IMPRESSIVE ARC of concrete at Hoover Dam is just 45 feet thick at the top of the span. But it swells to 660 feet at its bottom to restrain more than 300 pounds per square inch of water pressure. Image: THE BUREAU OF RECLAMATION
"Shh!" Gregory Burkhart hushed his tour group into silence as an alarm buzzer sounded. All 20 gazes fixed on him as he raised a hand and looked around. The floor shuddered and started to quiver as a roar filled the cavernous hall. It was the groan of the Colorado River, surging through giant pipes beneath our feet, as it was tortured for its potential energy.
Seventy years ago the green waters of the Colorado ran wild down the 275 miles from the Grand Canyon in Arizona to Black Canyon, here on the border of Nevada. The river flowed unhindered through a sheer 700-foot gorge it had carved out of the sharp-crested Black Mountains. It was warmed by summer sun that at times would bake the canyon to well over 120 degrees Fahrenheit. And it was protected from humans by an utter lack of roads, water and electricity.
This article was originally published with the title The Power of Gravity.