Scientists flooded the Grand Canyon in the US state of Arizona on Sunday morning to try to restore beaches, native fish and plants that have been wiped away by sediment-free water flowing from a man-made dam.
A torrent of gushing water raced down the Colorado River and into the canyon as four giant steel tubes at the base of Glen Canyon dam were opened.
Scientists hope the flood will return natural sediment along the Colorado River to the canyon.
Twenty experiments will be conducted during the flow test, including archaeological, biological, and hydrological studies.
An estimated 800-thousand metric tons of sediment were expected to be stirred up during the 90-hour run.
On Monday morning, when the waters are expected swell the highest, scientists will begin a four-day rafting trip down the Colorado River to see what the immediate effects of the high flow test were.
Before the construction of the Glen Canyon dam 40 years ago, natural flooding built up backwaters, eddies and sandbars with silt distributed from the Colorado's tributaries, landscape features within the river considered essential to native plant and fish species, including the humpback chub and the razorback sucker.
The dam forever altered the landscape.
Four of the canyon's eight native fish species have disappeared and prospects for the fifth, the endangered humpback chub, are grim.
Only about seven per cent of the sediment in existence before the dam was built is still there.