Lehi’s vision, as Nephi reported it, didn’t emphasize the river as a dividing line. For Nephi, however, it had been part of his vision that showed it to be more than a line, but an awful gulf. Nephi even notes that his father didn’t notice the filthiness of the water because he was so focused on other aspects of the vision.
The nature of allegories and visions is that they can have different interpretations. Nephi sees a different vision than his father did, not because there are two truths, but because there are two different beings, with different life experiences, seeing the same vision. The symbols can contain multiple messages, and some will see one meaning in the symbol and another person a different one.
As Nephi teaches his brothers, he teaches them according to his own vision, not his father’s. The questions Laman and Lemuel ask are ostensibly about Lehi’s dream, but all of them function in the way Nephi tells the story to emphasize Nephi’s growing position in the family. He teaches his brothers, when they should have taught him. Nephi teaches his vision, rather than Lehi’s, because Nephi will increasingly be the conduit through whom Jehovah guides the family.