Scientists have begun releasing floods of water into the Grand Canyon in an attempt to boost local wildlife.
The dam water currently contains thousands of tons of sediment
In an operation lasting 90 hours, huge jets of water are pouring from a dam, taking with them hundreds of thousands of metric tons of sand and silt.
It is hoped the sediment will recreate the sandbars, backwaters and beaches needed by fish, birds, snails and plants that are currently dying out.
Four of eight fish native to the Grand Canyon have already become extinct.
"The Glen Canyon dam, on the Colorado River, was built 40 years ago," said AB Wade, a spokesperson for US Geological Survey, which is participating in the experiment.
"The ecosystem has been compromised because of the dam," she told the BBC, because up to 90% of the sediment builds up behind the dam instead of being released into the canyon.
Torrential rains in October agitated sediment into the water in Glen Canyon dam, and scientists are now acting quickly to release the estimated million tons of sediment into the canyon below before they settle.
"This is an effort to mimic what nature doesn't have a chance to do," said Ms Wade.
Scientists say a particular focus of the initiative is the humpback chub, a native fish which is endangered.
Other species which may be helped by additional sediment deposits include the south-western willow flycatcher, which eats vegetation that grows in sediment, and the Kanab amber snail, which lives in the vegetation.
The Park Service has warned Grand Canyon enthusiasts that their favourite campsites could be submerged by as much as 4.5m (15 feet) of water - and to stay away.
On Sunday morning, four 2.5-metre-wide steel pipes on the dam were opened for 90 hours.
At their peak on Monday morning, they were releasing water at a peak volume of 1,161 cubic metres (41,000 cubic feet) per second through the canyon.
Some of the 50 scientists involved in the project will boat downriver to test whether sediment deposits are building up as hoped.
Scientists hope to save the humpback chub
A similar experiment in 1996 failed to re-establish long-lasting beaches and sandbars, and though a different technique is being used this time Ms Wade stressed scientists could not be sure of the results.
"This is an experiment," Ms Wade said.
"If it works out, we are likely to consider more releases over the next 18 months. This is just the start."
The Grand Canyon National Park is a World Heritage site and federal law says local authorities must do all they can to preserve it.