JEFFERSON CITY, Mo. -- As Union Electric Co. prepared to open a mountaintop hydroelectric plant in 1963, some workers raised concerns about a staffing schedule that left no one onsite overnight to prevent potential problems from escalating into a disaster.

But state utility regulators sided with the electric company, ruling the plant was not inherently dangerous and required no overnight staff.

Four decades later, on a December 2005 night without any staff onsite, the Taum Sauk reservoir began overflowing and quickly collapsed. More than 1 billion gallons of water rushed down the mountain, wiping out a state park and seriously injuring the park superintendent's family.

The concerns of 1963 resurfaced for the first time Friday as the Missouri Public Service Commission wrestled with its own investigation into the reservoir collapse and whether AmerenUE -- the successor to Union Electric -- has displayed a systematic disregard for safe service.

July 30 deadline

Commissioners on Friday gave their investigative staff until July 30 to report back on the Taum Sauk incident, leaving the potential that a broader review of Ameren's management policies could extend further.

AmerenUE president and CEO Thomas Voss said the investigation is unnecessary and unreasonable. Ameren attorneys and some public service commissioners also questioned whether they have jurisdiction to pursue penalties against Ameren for the collapse, since the hydroelectric plant is regulated by the federal government and Ameren has agreed to pay a $15 million federal fine.

Even if Ameren staff had been onsite the night of the disaster, it's unlikely they could have stopped it, Voss said.

As was approved in 1963, the Taum Sauk hydroelectric plant was being monitored overnight by staff at Ameren's hydroelectric plant at the Lake of the Ozarks, about 120 miles to the northwest. But because water sensors were raised too high at Taum Sauk, employees -- whether onsite or not -- likely would not have realized it was overflowing on Dec. 14, 2005, Voss said.

Over in 10 minutes

Even if an employee had been walking around the reservoir, he still might not have been able to prevent the deluge.

"I question whether someone who was onsite at night could react, because you can't see the whole reservoir" at once, Voss said, "and then to be able to get to a switch fast enough -- the whole thing was over in about 10 minutes."

But Public Service Commissioner Steve Gaw, who read an excerpt from the 1963 PSC case during Friday's meeting, said it seems union workers had made a "prophetic request."

The International Union of Operating Engineers, Local 148, had wanted to have operators at the Taum Sauk plant 24 hours a day, seven days a week. Their plan would have cost about $80,000 annually, according to the commission's 1963 decision.

The union swayed just one of the five commissioners; three took the plant's position, and one did not participate in the decision.

Although alone in his opinion, Commissioner William Barton said he would have required overnight staffing, at least until experience had shown the plant could run automatically without danger to the public.

It's unclear, even through hindsight, whether the Taum Sauk plant still would have been staffed overnight in 2005 if Barton's opinion had prevailed 42 years earlier.

But "at least one commission's foresight was clear at the time," Gaw said," and unfortunately, there weren't others who were listening."