GRAND CANYON NATIONAL PARK, Ariz. – Scientists flooded the Grand Canyon yesterday to restore beaches and save fish and plants that have been disappearing since sediment-free water began flowing from a man-made dam 40 years ago.

A torrent of gushing water raced down the Colorado River and into the canyon, carrying badly needed natural sediment with it, as four giant steel tubes at the base of Glen Canyon dam were opened.

“The sediment, sand, mud, and silt play an important role in the ecosystem,” said Chip Groat, director for the U.S. Geological Survey.

An estimated 800,000 metric tons of sediment were expected to be stirred up during the 90-hour run.

Four decades ago, before the dam was built, natural flooding built up backwaters, eddies, and sandbars with silt distributed from the Colorado’s tributaries.

The construction of Glen Canyon dam upstream forever altered the canyon: Four of eight native fish species have disappeared and prospects for a fifth, the endangered humpback chub, are grim. Only about 7% of the historical sediment from before the dam was built remains.

Twenty experiments will be conducted during the test, including archaeological, biological, and hydrological studies.

Today – when the waters are expected to swell the highest – scientists will begin a four-day rafting trip in the canyon to see what the immediate effects of the high flow test were.

In 1996, officials flooded the canyon in an 18-day water release, although only about five of those days produced high floods.

The Interior Department had been studying the dam’s effects on the canyon and had learned that beaches were washing away.

But scientists overestimated the sediment levels in the beds of the tributary rivers that flow into the Colorado below the dam, and sediment redeposited by some of the flooding was only eroded away by other floodwaters.