The 16-foot-high section of the dam on Tempe Town Lake near Arizona State University's campus broke open at about 10 p.m. Tuesday. There were no immediate reports of any injuries and authorities said no structures were in immediate danger.
"All of a sudden, we heard this ka-boom and the ground started shaking," said 13-year-old Lukas Henderson, who was biking on the northside of the lake with his sister and father.
Witnesses said the dry Salt River filled as far as the eye could see within seconds, and small animals could seen scrambling away from the floodwaters.
Warning sirens began wailing within minutes, and officers rushed along the riverbed to warn anyone - particularly transients known to camp on the river bottom during the summer - of the approaching water.
Water was flowing at 15,000 cubic feet per second, equivalent to the amount released during heavy storm flows, Tempe Mayor Hugh Hallman said. The lake was expected to continue draining into Wednesday morning.
Tempe spokeswoman Kris Baxter-Ging said its unclear how the rubber dam burst, but she said workers were speeding up an already under way effort to replace the dam's bladders. The project had been delayed earlier this year by winter flooding, she said.
The lake has four inflatable dams on both ends and the dam sections were supposed to last for 25 to 30 years.
However, Tempe officials determined in 2007 that Arizona's harsh sun and dry climate was taking a toll on the rubber dams and might have to be replaced in a few years. The city inspects the dams about once a month and repaired two tears in 2002.
Tempe officials said in April 2009 they intended to ignore a safety recommendation from the makers of the rubber dams because sufficient safeguards were already in place to prevent the dams from deflating.
The lake can hold up to 1 billion gallons of water, Baxter-Ging said.