Metaphor again: glossed in some commentaries as "the grave" (cf late Latin "lacus", pit). "Earthen lake" also has a specific physical meaning: it is a body of water fed by rainwater rather than by springs or watercourses, having an earthen rather than clay bed. I think that the term is used mostly in America these days. Both senses may perhaps be implicit: the song has every appearance of a literary origin, and complex metaphor is not unlikely.

Bronson (Traditional Tunes of the Child Ballads, 1959, I, 308) follows earlier scholars in suggesting that The Three Ravens is descended from the same ancestral song as The Corpus Christi Carol, the latter being a "pious adaptation" of it. David Fowler, by contrast (Literary History of the Popular Ballad, Durham NC: Duke University, 1968, 58-64) sees Three Ravens as "a secularised, chivalric Pieta" based on Corpus Christi.

There is no final word on that subject, so far as I know, and there probably never will be; for all the romantic ideas (full of Grail Knights and the like) that have been put about on the subject of both songs over the years. Fowler's explanation of Corpus Christi, taking into account the mediaeval "figurative imagination", is elegantly simple (though not easy to summarise adequately) and well worth looking at. How far it might also apply to Three Ravens is moot, of course.