Yes, it's true. Most of the water that flows from the Mississippi into the Gulf of Mexico comes from rain. And most of that rain comes from ocean evaporation. Sure, there's some input from aquifers, and some recharging of aquifers. And there's evaporation from the Mississippi, its tributaries, and all of the associated lakes, wetlands, and so forth.

But the fact remains that the Mississippi is part of the global water cycle. And cycles are circles.