SACRAMENTO — California's capital city may be best known for politics, but it has another claim to fame: It's America's most flood-threatened city not named New Orleans.

A recent state report predicts that the right combination of unlucky weather conditions could put some parts of the city under more than 20 feet of water, causing a $25-billion disaster that would cripple state government and ripple through the California economy.

Authorities are racing against time to strengthen the earthen levees that ring nearly the entire city to hold back the swollen American and Sacramento rivers.

"Every winter we hold our breaths and hope this isn't the year something happens before we can finish the work," said Sacramento County Supervisor Roger Dickinson. "There is a sense of the clock ticking."

When heavy rain begins to fall, folks here peer nervously at the sky and riverbanks. And Stein Buer -- the person perhaps most responsible for their fates -- frets and prepares.

"I never sleep during storms," said Buer, executive director of the Sacramento Area Flood Control Agency, which is working with the state and federal governments in a multibillion-dollar effort to avert catastrophe. "It's the nature of my responsibility."

Worst-case scenarios project 500 dead, 102 square miles flooded, 300,000 people uprooted, an international airport and state agencies under water, and years of recovery.

To avoid that outcome, Buer has plotted strategy, navigated bureaucracy, even joined crews tossing sandbags.

He isn't going it alone. The U.S. Army Corps of Engineers, the Federal Emergency Management Agency and the federal Bureau of Reclamation have all stepped up prevention efforts since Hurricane Katrina devastated the Gulf Coast in 2005. State flood experts and Gov. Arnold Schwarzenegger, meanwhile, are pushing to buttress the Central Valley's 1,600 miles of levees.

The aim, with the help of nearly $5 billion in state bond money approved in 2006, is to double Sacramento's flood protection over the next decade.

Work began recently on a $683-million Folsom Dam spillway channel that would more quickly lower the lake as a mega-storm approached the American River's 18,000-square-mile watershed. Along the Sacramento River, which drains 23,000 square miles of Northern California, crews have reinforced aging levees near some of the most flood-prone neighborhoods.

But in Sacramento's fastest-growing neighborhood, big trouble still looms -- even without a flood.

Amid farm fields north of downtown, the subdivisions of the Natomas Basin spread along the lazy curves of the Sacramento River. A century ago, the river would reliably overflow and turn Natomas into a vast inland sea. Today, flood waters could be 23 feet deep if it weren't for the levees.

Although they were upgraded during the 1990s, those earthen walls are now deemed at risk by federal regulators, mostly because the standards have toughened since Katrina.

The upshot for Natomas and its 70,000 residents is a de facto building moratorium and higher flood insurance rates set to kick in at year's end.

Builders are squawking. Homeowners are uneasy. Environmentalists question the wisdom of ever building there.

"To me, floods are the biggest risk to public safety we face and the biggest risk to our economic viability," said Sacramento Mayor Heather Fargo, head of the flood control agency's board. "In our 12-step recovery, we are way beyond denial."

Sacramento has spent 150 years getting to this point. Settlers experienced the first floods not long after John Sutter built his fort near the rivers' confluence. High waters caused an epidemic of cholera. In the flood of 1862, Gov. Leland Stanford traveled to his inauguration via rowboat.

Civic leaders hauled in dirt to raise downtown streets by 10 feet, turning first floors into basements. Levees narrowed the rivers, boosting their velocity to help wash away gold-mining debris flowing out of the Sierra.

But pinching the waterways laid the ground for the trouble to come. A constricted river is a river that wants to escape.