Pat Mulroy has a problem: Las Vegas only has enough water to sustain its phenomenal rate of growth until 2013, and the Basin and Range groundwater project likely won't come online until at least five years after that.

To help bridge the gap, Mulroy is pressing forward with another water project, a $1 billion plan to divert water from the Virgin and Muddy rivers east of Las Vegas. But there's one hell of a catch: Both are tributaries of the Colorado River, and reaching for their water could trigger a fight that other Colorado River Basin states have spent decades cracking their knuckles for.

Six years of drought and the emerging data from tree-ring studies in the region indicate that there is far less water in the river than the negotiators of the Colorado River Compact thought in 1922 (HCN, 3/21/05: What's worse than the worst-case scenario? Real life). And the Upper Basin states — Utah, Wyoming, New Mexico and, most prominently, Colorado — are beginning to parse the accounting of the river much more scrupulously than ever before.

"There are questions that have just been out there that haven't been answered," says Jim Lochhead, an attorney in Glenwood Springs, Colo., who represents a consortium of Colorado cities and water districts. "It's getting to be about that time."

One of the biggest points of contention centers on the tributaries that feed into the Colorado River in the Lower Basin states of Nevada, Arizona and California. The Compact is vexingly ambiguous about how water in those rivers should be charged against each of the three states’ share of the Colorado. Right now, much of it simply isn’t; in large part, the Lower Basin tributaries are counted as separate waterways entirely.

The Upper Basin states argue that those rivers ought to be considered part of the Colorado, just as Upper Basin tributaries such as the Gunnison River are. By that measure, the Lower Basin states are already using more tributary water than they have rights to. Colorado has periodically threatened to sue to bring that water onto the books, and if Mulroy moves on the Virgin and the Muddy, the entire "tributary issue" will come to the fore.

"That’s the whole enchilada, right there," says Lochhead. "The tributary issue is the basic, fundamental thorn that has been there since 1922."

In July, the state of Colorado warned the Bureau of Land Management — which is currently preparing an environmental impact statement on Mulroy’s Virgin and Muddy proposal — that it "believes that the Lower Basin, as a whole, has developed more ‘Colorado River System’ water than it is legally entitled to." It went on to say that it has "significant concerns about the development of any additional surface water supplies from the Colorado River or its tributaries in the Lower Basin."

But no state — with the possible exception of Nevada — wants to end up in court. "The downside for everyone in taking (the Compact) to court is (the chance) that the Supreme Court tosses the whole thing out," says Eric Kuhn, the general manager of the Colorado River Water Conservation District. "There may be a legitimate and rational decision made by the Upper Basin states not to litigate the issue, because the consequences might leave us worse off."

Instead, the Upper Basin states are quietly suggesting that Mulroy go after the Basin and Range groundwater rather than the Virgin and Muddy rivers. The groundwater project, in the words of Scott Balcomb, Colorado’s lead negotiator, "would postpone indefinitely the need to fight over the shortage" on the Colorado River. But it would also put Mulroy right back at square one — pursuing a groundwater project that, at the earliest, won’t be ready until five years after her current supplies are fully committed.

Mulroy’s designs on the Virgin and the Muddy may, however, actually help her scare up some charity from the other states. In an apparent effort to stay out of court, they have indicated recently that they may be willing to help. In August, representatives of the seven states sent a letter to Interior Secretary Gale Norton proposing a raft of measures to "avoid political and legal confrontation over the meaning of fundamental aspects of the Law of the River." The states are now seeking, among other things, Norton’s help in laying a framework that would allow them to exchange water between themselves. That would clear the way for other states to pitch in with some of their own Colorado River water to tide Mulroy over until the groundwater project is ready to go.

Doing that would put off the final reckoning on the Colorado River. But it will also allow Las Vegas to keep growing, to the tune of some 100,000 new people each year — which means that the next time Mulroy goes for the Colorado, the stakes will be much, much higher.