Intensified development in flood-prone parts of Missouri and California significantly raises the risk of New Orleans-style flooding in urban areas on the Mississippi and Sacramento rivers, researchers said Saturday.

Around St. Louis, where the Mississippi River lapped at the steps of the Gateway Arch during the 1993 flood, more than 14,000 acres of flood plain have been developed since 1993. That has reduced the region's ability to store water during future floods, said Adolphus Busch IV, a scion of the Anheuser-Busch brewing family and chairman of the Great Rivers Habitat Alliance.

Efforts to protect St. Louis from floods may have increased the risk, said Nicholas Pinter, a professor at Southern Illinois University. Pinter said as much as 85 percent of the Mississippi in St. Louis is confined behind levees, which have raised flood levels up to 12 feet higher than they were just a century ago.

That parallels the situation in New Orleans, which suffered catastrophic flooding when levees failed in the wake of Hurricane Katrina last summer.

Details about the research were being presented Saturday at the annual meeting of the American Association for the Advancement of Science.

In California, development in the Sacramento-San Joaquin delta, where flood control efforts have continued since the mid-1800s, also represents a significant risk, said Jeffrey Mount of the University of California, Davis.

Mount estimates a two-in-three probability over the next 50 years of a catastrophic levee failure in the massive delta region east of San Francisco.

Even a moderate flood could breech the delta's levee system, and a larger breech, perhaps following an earthquake, would inundate the whole region, Mount said.

The Sacramento-San Joaquin delta receives runoff from more than 40 percent of California. The delta covers 738,000 acres, crisscrossed with hundreds of miles of waterways. Much of the land is below sea level and relies on more than 1,000 miles of levees for protection against flooding, according to the California Department of Water Resources.