The federal government and three western states joined forces yesterday to launch a 50-year, $620 million project to restore wildlife habitat and protect endangered species along 342 miles of the lower Colorado River that have been heavily stressed by development.

Officials described the project, in the works for nearly a decade, as an attempt to balance the needs of imperiled animal and plant populations against those of human consumers who depend on the same water supply. The cost will be split between the federal government and the states.

With its goal of creating more than 8,100 acres of new marshes, forests and pools, the plan ranks as the largest river habitat project ever proposed under the Endangered Species Act, said H. Dale Hall, southwest regional director for the U.S. Fish and Wildlife Service.

Interior Secretary Gale A. Norton, in an interview, called the plan "unprecedented."

"This provides predictability for both the species and the water users," she said. "We can do a lot more working together than responding crisis by crisis to endangered species."

But environmental groups, some of which walked away from negotiations over the project under the Clinton administration, said the plan does not go far enough.

"The federal government and the states are trying to apply a Band-Aid approach over a 50-year period, and I fear the Band-Aid won't last that long," said Jennifer Pitt, a scientist at the advocacy group Environmental Defense.

The lower Colorado, which stretches from north of Lake Mead to the Mexican border, provides water and hydropower to Arizona, California and Nevada. It is also home to several endangered species, including the southwestern willow flycatcher, a migratory bird, and fish such as the bonytail and humpback chubs and the razorback sucker.

Because they are preyed on by nonnative species introduced by humans and because their traditional habitat is disappearing, several species have struggled to survive. Federal and state officials fear a further decline might trigger provisions of the Endangered Species Act that could force them to cut off water to residents.

"If we don't do something, the resources will continue to struggle," said Bennett W. Raley, assistant interior secretary for water and science. "It's far better to do good things than to do nothing, waiting for perfection."

Administration officials, who say they are largely adhering to a plan crafted under President Bill Clinton, acknowledge they are not attempting a radical overhaul of a river that has been greatly altered by development over the past century. Instead, they say, they aim to mitigate some of the effects by creating more backwater pools and marshes where endangered species can thrive. Officials also plan to tear out many salt cedar trees that line the shore, as the nonnative species may drain water from the river.

"It's impossible to go back to a natural Colorado River, given the number of people who rely on it," Raley said.

Twenty million residents in Arizona, California and Nevada use the river for drinking water, Hall said, and it irrigates 1.7 million acres of agricultural land.

Dennis Underwood, vice president for the Metropolitan Water District of Southern California, said his agency considers its $75 million contribution well worth it.

"It increases the reliability of our water and power resources," said Underwood, whose district serves 18 million Californians. "This is an overall comprehensive program."

Faced with a drought that has lasted five years, officials are eager to get started. Lake Mead, the reservoir that lies behind Hoover Dam and supplies the lower Colorado basin, has dropped 88 feet since January 2000 and is now at its lowest point since 1965.

The government has undertaken restoration projects higher up along the Colorado River, dating as far back as 1988.

Researcher Don Pohlman contributed to this report.

Visitors view the Red Wall Canyon in Arizona. A drought along the Colorado River has persisted for five years, adding to the project's importance.