By Kathryn Westcott
A multi-million dollar project that involves building one of the largest concrete arch bridges in the world, in the shadow of the Hoover Dam will reach a major milestone in the next few weeks.
Construction workers will close the six-foot gap before the end of the month
The Colorado River Bridge - which at its highest point will reach almost 900 feet (274m) above the river - is being built to take the pressure off the narrow, winding road that runs across the Depression-era dam.
Just six feet separate the two concrete fingers of the arch that will form the backbone of the bridge. One stretches out from the terracotta red canyon walls across the river from Arizona, the other from the Nevada side.
The fingers will meet when the final section of the 1,060ft (323m) arch is put in place before the end of the month.
A four-lane highway supported by concrete pillars will sit on top of the arch. It will provide a new key route between Las Vegas and Phoenix.
The project is scheduled to be completed late next year.
The bridge will stand a quarter of a mile downstream from the Hoover Dam, which when it was completed in 1936 was both the world's largest electric-power generating station and the world's largest concrete structure.
"It's been a great honour for everyone involved to build something worthy of standing in the shadow of the Hoover Dam," project manager Dave Zanetell of the Federal Highway Administration told the BBC News website.
Everything about the job is special. Big bridges are really tough and this has represented all those challenges but it has been exacerbated by the extremes of the site. It is extremely harsh terrain, a physically demanding place."
Some 1,200 workers have toiled in the searing heat - which frequently reaches 110 degrees Fahrenheit (43 degrees Celsius) - and high winds, 1,000ft (305m) above the river.
Steel cables holding the arch in place, will be removed during September
The 1060ft arch will form the centrepiece of one of the longest concrete arch bridges in the world
Concrete towers will hold up the bridge roadway. The tallest will reach 280ft in height
More than 17,000 cars and trucks are expected to use the new bridge on a daily basis once it is completed next year
Many have been hoisted above the gorge in cages and transported along the steel cables that form part of a high-line system supported by temporary cranes on both sides.
"This has truly been a team effort at every level," said Mr Zanetell. "It is a testament to how successfully teams of engineers, architects and the best of our construction industry can overcome many challenges and together to focus on a single goal."
Work, however, was set back a year in 2006, when two 280ft steel construction cranes collapsed amid high winds.
The bridge is the centrepiece of the $240m Hoover Dam bypass project, which involves creating a major commercial route between the states of Arizona, Nevada and Utah.
The old road over the dam, which had originally been built as a construction access road, is now deemed to be unsafe.
Building the approaches to the bridge involved blasting into the immense rock formations to build five miles of roadway and seven bridges.