Some trivia about the Mississippi River: the US Army Corps of Engineers keeps it flowing below Simmesport. If it wasn't for them, most of the water would heve been diverted to the Atchafalya River Basin - returning New Orleans to its condition of salt-water bayous (Actually, I suppose that's not very trivial if you happen to live in Louisiana)
Long article here
http://www.newyorker.com/archive/198...?currentPage=1
So.. I'm tempted to say that, yes, the Red River is essentail for KFS/Texas trade - but so are overland routes (there are possibilities in Arkansas)