It would require a tremendous amount of energy. Lake Powell is more than 1km above sea level, which means each liter of water would require 10kJ just in gravitational potential energy.

The reclamation effort is 1 maf (million acre-feet) of water over a year, which if supplied continuously from sea level with perfectly efficient pumping and transport corresponds to ~10 GW of power. A typical nuclear power plant generates 1GW, so without any transport losses, you'd need 10 new plants running full-time just to move the water up the hill.

Add in the desalination challenges and transport losses, and you're talking what, 20-40 new nuke plants?