VOA慢速英语2010年-EXPLORATIONS - Hoover Dam: Taming the（在线收听）

STEVE EMBER: I’m Steve Ember.

SHIRLEY GRIFFITH: And I’m Shirley Griffith with EXPLORATIONS in VOA Special English. Today we tell about Hoover Dam. It was the largest and most difficult structure of its kind ever built when work started in nineteen thirty-one. A new bypass bridge that opened this month near the dam is also a wonder of engineering.

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STEVE EMBER: Our report about Hoover Dam must begin with the Colorado River. This river made the dam necessary. The Colorado River begins high in the Rocky Mountains. It begins slowly, during the dark months of winter. Heavy snow falls on the Rocky Mountains.

The snow is so deep in some areas that it will stay on the ground well into the hot days of summer. But the snow does melt. Ice cold water travels down the mountains and forms several rivers -- the Gila River, the Green River, the Little Colorado, the San Juan, the Virgin and the Gunnison Rivers. These rivers link together and form the beginnings of the Colorado River. The Colorado River flows through the states of Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada and California. Then it crosses the border into Mexico.

SHIRLEY GRIFFITH: The Colorado River has always been extremely powerful. The river created the huge Grand Canyon. Beginning thousands of years ago, the violent water cut hundreds of meters deep into the desert floor of Arizona. The power of this river has also been demonstrated in more modern times. Between nineteen-oh-five and nineteen-oh-seven, the Colorado River caused great amounts of flooding in parts of Arizona and California.
　
Hoover Dam at night

Huge amounts of water ran into a low area in the dry, waterless desert that had once been an ancient lake. In two years of flooding, the Colorado River filled the ancient lake. That lake is called the Salton Sea. Today, it is about fifty-six kilometers long by twenty-four kilometers wide. It is even larger in years of heavy rain.

STEVE EMBER: The flooding that created the Salton Sea also flooded homes, towns and farming areas. Many people were forced to flee their homes. Government leaders knew they had to do something to prevent such floods in the future. In nineteen eighteen, a man named Arthur Davis proposed building a dam to control the Colorado River. Mr. Davis was a government engineer. He said the dam should be built in an area called Boulder Canyon on the border between the states of Arizona and Nevada.

SHIRLEY GRIFFITH: Building the dam would not be easy. The people of seven states and the people of Mexico needed and used the water of the Colorado River. Much of that area is desert land. Water is extremely important. Without water from the Colorado River, farming is not possible. Without water, life in the desert is not possible.
　
Workers drill into the canyon wall above the Colorado River

On November twenty-fourth, nineteen twenty-two, officials signed a document in Santa Fe, New Mexico. That document is called the Colorado River Compact. The document tells how the seven states would share the water of the Colorado River. It was agreed this could be more easily done with the aid of a dam. Later an agreement was signed with Mexico to supply it with water from the Colorado River.

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STEVE EMBER: The area chosen for the dam was called Black Canyon. The walls of Black Canyon rise almost two hundred forty-three meters above the river. An ancient volcano formed the rock in Black Canyon. Engineers decided the rock would provide a good strong support for the proposed dam.

However, the area also presented problems. The nearest railroad was sixty kilometers away. There was no electric power. And, in the summer, desert temperatures could reach as high as forty-eight degrees Celsius.

A great deal of work was done before operations started on the dam. Workers built a town called Boulder City to house employees working on the dam.

They built a large road from Boulder City to the area of the dam. They built a railroad from a main line in Las Vegas, Nevada to Boulder City. They built another railroad from Boulder City to the dam area. And they built a three hundred fifty kilometer power line from San Bernadino, California. This provided electric power to the area where the dam was being built.
　
The newly completed Mike O'Callaghan-Pat Tillman Memorial Bridge, with Hoover Dam in the background

SHIRLEY GRIFFITH: The work on the dam began in April of nineteen thirty-one. Workers called “high scalers” were some of the first to begin building the dam. They were suspended from ropes as they used heavy air-powered hammers to break any loose rock away from the face of the canyon walls. When they could not use hammers, they used dynamite. One high scaler became very famous. His name was Arnold Parks. He caught another worker who had fallen off the top of the canyon.

Mr. Parks held the worker to the wall of the canyon until others came to help. Today, visitors can see a statue of the men who worked as high scalers to build Hoover Dam. The high scalers worked on the sides of the canyon. Other workers dug huge tunnels deep in the floor of the canyon. This was done to permit the Colorado River to flow away from the construction area. This had to be done so the floor of the dam could be built.

On June sixth, nineteen thirty-three, workers poured the first load of a building material called concrete. Men in two special factories worked day and night to make the concrete for the dam. Huge equipment moved millions of tons of rock and sand. In the summer months, the terrible desert heat slowed the work but did not stop it. Men who worked at night on the dam suffered less, but the heat was still as high as thirty degrees Celsius.

STEVE EMBER:

Slowly the great dam began to rise from the floor of the canyon. From the canyon floor it reaches two hundred twenty-one meters high. Workers poured the last of the concrete on May twenty-ninth, nineteen thirty-five. They had used almost four million cubic meters of concrete. Workers also used more than twenty million kilograms of steel to strengthen the dam’s concrete.

SHIRLEY GRIFFITH:

The work was dangerous for the more than five thousand men who worked on the structure. The extreme temperatures, falling objects and heavy equipment caused accidents. The workers were provided with medical care and two emergency vehicles to take them to a new hospital in Boulder City. However, ninety-six men lost their lives during the building of the great dam.

The companies building the dam had been given seven years to complete the work. They did it in only five. President Franklin D. Roosevelt dedicated the dam in September of nineteen thirty-five, although it was not completed until the next year. However, work on the dam continued so as to use the river to produce large amounts of electric power.

Today, seventeen huge turbine machines produce hydroelectric power. About one point three million people in Arizona, Nevada and southern California depend on the dam for their electricity.

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STEVE EMBER: When Hoover Dam was finished, it was the largest and tallest dam in the world. It was also the largest power producer that used water to make electricity. Today this is no longer true. Taller dams, larger dams and a few that produce more power have been created. But Hoover Dam remains an extraordinary example of engineering. In October, two thousand ten, another major engineering effort was completed about five hundred meters south of Hoover Dam. A concrete bridge was built about two hundred seventy-five meters above the Colorado River. The bridge connects Arizona and Nevada.

The official name of the Hoover Dam bypass is the Mike O'Callaghan-Pat Tillman Memorial Bridge. Mike O’Callaghan was governor of Nevada in the nineteen seventies. Pat Tillman was a football player who quit the Arizona Cardinals to join the Army. He died in Afghanistan in two thousand four.

SHIRLEY GRIFFITH: The bridge is about five hundred eighty meters long. It is the largest single-span concrete arch bridge in the Western Hemisphere. Each of the one hundred six cast concrete pieces of the bridge was built in mid-air with the support of huge steel cables. Building took place on both sides of the river. For the curve to be perfect, the two sides had to meet within two and a half centimeters of each other.

Before this new bridge, the only way to cross the river in this area was on a small two-lane highway extending across the dam. This road was too small for the amount of traffic in the area. The new bridge keeps the majority of vehicles further away from the dam. So experts hope to protect it from possible attack or harm.

The new bridge is also giving visitors to Hoover Dam a fresh view of this important structure.

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STEVE EMBER: This program was written by Paul Thompson and Dana Demange, who was also the producer. I’m Steve Ember.

SHIRLEY GRIFFITH: And I’m Shirley Griffith. You can comment on this program on our website, voaspecialenglish.com. Join us again next week for EXPLORATIONS in VOA Special English