Mike O'Callaghan–Pat Tillman Memorial Bridge

Feel the wind first. It comes up the canyon in long, dry pushes, warm even in the shade, and it carries the faint hum of traffic. Under your feet is a concrete walkway. Beside you is a waist-high barrier. Beyond that barrier there is nothing at all for a very long way down. Far below, a thin green ribbon of water moves between walls of dark volcanic rock. This is Black Canyon, and you are standing on one of the highest bridges in the United States.

The river below is the Colorado. It forms the border between two states here. Behind you, roughly, is Nevada. Ahead is Arizona. The bridge joins them in a single long leap, carrying the highway between Las Vegas and Phoenix. Most drivers who cross it never see what you are seeing now. The road deck has high walls, and at highway speed the canyon flashes past in a second. Only people who park and walk out get this view.

And what a view it is. Look upstream, to the north side of the canyon. There sits Hoover Dam, a pale curved wall of concrete wedged between the cliffs. It was finished in the 1930s, during the Great Depression, and for decades it was one of the great engineering achievements of its age. From this walkway you see it the way few people ever did before 2010 — from above, and almost face on.

Here is the first surprise. For most of the twentieth century, the main highway did not go around the dam. It went over it. Cars, buses and heavy trucks crawled across the top of the dam, then wound down tight hairpin curves on either side. Traffic jams were common. Accidents happened on the steep turns. And as traffic grew, the narrow road on top of a national landmark became a serious problem.

After the attacks of September 2001, security concerns added urgency. Large trucks were barred from crossing the dam, and long detours followed. A bypass had already been studied for years. Now it moved forward. Construction on the bridge began in the middle of the 2000s, and the finished structure opened to traffic in October 2010. Officially, the whole project is often called the Hoover Dam Bypass.

The bridge carries two names, and both belong to men with deep ties to these two states. Mike O'Callaghan was a governor of Nevada in the 1970s. Before politics, he served in the Korean War and lost part of his leg there. Pat Tillman was a professional football player for the Arizona Cardinals. After the attacks of 2001, he left his football career to join the Army Rangers. He was killed in Afghanistan in 2004.

So the name honours two veterans, one for each side of the river. Near the walkway you will find interpretive panels that tell more about both men. It is worth pausing there. Many visitors photograph the dam and leave without ever learning who the bridge remembers. Their stories give this concrete span a human weight that the engineering alone cannot.

Now look down at the structure itself, if you can lean out safely and see it. The bridge rests on a great arch. Actually, it is a pair of arches, side by side, made of concrete. Tall concrete columns rise from the arches to hold up the road deck. The deck itself uses steel girders. Engineers describe it as a composite of concrete and steel, and it was the first bridge of its kind in the United States.

The second surprise is in the numbers. The main arch spans roughly 1,060 feet, or about 320 metres, from one canyon wall to the other. When it opened, it was described as the longest concrete arch bridge in the Western Hemisphere. The deck stands nearly 900 feet above the river. That is taller than many skyscrapers. Only a small number of bridges in the country stand higher above the ground below.

Building it was a slow and difficult job. The canyon walls are steep and the winds are strong. Workers could not simply put up scaffolding from the riverbed. Instead, cranes ran along heavy cables stretched high across the canyon, lowering materials into place. The two halves of the arch grew out from each cliff, inch by inch, until they met in the middle. During construction, high winds once brought down part of the crane system, and the work was delayed.

Think about that as you look at the joint where the arch halves meet. For months, two great curves of concrete reached toward each other over empty air. Each side had to be held back by cables until the final pieces closed the gap. When the arch was complete, it could finally carry its own weight. It is a simple shape — one of the oldest in building — doing its work on a modern scale.

A few practical words before you go further. The walkway is long, and it has no shade. In summer, this canyon is one of the hottest places in the region, and the reflected heat from the concrete is intense. Bring water. Wear a hat. The approach from the parking area includes stairs and a sloped path, so allow time. Hold phones and hats firmly. The wind here can be sudden, and anything dropped will not come back.

Before you leave, stand still for a moment. Listen. The traffic hums behind the wall, steady and unbroken — trucks and cars that no longer climb over the dam. Below, the river moves without a sound you can hear from this height. Look once more at the dam, then at the dark rock walls, then at the curve of the arch beneath you. Two eras of engineering face each other across this canyon, and you are standing right between them.