river that wants to be the Mississippi River
Water likes to follow the path of least resistance, the path that dissipates
its potential energy the quickest -- that is, the steepest path to the
sea. The Atchafalaya is much shorter (142 mi versus 315 mi),
starting from the same height, and thus much steeper than the lower Mississippi. The Atchafalaya now carries 30% of the Mississippi River's flow.
Left to nature, it is only a matter of time before the river changes course.
Of course things are rarely left to nature. Levees built along
the river, natural and otherwise, keep the river within its current course.
But most importantly, the Old River Dam (follow the link) keeps
Old Man River, or 70% of him at least, flowing past New Orleans, and not down the Atchafalaya.