Lake Mead's decline: not just a Vegas problem.

• Author: Best, Allen
• Position: PLANET-PROFIT REPORT - Travel narrative

The day we visited Lake Mead, near Las Vegas, the reservoir was at 42 percent of capacity and dropping, as it has been for most of the last 10 years. It was just before Christmas, gray and chilly. I was glad to have a winter jacket.

Vegas is a gambling town, and the house knows the odds. That's why Pat Mulroy, who heads the Southern Nevada Water Authority, surely calculated the chances before pulling the lever on a $700 million tunnel into the bowels of the reservoir. Having looked over the horizon, she doesn't like what she sees. And if she doesn't, Colorado shouldn't either.

Lake Mead began filling 75 years ago after completion of Hoover Dam, a triumph of 20th century America. The dam tamed the wild, unruly Colorado River, in one giant step allowing a more measured civilization in the uneven geography of wet and dry, high and low of the American Southwest. It enabled others: Green Mountain, Blue Mesa and Dillon in Colorado, and others outside the borders. Among the newest, and largest, is Lake Powell, located on the upstream side of the Grand Canyon from Las Vegas.

All those projects assumed plentiful water - and indeed, this year, with the Rocky Mountains laid deep in snow, there may be a big runoff. But that hasn't been the trend lately, nor is it likely to occur often in the future - hence the Big Bet by Vegas with its new tunnel. The city gets 80 percent of its water from Lake Mead using two funnels. Now it has sunk a shaft for a third tunnel. This new tunnel will dive deeper, under the reservoir, emerging at a level slightly above the original course of the Colorado River, in what is called the dead pool. A dead pool exists when a dam has so little water that there is no way to release it. If things go badly, that will be the case at Lake Mead in maybe 15 years.

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Drought in the Colorado River Basin has prevailed since 1999, interrupted by only a few average or above-average years. That decline has left a chalky, bathtub ring around Lake Mead, a visual reminder of what once was. This year's flows might eclipse a portion of the ring, but only temporarily. Several dozen climate models show unusual consensus in predicting markedly higher temperatures for the Colorado River Basin. Precipitation being equal, this greater heat will result in a 10 percent to 20 percent decline in river flows by mid-century. Some even question whether both Mead and Powell will be justified. Evaporation losses would be less with just one.

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