At sunrise this morning, the US Interior Secretary, Bruce Babbitt, will unwind a giant valve at the base of the dam and unleash a swirling torrent that scientists hope will recreate what used to be a regular phenomenon in the canyon at this time of year: a spring flood.
The hope is that the seven-day surge will do for the canyon what nature used to do for itself: trees and scrub will be scoured out and huge quantities of silt will be transported downriver, rebuilding sand beaches and creating new pools for fish to spawn.
"This is about restoring one of the most amazing, most beautiful places on earth," Mr Babbitt argued. "We've gotten all the groups to work together - environmentalists, power users, Native American tribes, irrigators - because we share a common purpose of protecting a sacred American place".
For the week that the valve stays open, the flow will be roughly equivalent to half of what passes over the Niagara in summer. In the Grand Canyon, river levels will rise by about 2ft. It will take roughly one day for the surge to travel all the way through the canyon to Lake Mead on the Nevada border.
Ecologists have long bemoaned the impact wrought by the dam which was built in 1963 to provide hydro-electric power and water for irrigation in six south-western states. The river is 20 degrees colder than it used to be and is clear rather than its natural turgid brown.
Scientists calculate that until 1963, spring floods would send some 65 million tonnes of sediment from Glen Canyon to Lake Mead. Since then, the flow of sediment has diminished to about 2.9 million tonnes a year.
The flood will also provide a special treat for river rafters. A limited number of rafting companies have been given permission to ride the unusually strong rapids for the week, even though the normal season does not start until April.
There may be some less welcome side-effects, however. Trout fisheries at the head of the Grand Canyon may suffer some damage and scientists fear that marsh areas that have become the habitats for rare species of frogs may be wiped out.Reuse content