A California dam could fail during an extreme storm and send water flooding into Mojave Desert communities that are home to about 300,000 people, authorities said.

The U.S. Army Corps of Engineers announced that it has changed its risk characterization of the Mojave River Dam from low to high urgency of action.

The earthen dam was built in the 1970s near the San Bernardino Mountains northeast of Los Angeles. It was designed for flood control and is usually dry.

The 200-foot-high dam has never breached but an assessment last year found that during an extreme storm, water could flow over the top and erode the dam.

That could threaten Apple Valley, Hesperia, Victorville, Barstow and even the tiny town of Baker, more than 140 miles downstream.

The chances of such a storm are only about 1-in-10,000, said Luciano Vera, spokesman for the Los Angeles district of the Army Corps of Engineers.

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