Congress isn't planning to take action on climate change any time soon. But if the planet keeps warming, a number of states won't be able to ignore the problem quite so easily. One good place to see this is in the Colorado River basin.

The Colorado River provides fresh water to nearly 40 million people in seven states out west: Arizona, California, Nevada, Colorado, New Mexico, Utah and Wyoming. A sizable chunk of U.S. agriculture relies on that water — about 15 percent of the nation's crops and 13 percent of its livestock. (Indeed, the vast majority of the river's water is used for irrigation and agriculture.)

But there's a problem: The Colorado River may soon no longer have enough water to satisfy the region's needs. Thanks to rapid population growth in cities like Las Vegas and Phoenix, water demand is surging. Meanwhile, the supply of water is dropping — and could keep dropping as climate change speeds evaporation, shrinks the snow pack in the Rocky Mountains, and makes droughts more likely. By some recent estimates, annual flows could drop up to 20 percent by mid-century.

The dilemma is laid out in a big new report from the U.S. Bureau of Reclamation, looking at the future of the Colorado River. The chart below sums things up. The authors of the study took the best estimates of future population growth in the region and paired them with estimates of future water supply. Trouble ensues:

There's a fair bit of uncertainty here — climate models still disagree on how sharply annual flows will drop in the future. And population growth is hard to predict. Some environmental groups have even suggested that states might be exaggerating their projected growth to qualify for more federal money for big infrastructure projects.

That said, the best estimates suggest that demand will continue to outstrip supply, much as it has in the past decade. By 2060, the report says, the median shortfall could reach 3.2 million acre-feet (or about five times as much water as Los Angeles uses each year). The amount of irrigated farmland is also expected to shrink.

When scientists talk about the need for "climate adaptation," this is what they mean. So how are these states going to deal with the water problem? The report has an in-depth analysis of different adaptation options, grouped into a few broad categories:

--Importing water from elsewhere. The report examined a bunch of zany proposals for bringing more water to the basin. One idea is to build a massive 600-mile pipeline from the Missouri River down to Denver. Sure, it would cost many billions of dollars and require large new power plants to pump the water, but why not? Or maybe ships could tow freshwater icebergs from the Arctic down to Southern California! Sadly, the report concluded that many of these schemes are unfeasible for now.

--Desalination. Another idea: If the fresh water's running out, why not set up some desalination plants to treat brackish water or seawater? There's already a $150 million desalination plant operating in Yuma, Ariz., to recycle salty irrigation water. The report doesn't rule this out, though these plants are pricey and would likely only be built in severe shortages. (What's more, desalination plants use a lot of energy, which means more carbon emissions, which worsens the problem... )

--Conservation and reuse. This is the option environmental groups tend to prefer, and there are dozens of different strategies here. Cities can recycle their "grey water," (say, using old bathwater to flush toilets or water golf courses). Land managers could kill off thirsty plant species like the tamarisk. Water managers could slow the pace of evaporation by placing covers over reservoirs and irrigation canals. Governments could even set up a system in which users trade water permits.

Add it all up, and the report concludes that there are probably enough realistic water-saving strategies out there to make up for the projected shortfall in the next 50 years. That's the good news.

The bad news is that none of this is cheap or easy. Right now, a city like San Diego pays about $800 for an acre-foot of water from the Colorado River. But desalinated water would cost up to $2,150 per acre-foot, according to the report. Recycling waste water costs up to $1,800 per acre-foot. Covering irrigated canals? As much as $15,000. That's one reason why state and local authorities have often resisted drastic measures.

What's more, the report notes, even if the right mix of adaptation strategies were adopted, the Colorado River Basin would still be highly vulnerable to droughts and water shortfalls in a certain portion of years. There's no way to stave off all possible calamities.

Further reading:

— Perhaps the best book on the history of the American West's water woes (and a look at how states have tangled over water rights) is Marc Reisner's "Cadillac Desert."

— Note that shortages are already occurring in many places. Las Vegas has been grappling with this issue since 2009, spending billions on new pipes into Lake Mead and forming "wastewater patrols."

— My colleague Juliet Eilperin recently looked at how low levels in the Colorado River affect power plants: "In the Colorado River’s 100-year recorded history, 1999 through 2010 ranks as the second-driest 12-year period, yielding an average of 16 percent less energy."