Communities just downstream of California’s Lake Oroville dam would not receive adequate warning or time for evacuations if the 770-foot-tall dam itself — rather than its spillways — were to abruptly fail, the state water agency that operates the nation’s tallest dam repeatedly advised federal regulators a half-decade ago.

The state Department of Water Resources informed federal dam regulators that local emergency officials “do not believe there is enough time to perform evacuations in the communities immediately downstream of the dam during a sudden failure,” according to a Feb. 8, 2011 letter reviewed by The Associated Press.

Absent “significant” advance warning, emergency responders instead would likely withdraw to safer ground and prepare for victims, said the same letter by the Federal Energy Regulatory Commission, which oversees safety of hydroelectric dams, in a summary of the state’s conclusions.

The catastrophic scenario of a sudden breach at California’s second-largest water reservoir, outlined between 2010 and 2012 in online archives of federal dam regulators, is a different and far graver situation than the concern that prompted sudden evacuation orders Sunday for 188,000 downstream residents.

Late Sunday afternoon, operators of the nearly half-century-old dam in California’s Sierra Nevada foothills became alarmed that the water cascading from the reservoir after a series of winter storms could roar uncontrolled down an emergency spillway toward towns downstream. They ordered residents of parts of three counties to leave.

Despite the troubles with the dam’s flood-control spillways, authorities have stressed they do not believe the dam itself is threatened. Sunday’s precautionary evacuation order was eased by midweek to allow people to return to Oroville and other downstream communities.