"The probability of a catastrophic levee failure in the Sacramento-San Joaquin Delta in the next 50 years is two in three," Mount said on the sidelines of the American Association for the Advancement of Science (AAAS) annual conference.

Mount gave a worrying presentation to the conference entitled "The Sacramento/San Joaquin Delta: the next New Orleans?"

He said one of the frequent earthquakes in California could destroy the levee system that has been built up since the middle of the 19th century, sending flood water over a wide area.

Mount said it could have a similar impact to the Asian tsunami in 2004.

The Sacramento-San Joaquin delta takes more than 40 percent of California's rainfall and covers some 280,000 hectares (700,000 acres). It is the main source of water for about 23 million people, of California's 34 million population.

But most of the land is below sea level and is protected by more than 1,600 kilometers (1,000 miles) of levees.

Another 5,600 hectare (14,000 acre) zone around St. Louis in Missouri faces a similar threat from the Mississippi river, according to Adolphus Busch, head of the Great Rivers Habitat Alliances.

Busch, a member of the Anheuser-Busch brewing family, said there had been excessive urbanisation in the zone despite major floods in 1993.

He said the housing and development had drastically reduced alluvial grounds that normally sponge up flood waters.

Nicholas Pinter of the University of Southern Illinois said that efforts to protect St Louis against flooding ultimately had increased the risks.

Eighty-five percent of the Mississippi is now held back by levees and the level of the river has risen by four metres (13 feet) since the start of the century, he said.

The situation is now similar to New Orleans, which was devastated last year after Hurricane Katrina smashed it water defences, and in the Sacramento region.

"In spite of 70 years of federal flood control efforts and nearly 40 years of federal flood insurance, the costs of flooding continue to rise and there is no federal policy to provide direction for future actions," said Gerald Galloway, an engineering professor at the University of Maryland.

Galloway said that development had to be restricted in risk zones and water defences had to be strengthened as the authorities in the Netherlands have done since disastrous flooding there.

"If we knew about Katrina 200 years ago, would we have done the same thing again in New Orleans?" Mount asked. "Well, in California we are reinventing our own Katrina as we speak."

Developers want to build 130,000 new homes near the Sacramento-San Joaquin Delta in the Central Valley, Mount said. Further north, Sacramento stands as one of the most at-risk for flooding among large metropolitan areas in the United States because of its location near rivers and the condition of its levees, he said.

Greater rainfall linked to global warming will only increase the risk of catastrophic flooding, according to Anthony Arquez, an expert at the National Oceanic and Atmospheric Administration.

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