I've wondered about this, but, political impossibilities aside, how long would it take the water to scour a new river bed large enough to carry the water of Mississippi??? Aren't we talking about geologic time here? If vastly more water were allowed to flow down the Atchafalaya tomorrow, wouldn't we just have massive flooding problems all along that river's banks???
"In the space of one hundred and seventy-six years the Mississippi has shortened itself two hundred and forty-two miles. Therefore... in the Old Silurian Period, the Mississippi River was upward of one million three hundred thousand miles long... seven hundred and forty-two years from now the Mississippi will be only a mile and three-quarters long. There is something fascinating about science. One gets such wholesome returns of conjecture out of such a trifling investment of fact."
-Mark Twain