Diminished by overzealous water withdrawals and a lengthy drought throughout the Southwest, Lake Powell, the meandering mass of water held by Glen Canyon dam, has slowly emptied in recent years. It now sits below half capacity.

NASA's Earth Observatory recently released satellite imagery showcasing the decade-plus drying-up of the reservoir that helps quench the thirst of more than 20 million Americans. The images above show Lake Powell, from 1999 to the present, as its water levels have slowly descended down the canyon walls.

The photographs were captured by satellites from NASA's Landsat program.

Part of an astounding array of hydraulic engineering that's tamed the Colorado for use by states of the Southwest, Lake Powell -- which straddles the border between Utah and Arizona -- and Glen Canyon dam not only provide tap water for households and irrigation for regional farms, they also produce 4.5 billion kilowatt-hours of hydroelectricity each year.

But all that production comes at a cost. With year after year of reduced precipitation and snowmelt runoff -- Lake Powell has now suffered 14 straight years of drought -- the reservoir has less water to share.

This year, the lake was charted at 42 percent of capacity; it's expected to end the year at 51 percent. Still, experts expect the lake's holdings to continue to shrink in the coming years -- vexing an already heated debate about how to provide the Southwest's growing population with enough water.