Millions of people in the Southwestern US are dependent upon the Colorado River and Lake Mead for water, as well as for power generated by the Hoover Dam there. Climate Change is threatening the entire way of life for the residents of these states, and will have nationwide consequences if the Southwest runs out of water. In California alone there are 77,500 farms producing more than 400 commodities, and two-thirds of the nation’s fruits and nuts. In Nevada and Arizona tourism alone generates billions.

Why can’t we get the states that draw water from the Colorado River as well as our Federal Government (and maybe Mexico since they get water too) to go all in on making massive desalinization plants in California and building a pipeline to Lake Mead to refill it? This would give everyone a larger share of water, create thousands of jobs, create economic security in not having to worry about the lake drying up, and would help tackle rising sea levels. It would also ensure that Hoover Dam continues to generate electricity.

Yeah, it would cost a lot, but if we can build things like the Panama Canal and Hoover Dam decades ago in remote regions with basic tech and equipment then certainly we can do this. We build pipelines for oil to refineries for hundreds of miles, this isn’t that different. 
We could power the desalinization plants with solar and/or wind power so that they don’t cause any air pollution. 
It may sound ambitious but if we don’t do something soon it’ll be too late.