall`\
   1. A cavity made in the earth by a torrent of water; a
      washout.
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   2. A watery appearance in the sky, accompanying the rainbow;
      a secondary or broken rainbow.
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            These water galls, in her dim element,
            Foretell new storms to those already spent. --Shak.
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            False good news are [is] always produced by true
            good, like the water gall by the rainbow. --Walpole.
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