But the mega drought affecting the western United States is sending reservoir levels plummeting towards deadpool -- the point at which the dam can no longer produce power. 'There isn't as much head so there isn't as much pressure pushing the water into the turbines, so there's less efficiency and we aren't able to produce as much power.' Thousands of workers toiled 24 hours a day to build what was then the largest hydroelectricity facility in the world. If it drops to 950 feet, the intakes for the dam will no longer be under water and the turbines will stop. It is fed chiefly by the huge snowpack that gets dumped at high altitudes, melting slowly throughout the warmer months. Boaters on Lake Mead, many of whom come from Las Vegas and its surrounding towns, say they are doing their part to protect supplies. Householders in southern California have grumbled about the fate of their luscious lawns since being ordered to limit their outdoor watering to one or two days a week at the start of the summer. Climatologist Steph McAfee of the University of Nevada, Reno, says the U.S. west has always been something of an improbability. The last two decades of drought are not, McAfee says, actually that unusual in climatic terms, according to tree ring reconstructions. On Lake Mead, boat seller Jason Davis manoeuvers his craft towards Hoover Dam, where thousands of tonnes of concrete loom over the water in graceful modernist lines, and a ring of mineral deposits shows where the water level used to be. 'It's like, out of sight, out of mind. Hey, we're using too much water.