The Power of Gravity
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.
Yet in 1931 this seemed to the U.S. Bureau of Reclamation the perfect spot to construct the tallest dam yet built. Today Hoover Dam is just one of 52 that restrain the waters feeding the Colorado River. Every year generators tap about 10 billion kilowatt-hours of energy from the flow. Although Hoover is no longer the largest dam in the world, it is a National Historic Landmark and one of the few giant dams that encourage visitors and that allow the public a glimpse of their innards.
A security clampdown last winter placed off-limits many of the interior passageways that were once open to tourists. But visitors can now walk at their own pace among the seven stations of the “Discovery Tour” that explain how this engineering wonder was built and how it holds back the water that makes Las Vegas and other southwestern cities habitable.
To begin the tour (which I took last August, before the country lurched into war), Burkhart crammed all 21 of us into an elevator sized for 10. The 506-foot descent from the crown of the dam into the bowels of the canyon wall was mercifully brief, but any claustrophobes in the group must have been dismayed to see the doors open into a dark tunnel of volcanic rock. As we made our way through the 250-foot tunnel to the power plant, some of us looked with concern at the rivulets of water seeping down the walls. “Don’t worry,” Burkhart said. “The water comes from natural springs, not from leaks in the dam. So don’t try to plug the cracks with your bubble gum.”
The dam does leak a little, however, as Burkhart elaborated later in the tour. “Concrete releases heat as it cures, and that can cause cracks,” he said. “This dam won’t be fully cured for about 1,500 years. So water finds its way through and is collected by a drainage system.” I asked how much water leaks in. “It varies,” he answered. “About 100 to 150 gallons–a minute.”
Especially after our confining journey through the canyon wall, the generator room was an awesome sight. Through the floor rose the top 30 feet of nine red generators, each 70 feet tall, weighing 2,000 tons and capable of producing 130 million watts of power. We were standing by one of these monsters when the alarm sounded and the building shook.
“That noise was the wicket gates opening up on the N7 generator,” Burkhart explained after the sound subsided. The water now spinning N7 up to 180 revolutions per minute, he said, was just seconds ago at the bottom of Lake Mead. From another station on the tour, you can walk out a gangplank over the water to one of the intake towers that rise from the lake bed north of the dam. The water that enters there gushes through a steel penstock pipe 30 feet in diameter, then swerves into a 13-foot-wide tube leading to the powerhouse. It sweeps by a massive butterfly valve at 22,000 gallons per second and spirals around the turbine case, its speed regulated by 24 wicket gates to produce precisely 60 cycles per second on the power lines.
“There is a building just like this one on the Arizona side of the river,” Burkhart said, as he flung open a door and pointed across the river. I squinted against the desert sun. The temperature at the visitor center was 104 degrees F in the shade. The railings on the overlook were too hot to touch. Even the backs of my hands were sweating.
