CORONA — The U.S. Army Corps of Engineers says a recent assessment has raised 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 in Riverside County southeast of Los Angeles.

The 96-mile river typically has little water flow except during winter storms and the dam is typically dry.

The dam was built in 1941. Engineers say an assessment this month indicates potential for poor performance of the spillway during a “significant flood event.”

Work on the spillway will begin in 2021.