True, you wouldn’t want to take much water from the Mississippi during normal or low levels, but how often does the mississippi go to flood stage? Seems like every five years or so. You can turn on the pumps and siphon off some of the excess volume to transport to Lake Powell to raise it’s levels. The extra volume there could be preserved for years.
And the desert dwellers would then have a conniption fit when you decide to turn them off.