How low will it go?

Eric Kuhn is not the person you'd expect to deliver ominous revelations about Colorado's future.

Kuhn runs the Colorado River Water Conservation District, which represents 15 counties on the state's Western Slope. Compared to some of Colorado's other water agencies, the River District leads a relatively low-profile existence on one floor of a small office building in the mountain town of Glenwood Springs. The district has 23 employees, a dog-friendly office policy and a vehicle fleet composed mainly of Subarus, all of which gives the enterprise the folksy charm of an REI catalog. 

At the age of 59, Kuhn is resistant to many of the most rudimentary conventions of the water business -- things like bolo ties and golf -- and he is not particularly partial to the back-slapping bonhomie in which even the staunchest water adversaries often engage. He favors jeans and Nikes, and sometimes leaves the office to walk a footpath along the river and turn over problems in his mind.

Kuhn has a warm personality and a fierce, focused intelligence. He devotes a great deal of energy to understanding things like the relative performance characteristics of various types of weather radar, and he has been known to wander off in the middle of a conversation when an important breakthrough materializes in his head. "He's one of these scary-brilliant people," says his wife, Sue. "It's just kind of how he's wired."

He also has something of the manner of a well-read naval officer -- which, it turns out, he once was. He spent six years aboard submarines, spying on the Soviets' nuclear weapons program. It's tempting to think that Kuhn's way of thinking about the Colorado River was shaped by those long deployments in the murky depths, when his submarine's survival depended on the intelligent interpretation of what little data could be gathered.

Kuhn has lately become obsessed with one of the bigger riddles hanging over Colorado's future, and his quest for answers has challenged some of the fundamental tenets of the state's water orthodoxy. 

"I've always been concerned that our reliable water supply is a lot less than what we've been suggesting," he says. "I think there's a lot less available than we thought there was."

More than 3 million people in Colorado -- roughly two-thirds of the state's population -- rely on water from the Colorado River. The river sustains alfalfa, apples and pears, Olathe sweet corn, and ski and ranch towns across the Western Slope. Yet its water may be even more important to the Front Range -- Denver and its cluster of urban and suburban satellites that lie hard against the eastern foothills of the Rockies.

More than a dozen tunnels channel water underneath the Continental Divide to roughly 2.5 million people on the Front Range. Moreover, the state is expected to grow by 2.9 million people over the next 25 years, and the Colorado River has long been seen as the only real source of water for the future.

So it's not surprising that water managers have, for years, privately asked: How much more of the Colorado River can the state use?

Publicly, the state's water managers tend to stick to a time-worn mantra: Under the terms of the interstate treaties that govern the river, Colorado still has as much as 1.5 million acre-feet left to develop. That's enough water for about 12 million new residents, more than four times the state's official population projection for the next quarter of a century.

But there has always been an unspoken acknowledgment of a much grimmer possibility. Six other states also rely on the Colorado River. And between the peculiarities of the so-called Law of the River and mistaken assumptions about the river's long-term performance, Colorado may have to content itself with little more than "the right to leftovers," as two observers once put it. In a severe drought, in fact, cities and farms within the state could be pitted against each other in a fight for water.

That always seemed like an abstract possibility, until just such a drought wrapped its hands around the river in 1999, and kept them there right up to the present day. And so Kuhn decided it was time he figured out just how much water was left for Colorado -- and, by extension, how long the state could continue to grow.

Until recently, "the thought in Colorado was that it would take another hundred years," Kuhn says. "But the water supply we thought we had isn't there."

• Water