Water is unleashed from Glen Canyon Dam towards the Grand Canyon in the United States - an experiment to mimic natural floods and recharge the ecosystem.
More than 300,000 gallons (1.1m litres) of water per second gushed from giant steel tubes into the river - the equivalent of turning on 1.3m garden hoses simultaneously.
US Secretary of the Interior Dirk Kempthorne, in the blue shirt at the railing, released the valves. He said the volume of water could fill the Empire State Building in 20 minutes.
The manmade flood will carry rich sedimentary residue to nourish the Grand Canyon area, improving the fish habitat in the river and rebuilding beaches.
Water levels along the Colorado river rose quickly after the flood was released. Since the dam was built in 1963, 98% of the sediment carried by the river has been lost.
Before then the river, near the Arizona-Utah border, was muddy, and natural flooding built up sandbars essential to native plant and fish species.
After the flood ends on Friday, it is hoped the water will leave behind sediment, and restore sandbars, as it goes back to normal levels.