Tempe Town Lake overnight became a bog after one of the four inflatable bladders at the west end of the lake exploded around 9:44 p.m., sending a wall of water into the Salt River bed.

On Wednesday morning swampy patches of earth could be seen in many parts of the lake bed, with standing water in the center.

Tempe spokeswoman Kris Baxter estimated that at least three-quarters of the about 1 billion gallons of water had drained overnight. Officials say the dam breach left some areas of the lake with three feet of water or less; the average lake depth is about 16 feet.

Related: Photos | Video | February: Tempe delays replacing four dams | Dam tore in 2002, but no water leaked | Tempe Town Lake timeline

Tempe City Councilman Corey D. Woods said the city had been planning to replace the four rubber bladders at both ends of the lake starting last spring. However, he said, heavy rains and subsequent releases into the Salt River prevented that work from getting started on time.

"We already have two bladders in hand and were prepared to do the earthwork but couldn't," Woods said. Water was being released into the river all spring, he said.

Tempe Fire spokesman Mike Reichling said, "Fortunately, we were prepared for this exact circumstance. Within the last few months we had a drill for what we would do if this happened.

Reichling said fire officials got a call within a minute after the bladder burst and notified emergency crews throughout the Valley. Helicopters flew the length of the Salt River bed to make sure no one was in the water's path.

Reichling said eyewitnesses told officials that when the bladder exploded, it sounded like a giant balloon popping, and said water immediately began flowing into the river bed out of the lake.

The river filled as far as the eye could see within seconds, witnesses reported.

Warning sirens started wailing within minutes, and officers rushed along the riverbed to try to warn anyone of the approaching flood.

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.

Kris Baxter, a spokeswoman for Tempe, said the city believed that Bridgestone's main concern was that boats on Town Lake could get near the dam and puncture it. The city responded to the concern by stringing a line of buoys across the lake to prevent boats from getting close to the dam.

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.

Federal river flow gauges downstream showed the sudden spike in water flow rates as the lake drained. The amount of water measured at Priest Drive rose from nearly nothing at 9:45 p.m. to more than 13,000 cubic feet per second an hour later.

Such a spike in river flow is comparable to winter flash floods on rivers in Arizona's high country.

The amount of water moving in the river was less than the amounts that flowed downstream during rainy winters in the past. In January 2009, the river flowed at more than 18,000 cubic feet per second at its peak, as Salt River Project spilled water from its reservoirs upstream. During the winter of 2005, the flow peaked at about 41,000 cubic feet per second.

But in those cases, the flow was increased gradually over days or weeks.

Water that flows downstream will eventually reach Painted Rock Dam northwest of Gila Bend. The flood control dam can contain about as much water as Roosevelt Lake, far more than could escape from relatively small Tempe Town Lake.

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."

"It just flooded over in seconds," he said. "It was a high wall of water."

Lucas Henderson, 13, of Tempe, was biking on the north side of the lake with his sister and dad.

"All of a sudden, we heard this ka-boom and the ground started shaking," he said.

Six- to 8-foot waves poured out.

"It was like, whoosh, and the lake started emptying," he said. "It was amazing."

A crowd of hundreds soon gathered Tuesday night on the south side of the lake to observe the spectacle. Witnesses said small animals climbed up the river's banks to escape the floodwaters.

The lake was expected to continue draining Tuesday night and into Wednesday morning.

Tempe Town Lake, containing 977 million gallons of water, had its 10th anniversary Dec. 12.

Repairs had been scheduled to begin on the dam, which was deteriorating more rapidly than expected, in early February. But a severe winter storm postponed the work.

Jeff Lane, an SRP spokesman, said SRP halted water releases on May 27. He said there has been virtually no rain or any other sources of water since that time.

The Salt River has had water releases for three years in a row, an unusual occurrance in itself, he said. But before the bladder burst, the last time there was a huge amount of water in the riverbottom was in 1993 and 1995. The problem seemed to be eliminated in 1996, when Roosevelt Dam was raised 77 feet and Arizona also began a 13-year drought.

Hallman said replacement dams were being built but had not yet been completed.

In 2009, the dams' manufacturer, Bridgestone Industrial Products, had urged Tempe to evaluate whether the safety measures at the lake were sufficient "to prevent injury and reduce the risk of loss of life" if the dams were to rapidly deflate. Bridgestone manufactured the eight inflatable dams that contained the lake.

The eastern dams upstream are submerged and have held up. But a plan to keep the western dams wet failed, exposing the rubber to scorching sun that has damaged the material.

Tempe and Bridgestone officials have argued over whose fault that is. In March 2009, the City Council approved an agreement for Bridgestone to replace the four damaged dams at the lake's western end.

But the question of people's safety on the lake if the dams were to deflate remained a sticking point. Bridgestone recommended that the city look at increasing the buffer zone between safety buoys and the western dams, adding warning signage and enhancing the alarm system to better warn lake users if the dams rapidly deflate.

The cost to temporarily replace the dams was estimated at $2.5 million earlier this year. Tempe at that time budgeted an additional $250,000 to fund the project's contingency costs. Bridgestone was expected to reimburse Tempe up to $3 million of the costs to replace the dam.

Republic reporter Shaun McKinnon contributed to this article.