Bob Martin, the deputy power manager at the Glen Canyon dam, gestures at the band of whitish, chalky residue running along the steep canyon walls towering above the Colorado River.

"That's where the water level used to be," says Martin. "It's fallen lower than even the lowest end of the scale thought possible when the dam was constructed."

The miles-long "bathtub ring," bleached white by water over years, is a glaring symbol of the crisis unfolding at the Colorado River basin, which is suffering its driest period for more than 12 centuries.

At the epicenter of this slow-burn environmental disaster embroiling the US south-west is Lake Powell, an artificial reservoir on the Colorado River created through the flooding of Glen Canyon in 1963.