water gall

1. A cavity made in the earth by a torrent of water; a washout. [ 1913 Webster ]

2. A watery appearance in the sky, accompanying the rainbow; a secondary or broken rainbow. [ 1913 Webster ]

These water galls, in her dim element,
Foretell new storms to those already spent. Shak. [ 1913 Webster ]

False good news are [ is ] always produced by true good, like the water gall by the rainbow. Walpole. [ 1913 Webster ]