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In 2007, Western states and the federal government signed an agreement specifying what happens when water is in short supply. When the level of Lake Mead drops below a certain elevation, more water is released from Lake Powell in a process called equalization.
In its latest study, the Bureau of Reclamation (BOR), which manages the system, put the probability of a 2011 equalization release from Lake Powell to Lake Mead at 97 percent. The release will likely be 11.63-million acre-feet instead of the usual 8.23 million, about 40 percent more than usual. The final decision won’t be made until April. If the release occurs as expected, there likely won’t be a water shortage in the lower basin states of Nevada, Arizona and California in 2012.
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A 10-year drought has caused Mead to recede, forcing the probable equalization. Those opposed to equalization say that, ultimately, it causes Utah and other upper-basin states to lose out on their share of the Colorado River, thus impeding their growth.
The situation could get worse. A 2008 study by research marine physicist Tim Barnett and climate scientist David Pierce of the Scripps Institution of Oceanography concludes there is a 50 percent chance that Lake Mead will be dry by 2021 if the climate changes as expected and future water usage is not curtailed. That could affect the water supplies of between 12 million and 36 million people as a net deficit of nearly 1 million acre-feet of water per year from the Colorado River system continues.
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http://www.sltrib.com/sltrib/outdoors/51263038-75/lake-powell-river-colorado.html.csp?page=1