Hoover Dam

A dam is a wall that holds back water.

Hoover Dam is in the United States. It stretches across a deep, rocky canyon. Behind it is a big lake.

Hoover Dam stands in Black Canyon, where the Colorado River forms the border between the states of Nevada and Arizona, about 30 miles southeast of Las Vegas. It is a curved wall of concrete 726 feet (221 meters) high, about as tall as a 60-story building, and 1,244 feet long at the top.

The dam holds back Lake Mead. The National Park Service says the lake was the largest artificial lake in the world for decades and is still the largest in the United States. The dam is a National Historic Landmark.

The dam spans Black Canyon on the Colorado River between Arizona and Nevada, about 30 miles southeast of Las Vegas (the Bureau of Reclamation's project page says 35). It is a concrete arch-gravity dam: Reclamation explains that the water load is carried both by gravity action and by horizontal arch action. It rises 726.4 feet from foundation rock to the crest roadway, is 660 feet thick at the base and 45 feet at the top, and weighs more than 6,600,000 tons.

Its reservoir, Lake Mead, has a total storage of 28,945,000 acre-feet when the water stands at 1,221.4 feet. Reclamation calls the dam a National Historic Landmark, and the American Society of Civil Engineers rated it one of America's Seven Modern Civil Engineering Wonders.

The dam is very thick at the bottom and thinner at the top.

Below the dam are two long buildings. Inside them, water from the lake helps make electricity.

The dam is much thicker at the bottom, 660 feet, than at the top, 45 feet. At the base, the water presses on it with a force of up to 45,000 pounds on every square foot.

At the foot of the dam, two long wings of the powerhouse sit on either side of the river. Big pipes carry water from the lake to the generators that make electricity, and then the water flows on down the river. On each side of the canyon is a spillway, a channel that lets floodwater pass the dam safely.

The thick-arch section, 660 feet at the base tapering to 45 feet at the crest, resists a maximum water pressure of 45,000 pounds per square foot at its base. Towers and ornaments on the parapet rise 40 feet above the crest.

Penstocks carry water from four intake towers on the lake side of the dam to the horseshoe-shaped powerhouse at its foot. Two spillways, one on each side of the canyon upstream of the dam, connect to two of the original diversion tunnels, so that in a flood the lake can overflow into them; Reclamation lists a spillway capacity of 270,000 cubic feet per second at a lake level of 1,232 feet. The National Park Service notes that each spillway can carry as much water as flows over Niagara Falls.

In the photo, the blue lake is behind the dam.

The river flows away below it, between tall red-brown cliffs.

The National Park Service says Hoover Dam still does what its builders hoped: it protects southern California and Arizona from floods, provides water for farm fields, and supplies water and electricity to Los Angeles and other growing cities in the Southwest.

In the photo, taken from a bridge downstream, you can see the curved face of the dam, the powerhouse wings at its foot, Lake Mead behind it and the Colorado River flowing away below, between the steep walls of Black Canyon.

In the National Park Service's summary, the dam continues to protect southern California and Arizona from floods, irrigate farmland and supply water and power to Los Angeles and other cities of the Southwest. It also had an unexpected result: for millions of people in the 1930s it became a symbol of what American industry and workers could do during the Great Depression, and in the early 21st century almost a million people a year still came to see it.

The photo, taken from the Mike O'Callaghan–Pat Tillman Memorial Bridge downstream, looks up Black Canyon at the curved downstream face, the two wings of the horseshoe-shaped powerhouse flanking the river, Lake Mead stretching behind the crest, and roads and power lines climbing the canyon walls.