The man-made flood, started Wednesday, was to continue for 60 hours at a rate of about 41,500 cubic feet per second, the Interior Department said. The water released from the power plant and bypass tubes at Glen Canyon Dam is expected to push sand built up at the bottom of the Colorado River channel into a series of sandbars and beaches along the river.

Scientists are monitoring how the high-flow releases affect the well-being of native fish, particularly the endangered humpback chub.

"This experiment has been timed to take advantage of the highest sediment deposits in a decade and designed to better assess the ability of these releases to rebuild beaches that provide habitat for endangered wildlife and campsites for thousands of Grand Canyon National Park tourists," Interior Secretary Dirk Kempthorne said in a statement.

The water was released at a rate that would fill the Empire State Building within 20 minutes, the agency said.