On Oct. 17, history was quietly made: The surface elevation of Lake Mead, a huge reservoir on the Colorado River near Las Vegas, dropped below its record low and continued to fall about a tenth of an inch per day over the following days. Those fractions of inches might seem insignificant, but when projected across the surface of the reservoir, they represent a lot of water. Although the surface of Lake Mead today is at almost 75 percent of its maximum level, the reservoir holds just 39 percent of its full capacity. The new low illustrates a trend that has implications not just for the almost 20 million people downstream who rely on the reservoir's water, but for the entire Colorado River watershed. If the decade-long drought and population growth continue, the water that runs the West will be ever more coveted, contentious and hard to come by.