PHOENIX — A new controlled flood at the Grand Canyon is set to begin today in an effort to return natural sediment along the Colorado River to the canyon.

Environmentalists have said for years that the unnatural flows of the Colorado River below the Glen Canyon dam were washing away sediment and that beaches were disappearing, along with fish and plants.

The simulated flood beginning today will allow scientists to see whether the dam — the root cause of many of the problems — can also help fix them.

It will consist of opening four giant steel tubes at the base of the dam, sending a torrent down the Colorado and into the canyon. An estimated 800,000 metric tons of sediment will be stirred up during its 90-hour run.

‘‘We're trying to mimic the role of all that sediment that used to be there before the dam,'' said Dennis Fenn, director of the Southwest Biological Science Center, under the U.S. Interior Department. ‘‘Water that goes through the dam is clear, and sediment-free. The sediment is trapped behind the dam and doesn't come down like it used to.''

Fenn said that only about 7 percent of the historic sediment before the dam was built is still there.

Glen Canyon dam, built 40 years ago just upstream from the Grand Canyon, 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.

Before Glen Canyon Dam's construction, natural flooding built up backwaters, eddies and sandbars with silt distributed from the Colorado's tributaries.

Those landscape features within the river were considered essential to native plant and fish species, including the humpback chub and the razorback sucker.

Officials have unleashed high flood waters before to see how the environment responds.

The Interior Department began studying the effects of the dam on the Grand Canyon in the early 1980s and soon found that the beaches were washing away. By 1996, the department had spent more than $100 million studying the canyon's environment.

That year officials flooded the canyon with an 18-day water release, although only about five of those days produced high floods.

‘‘We learned a lot from that study,'' Fenn said.

He said a major problem with that study was that scientists overestimated the sediment in the bed of tributary rivers that flow into the Colorado River below the dam. The initial high flood waters redeposited sediment in the Grand Canyon. But steady, lower flood waters began undoing the good, eroding the moved sediment.

In the summer and fall of 2000, a high water flow was released through the dam to flush away nonnative fish and redistribute sand on beaches. Then a low, steady flow was maintained in an attempt to promote conditions that would encourage an increase in the native fish.

Again, researchers found that keeping the Colorado River flow steady and low below Glen Canyon Dam didn't accomplish many of the things it was supposed to accomplish.

Further, that study cost utilities around $32 million, contributed to rafting accidents and didn't help produce a population upsurge for such endangered native fish species.

This latest dam drain is estimated to cost just under $2 million in potential power generation, said Clayton Palmer, a resource manager for the Colorado River Storage Project office.

‘‘It's the value of the water that won't go through the power plant,'' he said, adding that this shorter test is cheaper than the one in 1996.

And this time, the Bureau of Reclamation — the government's dam-managing agency — plans to release a surge of water for a much shorter period of time, hoping to avoid the erosion of redeposited sediment.