FENTON — Anyone who has ever lived within the reach of a Midwestern river knows the drill: It rains a lot. When it does, water swells. Buildings are threatened, and out come the sandbag crews.

"It's just part of the Midwest ideology," said Tom Kientzy as he leaned over to relieve his aching back after hours of lifting. "You just come to help out."

As the Meramec River bulged into Fenton Creek on Thursday, about 500 people showed up, shovels in hand, ready to help protect the old business district. On Friday, as many as 1,000 people followed, throwing themselves into a collective act of preemption and hope that is virtually embedded in a floodplain dweller's DNA.

"If it were my house, I would hope they'd help me — and they would," Kientzy said. "We're total strangers here."

To each other, maybe. But not to the repetitive, almost rhythmic process of building walls made of sand.

Most of the sandbaggers here, or in nearby Valley Park, or anywhere else within sight of a rising Missouri river, have done this before.

"I did this when I was little, in 1993," said Dan Ferrari, now 22, who traveled from Hazelwood to Fenton, where he helped shore up a little butter-yellow house. "The last time I was pretty much just holding the bags open for people. This time I can really help."

A fleet of dump trucks brought the sand from several area quarries to downtown Fenton, where, in turn, city trucks shuttled loads of sand to awaiting baggers.

"Sand, sand and more sand," said Jeff Jasper, driving a city dump truck in Fenton. "It's nonstop."

Jasper, working a 12-hour shift, slurped Dr Pepper as his radio crackled constantly. "Do you copy? We need trucks down here. … Getting low on sand."

Across the Meramac valley, the routine played out over and over again Friday. The shoveler loads three scoops in a bag, another person ties it off, then another sends it down a line of hands as a wall slowly builds. Shovels scrape along the asphalt. Bales of plastic bags — stamped "Made in Indonesia" in blue ink — crumple in the breeze.

"There's people from all over doing this," said Mary Seibert, whose insurance company building was girdled with sandbags by midafternoon. "It's unbelievable the people that come out to help. It's a weird feeling. It's amazing."

Even those who couldn't lift any sand came to help. Steve Vanhoogstraat spent four hours sandbagging on Thursday. But, with a hip that has been replaced three times and an injured shoulder, he finally had to call it quits. He decided to lend his talents as a deejay to the effort.

"I thought everyone works better to music," he said, standing next to a pair of giant speakers. "Yesterday, when I started playing, you could see the whole mood shift a little. People started bopping their heads and moving a little faster."

Indeed, the whole atmosphere was almost festive, with the smell of grilled hot dogs wafting and Bob Marley's "Jammin'" in the air.

But today, as the river is expected to crest to historic highs, the efforts of thousands of volunteers — and the hundreds of thousands of sandbags they filled — will meet their test.

Stephen Deere of the Post-Dispatch contributed to this report.