The Colorado river and its tributaries took a hundred million years or two to carve the Glen Canyon out of the pink and scarlet sandstone which marks out the American southwest.

Its myriad gorges, sheer cliffs and towering spires remained a largely hidden secret. Prehistoric peoples farmed part of the canyon and Navajo Indian communities built camps close to the river, but few modern Americans ventured there besides explorers until the canyon disappeared under a man-made wonder, the vast Lake Powell, with the construction of Glen Canyon dam half a century ago.

Almost immediately, environmentalists and archaeologists mourned the loss. A final burst of exploration had turned up thousands of ancient ruins and drawn a belated focus on the canyon's stunning natural architecture.

"Glen Canyon died in 1963," wrote the renowned conservationist David Brower, who founded Friends of the Earth. "Neither you, nor I, nor anyone else knew it well enough to insist that at all costs it should endure. When we began to find out, it was too late."

But Lake Powell, the US's second largest reservoir, proved its own marvel. It draws about three million tourists a year to boat, swim and take days on the water, exploring the crevices and side canyons of a lake that stretches nearly 190 miles across the border between Utah and Arizona. The otherworldly landscape of monumental rock piles and soaring sandstone cliffs has provided the backdrop for scenes in Planet of the Apes and Gravity, and for episodes of Doctor Who.

After the dam was constructed at the southern tip of the canyon, the lake took more than a decade to fill with melting snow from the Rocky mountains flowing down the 1,450-mile Colorado river. That brought its own natural phenomenon.