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A new report warns that in the future, draughts in the West will be longer and more severe. That means water levels in the Colorado River could reach even lower levels than those seen now, after seven years of draught.

As NPR's Elizabeth Shogren reports, that is grim news for the millions of people who rely on the river for water.

ELIZABETH SHOGREN: Bob Gripentog sees the impact of the draught everyday. He manages a marina on Lake Mead, the massive reservoir behind the Hoover Dam on the Colorado River. Gripentog says it's abundantly clear that demand for that water has far exceeded the supply. You just have to look at the cliffs around the lake.

BOB GRIPENTOG: You'll see a big white, like a bathtub ring around it. That's alkali that was deposited when the water was high. That's about 100 feet high right now.

SHOGREN: In fact, the lake is 90 feet lower than historic averages. That's bad news because the water stored in Lake Mead nourishes agriculture in much of the southwest, and it keeps taps flowing in southern California, Nevada, Arizona and Colorado.

The lake is so low that Gripentog had to move his marina 12 miles to deeper water. And just last week, part of another marina - boats and all - moved to his harbor.

GRIPENTOG: We actually floated them just like a tugboat, pushing a barge down the river. And it's pretty good sight when you look out across here and all of a sudden there's a marina moving down the lake.

SHOGREN: The low water levels on the Colorado were of great concern to officials at the National Academy of Sciences, which advises the government. The officials asked a panel of experts to figure out what the latest signs had to say about the future of the river.

In particular, the experts looked at studies of tree rings. The ring showed rainfall patterns over the past several hundred years. It turns out that the last century was relatively wet. But before that, tree ring showed the droughts were longer and more intense.

Connie Woodhouse says several droughts over the last 500 years were far worst than anything in modern history. She's a tree-ring expert from the University of Arizona in Tucson who was on the panel.

CONNIE WOODHOUSE: This record suggests that these trims of droughts that we see in the long term record could occur in the future, given just natural variability by itself.

SHOGREN: And the report says climate change, likely, will make future droughts even worse. Ernst Smirden(ph), also from the University of Arizona, chaired the panel.

ERNST SMIRDEN: The preponderance of scientific evidence certainly suggest that the warmer temperatures will reduce Colorado River-flow and water supplies in the future. We think, in all probability, there will be droughts in the future that will be more severe than anything that we have experienced.

SHOGREN: Smirden predicts that the current system for divvying up water from the Colorado will have to change. Conservation efforts probably can't save enough water over the long term to satisfy everyone. He says farmers, probably, will have to give up water to send to cities, and policymakers will have to make painful tradeoffs.

SMIRDEN: The Colorado River is a lifeblood of the people that live in Arizona, and southern Nevada, and the upper basin of Colorado. These are states and regions that are growing much more rapidly than any place else in the United States, so this is a very, very serious problem.

SHOGREN: The expert panels says state and federal officials need to be realistic about how much water the region will need in the future, and it urges them to plan for what they'll do if the Colorado River provides much less water than it does today.

Elizabeth Shogren, NPR News, Washington.

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