...I'd write my love a letter that she'd understand;
   I'd write it by the river where the waters o'erflow...Ritchie vsn.

The differences here may be just a matter of natural changes over the years- someone filling in a gap when he/she cannot remember just how the words lay, or something like that. A "corruption" in the wording, the scholars describe it. But who knows what the "original" poem said? And it's all pretty...