Still, that hasn't been a problem -- yet. Lake Mead and Lake Powell provide the backup capacity that ensures enough water for every state. "But what makes that sustainable is only these rare -- and maybe becoming rarer -- big, wet (El Nino) events that fill Lake Powell," says Kuhn. In the middle of an extended drought, a powerful El Nino could temporarily boost reservoir levels. But if the drought continues, levels will continue dropping again. "Then," he says, "you're waiting to see if you have another wet year."

After nearly a decade of drought, the reservoirs are half empty. If they continue to drop, that will touch off a fight over what little water is in the river, like creditors battling over the carcass of a bankrupted company. And, owing to the 75-over-10 provision, California and the other Lower Basin states get first dibs on their 7.5 million acre-feet. If conditions get bad enough, the Lower Basin states could make a legal "call" on the river and demand that the Upper Basin not take any of its Compact water until the 10-year average once more rose above 75 million acre-feet.

Legal scholars David Getches and Charles Meyers described the predicament poignantly 23 years ago: "How to build a future on the right to leftovers?"

While there may not be much water left for new projects in Colorado, there is also a high risk that existing users -- anyone whose rights post-date the 1922 signing of the Compact -- will be "called out" and cut off to meet the Lower Basin's demands. That could set off a civil war within the state.

To comply with a call, Colorado and the other Upper Basin states would have to shut down their own water users to satisfy their share of the "delivery obligation." Currently, that would be done according to the rules of prior appropriation: More recent, or "junior," users would be shut down first; then the state government would work its way backwards, shutting down increasingly senior rights until the delivery obligation was met.

But some of the most junior -- which is to say, the most at-risk -- water users are the water agencies that pipe Colorado River water under the Continental Divide to supply the Front Range. Chips Barry is the manager of Denver Water, the state's largest urban water agency, which supplies about 1.3 million people. In a worst-case scenario, "every right on the Colorado River, post-1922, gets called out," he says. "Now if that happens, that's a disaster for the state of Colorado, and for Denver Water."

Denver would implement strict water conservation and efficiency measures, but beyond that its only option would be to buy water rights around the state that were in use prior to 1922. And every acre-foot that Denver tried to grab would be one that someone else in Colorado already uses.

That would pit the state's cities against its farmers, and put the problem straight back in Kuhn's lap: Most of those pre-1922 rights are held by farms in the River District's territory. And so Kuhn is already thinking about how to prevent the Front Range from buying up farms on the Western Slope, taking their water, and leaving them dry.

The plan -- which he's been developing with Denver Water and several other water agencies -- is to create a water "bank" that bundles together pre-1922 water rights on the Western Slope. Those could then be used to cover deficits on the Front Range in a call. The bank would sign agreements with farmers in advance, and pay them to irrigate, say, only half their ground in a bad year. The water that would have been used on the other half would then go to cover critical municipal uses. When the drought ended, the farmers would get back full use of their water.

Kuhn insists that such a deal can only come if the Front Range cities put aggressive water-austerity measures in place and use Western Slope water for only the most critical uses. But he acknowledges that it would be impossible for the Western Slope to simply stonewall the Front Range if its cities are clamoring for water in an emergency. "You can ask them to brown their lawns, but their fire hydrants still have to be charged," he says. "You can't ask them to go without (drinking) water."