The fact that the rod of iron is so close to this river is significant. Remember that Nephi later describes a terrible gulf that this river runs through. The depths of the river represent the depths of hell. Therefore, the depths of hell are not far from the rod of iron; it doesn't take many steps off the strait and narrow path before one falls into the terrible gulf and the river. Mormon wrote, thus we can plainly see that after a people have been once enlightened by the Spirit of God, and have had great knowledge of things pertaining to righteousness, and then have fallen away into sin and transgression, they become more hardened, and thus their state becomes worse than though they had never known these things. (Alma 24:30)