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A new assessment by the U.S. Army Corps of Engineers raises concern about the potential for failure of the spillway of a flood control dam on a Southern California river where 1.4 million people live downstream.

The Corps said Thursday it has changed the risk characterization of Prado Dam from moderate urgency to high urgency.

Prado Dam is located on the Santa Ana River in the city of Corona about 35 miles (55 kilometers) southeast of downtown Los Angeles.

The 96-mile (154-kilometer) river begins in the San Bernardino Mountains and runs through inland Southern California and Orange County to the ocean.

It typically has little water flow except during winter when storms can turn it into a raging torrent that historically caused serious floods.

The dam, which otherwise is typically dry, was designed in the 1930s and constructed in 1941. It has never experienced a storm large enough to cause water to flow over the spillway, the Corps said.

The new evaluation was conducted this month.

“Risk factors identified indicate the potential for poor spillway performance, which could have adverse impacts to the downstream population, if a significant flood event occurs,” the Corps said in a press release.

Downstream from the dam are 29 cities, with more than $61 billion worth of property, it said.

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

The Corps said it is working on interim measures to reduce the spillway risks and modification of the existing spillway is expected to begin in 2021.

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