250,000 gallons per second, actually. The elevation of Lake Powell is currently about 3650’, and the elevation of the point near New Orleans can’t be much above sea level.
I’ll leave to the engineers an estimate of pumping costs to raise 13.5 trillion gallons of water more than 3600 feet and to push it through however many miles of pipeline.
You have to raise it 7500 feet to get it over the continental divide.