One of the largest water conservation projects in the U.S. is under way in the Arizona desert, where construction crews are replacing a 23-mile section of the All American Canal, a 70-year-old channel just north of the Mexican border. The earthen canal, which carries more than 3 million acre-feet of water annually from the Colorado River to the Imperial Valley, seeps badly in the sandy dunes around Yuma, Ariz., losing enough water to supply the needs of 500,000 households in Southern California. The canal's new section, lined in concrete, is expected to be completed in late 2010 at a cost of about $285 million.

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