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 Friday.

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 Corps says it estimates that only 16,000 people in those communities would be affected by flooding.

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 (61-meter-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 (225.3 kilometers) 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.

However, "all it takes is one event ... one Katrina, one Hurricane Harvey," Vera said. "These storms are happening more and more, so this is our way of looking toward the future."

The corps has been working with local communities on emergency preparation plans and will also begin a study on upgrading and strengthening the dam, Vera said.

Since 2005's devastating Katrina, the corps has been looking at all of its 700 dams nationwide.

In May, the corps upgraded the risk characterization of Prado Dam to high urgency. That dam is located on the Santa Ana River in the Los Angeles suburb of Corona. Dozens of Southern California cities with about 1.4 million people live downstream.

Work to improve the dam has been under way since 2002 to increase the amount of floodwaters and sediment it can store.

In 2017, some 200,000 people in three Sierra Nevada counties were forced to evacuate after spillways at the Oroville Dam crumbled and fell away during heavy rains.

Flooding didn't happen, however, and the dam has since been repaired.