I’m going to be criticized, but here goes: Build a huge water pipeline from the Missouri and Mississippi Rivers to the Colorado River. In years when those rivers flood — and only in those years — pipe as much water as possible to the Colorado River (and any other western rivers on the way that are in need).
We can then fill up Lake Powell and Lake Mead and quench the thirst of Las Vegas, Phoenix and Los Angeles. The best part: Make California, Arizona and Nevada pay for it since it is mostly for their benefit.
West Valley City