With Tempe in the grip of a suffocating heat wave, flooding might be the last thing on your mind.

But as recent flooding in the Midwest proves, not only can the worst case happen, sometimes the unthinkable happens.

Consider Cedar Rapids, Iowa.

In more than 150 years of record-keeping, the Cedar River had never gone more than 8 feet above flood stage. Last month, it exceeded that record by 10 feet, an event that left weather experts and public safety officials all but speechless.

No one came close to predicting the ultimate scope of the calamity because a unique chain of events rendered previous forecasting methods useless.

Before this summer, Cedar Rapids could always guess how high its flood crests would be by looking at what the river did upstream in Waterloo. But this year, torrents of rain fell between the cities, and no one had ever planned for that.

In Tempe's case, and for the Valley as a whole, the nightmare scenario - albeit a highly improbable one - begins about 40 miles up the Salt River Valley as the buzzard flies.

There lies the massive Roosevelt Dam, nearly 100 years old.

At peak capacity, it can hold back almost 1.7 million acre-feet of water.

Below Roosevelt are three smaller dams, whose lakes push the total capacity of the Salt River system to more than 2 million acre-feet - enough water to flood 3,125 square miles a foot deep. Tempe is about 40 square miles, Mesa 130 square miles.

Water has never breached or topped Roosevelt Dam, but it came frightfully close in February 1980.

After a storm poured 10 inches of rain on mountains upstream of the reservoir, engineers opened the floodgates to make room for the torrent. The dam came within inches of overflowing, which engineers believe would have triggered a catastrophic collapse.

The Salt River Project expanded and strengthened the dam in 1996, and experts say that greatly reduced the dangers of failure.

Charlie Ester, manager of water resource operations for the Salt River Project, said federal and local agencies have invested heavily in dam safety since the 1970s.

"We don't have the same system we had here 30 years ago," Ester said. "It's been greatly improved."

Ester said after its renovation, Roosevelt Dam now can handle water from a storm of the sort that might occur, statistically, once in millions of years.

Even so, the scenario is on the minds of disaster planners.

Joe Muñoz, spokesman for the Flood Control District of Maricopa County, said planners recently went through an exercise with a Roosevelt collapse as its centerpiece.

Should that happen, Muñoz said, "It would break every dam below it," unleashing more than 630 billion gallons of water onto the Valley.

People would have perhaps four to five hours to get out of the way, Muñoz said, before Tempe and then downtown Phoenix would be swept with churning walls of water, mud and debris.

Ester said the Valley's biggest flood danger comes not from the well-tamed Salt River, but from the Verde, which feeds into the Salt River Valley northeast of Mesa.

The Verde's dam system is less extensive than the Salt's, Ester said, and spillway failures could dump 500,000 to 600,000 cubic feet of water per second into the Salt channel.

That would dwarf the biggest flood in recent years, when 124,500cubic feet per second roiled through the Valley in January 1993.

But again, Ester said, "The likelihood of that happening is very tiny; much, much smaller than the flood that's going on now on the Mississippi."

"We're not geographically located to get floods of that magnitude like they're getting in the Midwest. That's not our hydrology."