A section of the inflatable rubber dam that forms Tempe Town Lake exploded shortly before 10 p.m. Tuesday, sending a wall of water into the Salt River bed.
The river filled as far as the eye could see within seconds, witnesses reported.
Related: Photos | February: Tempe delays replacing four Town Lake dams | Tempe Town Lake timeline
Warning sirens started wailing within minutes, and officers rushed along the riverbed to try to warn anyone of the approaching flood.
There was no immediate word of any casualties or injuries.
In April 2009, Tempe officials said they intended to ignore a safety recommendation from the makers of Town Lake's rubber dams because sufficient safeguards already were in place to prevent the dams from deflating.
Tempe Mayor Hugh Hallman rushed to the scene Tuesday and said authorities didn't yet know why the dam failed. The top concern was public safety, so city officials alerted all the municipalities downstream, along with the Maricopa County Sheriff's Office, he said.
Hallman said the other cities were checking their sections of the river for anyone in the water's path. Helicopters were scanning the river, along with emergency personnel.
Hallman said transients are known to camp on the river bottom this time of year.
The mayor said the flow was 15,000 cubic feet per second, equivalent to the amount released during heavy storm flows.
He added that the lake is now closed to the public.
Tempe police said no structures were in danger. Sheriff's deputies and police officers were riding ATVs along the river to see if anyone was in peril.
The lake stretches along the Salt south of Loop 202 from east of Rural Road to west of Mill Avenue.
Preston Swan, 24, of Tempe, witnessed the dam's collapse.
"It sounded like a big explosion," he said.
He and some friends were riding bikes in the area when a section of the recreational lake's western dam erupted. It collapsed immediately and water instantly surged into the riverbed.
Maureen Howell, 24, of Tempe, said she called 911 and that police arrived almost immediately.
"We stuck around because we thought this was a once-in-a-lifetime event," she said.
Philip Kanemeyer, 23, of Tempe, said the sound of the explosion was so loud that he ducked. "I saw pieces flying 10 feet into the air."
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