A plan to flush part of the Colorado River with a tidal wave of water and sediment to save an endangered fish and restore the beaches of Arizona's Grand Canyon was approved yesterday by the federal government.

The flooding is the second half of a two-part experiment scheduled to begin next month, when pulses of water will be released southward from the Glen Canyon Dam near the Arizona-Utah border to disrupt the spawning of non-native rainbow and brown trout. The trout have overwhelmed and eaten the native humpback chub, reducing the population from millions to fewer than 2,000 adults.

At the same time, biologists armed with electric wands will shock a six-mile stretch of the Colorado near its confluence with the Little Colorado River to force stunned trout to the surface for removal. They hope to cut the trout population by two-thirds over a two-year period.

The more dramatic portion of the experiment is to come in early 2004, when engineers release a torrent of water to flush sediment stored at the mouth of the Paria River into the Colorado to rebuild beaches and sandbars in the canyon. Scientists had hoped to flood the canyon as early as next month, but drought conditions prevented a sediment build up in the Paria.

"The beaches are important, but our main concern is the chub," said Joe Alson, superintendent of Grand Canyon National Park. "We're heading down a slope and getting to numbers that may not allow recovery. You hate having a species disappear."

The Glen Canyon Dam was built 40 years ago to create the reservoir known as Lake Powell, which extends northward into Utah. However, in taming the river, the dam altered its seasonal characteristics, bottling up sand, silt and clay from spring flooding and preventing warm summer waters from reaching the chub.

Determined to reverse the dam's environmental damage, then Interior Secretary Bruce Babbitt ordered the floodgates opened for one week in 1996. But within a year, the beaches began disappearing and the chub population declined further.

In April, a panel of scientists and river users recommended a plan that more accurately mirrors the river's pre-dam conditions. Three federal agencies - the Bureau of Reclamation, the National Park Service and the U.S. Geological Survey - evaluated and approved it.

"We moved pretty quickly on this," said Barry Wirth, a Bureau of Reclamation spokesman.

Environmental groups worry that the plan doesn't go far enough. "It's a baby step. It illustrates that we have too few tools in the tool box," said Geoffrey Barnard, president of the Grand Canyon Trust.

The trust wants the government to more aggressively regulate water temperatures and sediment flow to protect chub.

Late last month, the group announced it will sue the U.S. Fish and Wildlife Service for violations of the Endangered Species Act in the agency's plan to protect four endangered fish, including the humpback chub.

The agency has determined that the chub will be recovered at 2,100 adults, but Barnard said that number wasn't sufficient. Earlier studies had declared the fish endangered when the population was twice as high.

"If it was endangered when there were 4,300 fish, why is it saved with 2,100 fish?" Barnard said.