Well, while I haven't heard a universal standard on it, I can tell you that I rarely conform to rivers unless they make sense.
I mean, if you take Paradox's Niger river, its two separate pieces. The bit near Gao and then that bit that we've been talking about. The Niger River is continuous and isn't in pieces. You can barely see it as well, and so I for one, ignore it. Where its at, its sort of irrelevant.
On Geography, placing Niani West of Jenne without the South component, isn't any better than having it South of Jenne without the West component.