[SunkenCiv]

The truth is, I'm also not subscriber, but found this irresistable snippet while looking for something quite different on that website. I may have bought the issue anyway... hmm, can't find it... ah, last week's pizza box... still fresh... mmm...

[colorado tanker]

That seems like a good guess. By the time the Mississippi formed, the Ouachitas would have eroded to their modern height, making it fairly easy for the huge amount of water coming available at the end of the ice age to break through.