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Reservoirs Tilt the Earth by Stephanie Gregory The Wild File, Outside Magazine, January, 2001

Q: I've come across an assertion that the weight of water backed up behind dams and stored in reservoirs has changed the degree of the tilt of the Earth's axis. Can this be true? - Rick Sylvester, Squaw Valley, California

A: Yes and no. the world's dams and reservoirs hold a total of ten trillion tons of water, the largest percentage of it in Canada and Russia. This has shifted the collective mass of the world's water closer to the North Pole than it would be if it were flowiing freely. This redistribution of mass, explains Benjamin Fong Chao, a geophysicist at NASA;s Goddard Space Flight Center, has increased the speed of the earth's rotation enough to shorten the length of a day by .02 millionths of a second over the past 40 years. "It's the same as an ice-skater putting her arms up over her head to spin faster," says Chao. Because Russia has built even more dams than Canada over the past 50 years, Earth's center of mass has shifted ever so slightly toward Russia. this has caused the axis to tilt a fraction of a degree away from the weight and the North Pole to move 27.3 inches in the general direction of Hawaii. (Imagine Chao's ice-skater with a one-pound weight in her left hand -- the extra mass makes her imaginary axis tilt an inch or two to the right to keep equilibrium.) The scale of those changes is pretty small, partly thanks to other forces than happen to be canceling out the dam effect, in particular "postglacial rebound," the gradual rise of landmass after the glaciers weighing them down have melted. All told, in the last 100 years the North Pole has tilted 32.8 feet toward the eastern United States.