The U.S. Bureau of Reclamation hopes this year's 60-hour pulse of floodwaters from Glen Canyon Dam will work better than the 1996 and 2004 experiments, which had mixed success in rebuilding sandy beaches and sheltering sandbars for endangered humpback chubs and other fish and wildlife.

"Those conditions were markedly different," said Randall Peterson, environmental manager for the bureau's Upper Colorado Region office in Salt Lake City.

This time, he said, the river may be primed for success because of exceptionally high volumes of sediment that tributaries below the dam have washed into the river in recent years. There is three times the 2004 level of sediment available to wash downstream - the highest levels in a decade, according to the U.S. Geological Survey.

The point is to see whether artificial floods can stir up sediments to adequately rebuild beaches and sandbars that have slumped since the government built the dam and began regulating the flow in the early 1960s. Loss of sandbars has left young chubs, one of a handful of the Grand Canyon's native fish, with fewer rearing places out of the current. Beaches provide both wildlife habitat and campsites for boaters who float through the national park.

The experiment will cost roughly $4 million in lost hydroelectric generation at the dam, Peterson said. Government agencies will spend $3.7 million from their budgets and from the dam's power-generation revenues on science projects during and after the flood.

The bureau's plan has drawn criticism from environmental groups and the National Park Service, though, with some advocates even calling it a public-relations "greenwashing."

The Grand Canyon Trust sued in an attempt to force the agency to repeatedly flood in spring but keep stable flows through the summer and fall to keep from disrupting fish reproduction and rearing. Executive director Bill Hedden said the five-year plan that the Bureau of Reclamation adopted instead assures just this week's flood test and then five years of flows that may vary widely with summertime electricity demand for air conditioning.

The government pledges stable flows in September and October each year, though Hedden said July and August are more important for young fish.