The earliest mention of this in the chanty literature—that I'm able to find at the moment!—is by C. Fox Smith in her 1927 collection. (I have looked at writing mentioning this song back to the 1860s, but, unless I didn't put it in my notes because I thought it wasn't notable, I don't find attention called to this feature.)

I'm not sure when in the year 1927 Smith's book was published, but the March 1927 issue of Gramophone had a review of a recent batch of recordings of chanties commercially released. The review covers 5 different groups' recordings (chanties were REALLY a popular fad at that time, it seems), and all contain renditions of "Rio Grande." About two, the author of the review notes that they pronounced Rio incorrectly--they sing Ree-o instead of Rye-o. I can't say whether the author knows this from experience with oral tradition or if he/she got it from a book like Smith's.

Doing a quick check, these field-recorded singers pronounce "Rye-o":

Mark Page - Carpenter Collection - at sea 1849-1879 - recorded in late 1920s

Joseph Hyson of Mahone Bay, Nova Scotia - Helen Creighton - recorded sometime between 1930s-1956

Leighton Robinson of Falmouth - Library of Congress, Sidney Robertson Cowell - went to sea 1888 - recorded in California, 1939

St. Vincentian whalermen of the 1960s sang "Royo Groun."


Myself attempting to sing in this style, "Royo Groun"