In the Southwest, the saying goes, the whiskey is for drinkin' and the water is for fightin'. And the biggest water fight in decades, which the Bush administration launched on New Year's Day, begins with a trickle of snow melting in the brilliant midday sun in Colorado's Never Summer mountain range.

A thousand miles downstream, the Colorado River roars through the Grand Canyon, irrigates a billion dollars worth of green vegetables in a California desert and waters the golf courses of Las Vegas, Phoenix, Tucson and Los Angeles. But at its headwaters here in Rocky Mountain National Park, the mighty Colorado is a placid stream you can cross with one big step. The Colorado starts small, but "it looms incredibly large in the American West," says water lawyer Joe Sax of the University of California's Boalt Hall law school. "That river is the lifeline of seven states. They're all growing, so they're all fighting harder than ever to get a share of the water."

A "compact" devised by the federal government in the 1920s allocated the water among the seven states of the Colorado River Basin: Wyoming, Colorado, New Mexico, and Utah in the "upper basin," and Nevada, Arizona and California in the "lower." For eight decades, six of the states used less than their allocated share, letting California take much more water than it was legally entitled to.

But a combination of rapid growth and repeated drought prompted the upstream states to demand that the compact be enforced. The Clinton administration ordered California to cut its thirst by Jan. 1, 2003.

California's water users refused to meet the deadline, so Interior Secretary Gale A. Norton enforced the Clinton order. She closed the spigots at Hoover Dam on New Year's Day, cutting California's annual share of Colorado River water by about 13 percent, or 260 billion gallons -- enough to supply about a million households for a year.

The western water dispute is one of the few areas in which the Bush administration has gone along with a Clinton-era policy on a major natural resource question -- and has won plaudits from the environmental community. The reasons, water watchers say, involve policy, politics and personalities.

"The political fact is, Bush is probably not going to carry California anyway" in the 2004 election, says Sax, who worked on the dispute as a senior official in the Interior Department under President Bill Clinton. "Meanwhile, all the upstream states are either in his camp already or winnable, so why not reward them?

"And look at the people involved: Clinton's Interior secretary [Bruce Babbitt] was an Arizona politician. Bush's Interior secretary is a Colorado politician. For people like that, it's only natural to battle California over water."

Beyond that, Sax says, "as a policy matter, it would be hard for Interior to do anything else. There's a clear compact in place and a Supreme Court decision enforcing that compact."

The California water users say they may sue. Los Angeles and San Diego say they will draw from water stored in reservoirs to make up for the diminished flow in their canals. Agricultural users have less stored water and may have to cut production.

The pressure to bring water to the driest corner of the United States has always turned politics upside down in the West. Conservative Republicans who routinely denounce "big spending" and "big government" fight tooth and nail to bring more big-budget government water projects to their states. Liberal Democrats in the region with otherwise pristine environmental records break with their green backers when it comes to proposals for new dams and concrete-lined canals.

All this is necessary because much of the western United States -- from the 100th meridian, just west of Wichita, to the coastal strip of California -- is desert, with 20 inches of rain or less per year. For all the variety of mountain, prairie, forest and salt flat, the region is marked by "one overmastering unity -- the unity of drought," as novelist and historian Wallace Stegner put it.

As nature designed it, the Colorado River flowed about 1,500 miles south and west to the Pacific from its headwaters on the western slope of the Continental Divide. Fed by such major tributaries as the Green, Gunnison, Yampa and San Juan rivers, the Colorado was powerful enough to cut through 200 miles of rock to form the Grand Canyon; it once poured billions of gallons each year into the Gulf of California in northern Mexico.

But today, a thick network of dams and diversion canals -- built over the past century to hydrate farms, factories and cities -- sucks up all the water before the river reaches its mouth. "It's a rare year now when the Colorado flows to the Pacific," says Bill Bates, a resource engineer with the Denver Water Board. "I think it may have happened once or twice about a decade ago, but certainly not in the recent drought years."

On the main stem of the river, Bates says, "there are thousands of diversions -- for agriculture use, for cities, for ski areas making snow. Every one of them has a formal water order saying how much water they can take. There are sluice gates to control the flow into each diversion and gauges to measure how much goes through the gate. Nowadays, we've got some gauges hooked up to wireless transmitters; that makes it easier to track the usage on the more remote diversions."

All of that infrastructure has made water storage and delivery reliable enough to attract millions of new residents to the Southwest. In the 1990s, the Colorado Basin states recorded the fastest growth rates in the United States. There are millions of new Westerners now, and every one of them uses water every day.

For that reason, nobody expects the Bush administration's crackdown on California to bring peace to the century-long battle over Colorado River water.

"The big fear is that this is going to produce a series of new water wars," says Jim Lochhead, a Glenwood Springs, Colo., attorney who has represented the state of Colorado in the dispute.

"It wouldn't be surprising if some of the California users sue Interior, and then you're going to have a legal and political battle for years. Even those of us who say the Bush administration did the right thing don't predict that the struggle is going to get any easier."

Just below the Continental Divide in Colorado's Never Summer mountain range, the Colorado River is little more than a trickle of snowmelt.Water from the Colorado River is used on a field in the Imperial Valley, one of the richest farming areas in the world.The Colorado River winds through the Chocolate Mountains on its journey. Says water lawyer Joe Sax: "That river is the lifeline of seven states."