"The Grand Canyon began to form at least 17 million years ago, according to a study that comes in the wake of more than a century of debate about the history of this remarkable crack in the Earth.

The "incision history" that carved out a canyon that has 277 miles of river, measures up to 18 miles wide and a mile deep, has been disputed in part because some of the more common methods for dating the geological event don't reach back more than about one million to three million years ago.

Now, Dr Victor Polyak and Dr Yemane Asmerom of the University of New Mexico, Albuquerque, and colleagues report in the journal Science how they have used improvements in methods to date cave formations, relying on the way certain isotopes of uranium gradually decay, via other elements, to lead.

They determined the ages of the canyon's mammillaries or "cave clouds" - carbonate deposits that form at or near the water table level. By tracking the deposits that marked the position of the water, they conclude that the canyon is oldest on its western end and opened up steadily to the east through erosion.

The eastern part of the canyon was cut at a much faster rate than the western part, so the eastern canyon was probably completely cut through in five to six million years.

The timing may not be too surprising to geologists, but the new study places the dates on a firm footing for the first time, showing how it took up to 17 million years to carve its course downward through the 1-mile depth of the canyon.

Attempts to estimate the rate valleys are carved out go back at least to the British geologist Archibald Geikie who in the 1880s guestimated it was about one foot in 1,200 years, which is in the mid-range of the new data for the Grand Canyon.

Yesterday, arcs of water unleashed from a dam coursed through the canyon in a 60 hour flood meant to mimic the natural ones that used to nourish the ecosystem by spreading sediment.

More than 300,000 gallons of water per second were released from Lake Powell above the dam near the Arizona-Utah border, enough water to fill New York City's Empire State Building in 20 minutes.

The water level in the Grand Canyon rose 2 feet to 15 feet in some places. After the flood ends in the next day or so officials hope the water will leave behind sediment and restore sandbars as it goes back to normal levels. Officials have flooded the canyon twice before, in 1996 and 2004.

The construction of the dam in 1963 put an end to the cycle of spring floods that would naturally flush out the river. It encouraged non-native fish to thrive while accelerating the extinction of four native fish species.

Before the dam was built in 1963, the river was warm and muddy, and natural flooding built up sandbars that are essential to native plant and fish species. The river is now cool and clear, its sediment blocked by the dam.

The change helped speed the extinction of the four fish species and push two others, including the endangered humpback chub, near the edge.

Shrinking beaches have led to the loss of half the camping sites in the canyon in the past decade. Since Glen Canyon Dam was built, 98 per cent of the sediment carried by the Colorado River has been lost, Grand Canyon National Park Superintendent Steve Martin said.

But he said one flood was not enough. Martin says manmade floods need to occur every time there's enough sediment to do so - about every one to two years depending on Arizona's volatile monsoon season. "The science is really clear that's what we need to do."

The Grand Canyon Trust, a Flagstaff-based group that has been critical of the federal Bureau of Reclamation's management of the dam, also is calling for more regular high flows.

Scientists will document habitat changes, the effects of higher water flows and determine how backwater habitats are used by the chub and other fish."

Source:http://www.telegraph.co.uk/earth/mai...icanyon106.xml