Howard S. answered 05/23/19
Sage's multi-dimensional approach brings joy and learning
"Rivers" are simply a typographical technique to make sure newspaper readers do not "escape" the page that the editors want them to read.
"Rivers" don't really apply to poetry at all. Poetry, spacing, different margins, etc. relate to the rhythm and meaning the writer wants her/his poem to have.
Examine some poets that do this and you will be able to apply it to your own work.