Rob Davis at Voice of San Diego has an excellent explainer this week on his community’s dependence on Colorado River water, and the central problem of what happens if Lake Mead gets really low:

If the Colorado consistently comes up short, no one knows who will cut consumption to keep Lake Mead from running dry. A short-term plan is in place from the time the reservoir hits 1,075 feet above sea level until 1,025 feet. Arizona and Las Vegas take that hit. (At 895 feet, the reservoir wouldn’t be able to distribute water and the Hoover Dam wouldn’t produce power. The lake is currently at 1,096 and climbing.)

Hitting those triggers would be monumental. At 1,025 feet, the reservoir would be a 25-foot drop away from cutting off Las Vegas. Someone would have to take less water to stop the decline. But there is no plan on how to ensure water gets to the big city in the middle of the desert — only an agreement that states will meet to negotiate. And with the vicious politics that can thrive around water, that meeting won’t be easy. It would require Mexico, California, Arizona or Nevada to give up some of their water.

I think Rob nails the central dilemma here – the question of now how will we use less water (taking ag land out of production, urban conservation, slow/no growth, etc.) but how the various parties on the river will allocate the shortages. That’s the horse – the shortage allocation process. The cart pulled behind, then, will determine how the individual water-using entities will conserve to make up their share of shortages.