E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Yacu-mama [mother of waters].
A fabulous sea-snake, fifty paces long and twelve yards in girth, said to lurk in the lagunes of South America, and in the river Amazon. This monster draws into its mouth whatever passes within a hundred yards of it, and for this reason an Indian will never venture to enter an unknown lagune till he has blown his horn, which the yacu-mama never fails to answer if it is within hearing. By this means the danger apprehended is avoided. (Waterton.)