The humorist Dave Barry once joked that all the major natural attractions of the American West – the Grand Canyon, the Badlands, the Rocky Mountains, and Robert Redford – were caused by erosion.
That may be a backhanded way of acknowledging that Redford, the veteran actor, director and environmental activist, could not be better placed to narrate a film looking at the potentially disastrous consequences of a long regional drought on the Grand Canyon.
The film, which opens at Imax cinemas across the United States later this week, gives viewers both an opportunity to savour the extraordinary wonders of the deep canyon, burrowed by the Colorado river across much of Arizona, and a chance to contemplate the vast changes already wrought on the ecosystem by a long dry spell and an ever-increasing demand for drinking and irrigation water from the region's burgeoning population.
"[The river] is endangered now on a number of fronts," Redford said in a brief interview with the Associated Press. "Its volume is shrinking because of a megadrought cycle that's now facing the South-west, and some scientists predict that it could last into the next century."
The film, Grand Canyon Adventure: River at Risk, follows the environmental activist (and presidential family scion) Robert F Kennedy Jnr, his anthropologist friend Wade Davis, and their two daughters as they make a river trip along the Colorado under the guidance of a Native American from the local Havasupai tribe.
Along the way, they compare the views they have with photographs taken in 1872 by the pioneering explorer John Wesley Powell so they can understand how much the landscape has changed in the intervening 136 years.
The film makes clear that every generation has a responsibility to pass on its concern about the Grand Canyon, just as Kennedy and Davis do to their daughters. It is a responsibility Redford takes seriously too – as his long resume of environmentally-themed films and forays into public activism attest.
Redford has himself navigated the Colorado river and keeps a boat on Lake Powell, a vast man-made reservoir – now at perilously low levels – at the eastern end of the Grand Canyon.
The Colorado river's water has been subject to furious disputes and fractious negotiation between the seven western states who rely on it to maintain their growing populations. Water exploitation, compounded by the construction of big dams along the length of the river and the development of desert cities such as Las Vegas, have had a profound impact on the river's ecosystems.
The drought that has afflicted the region since the late 1990s has only heightened the stakes. Last week, in response to concerns about silting and the loss of habitat for fish and other wildlife, the federal government opened the jet tubes at the Glen Canyon dam for 60 hours.
The Secretary of the Interior, Dick Kempthorne, said: "This experiment has been timed to take advantage of the highest sediment deposits in a decade and designed to better assess the ability of these releases to rebuild beaches that provide habitat for endangered wildlife and campsites for thousands of Grand Canyon National Park tourists."
In recent years, the Grand Canyon has been held up by creationists as "proof" of the majesty of God's design. Some of them have even sought to argue that the site is no more than a few thousand years old.
A scientific study published in the journal Science last week, however, suggests the Grand Canyon could be as much as 17 million years old, three times older than previously thought.