The reservoir that ruptured sits atop 1,590-foot (485-meter) Proffit Mountain near Lesterville in Missouri's Ozarks and feeds the 440-megawatt Ameren Corp. plant. The breach of the reservoir's concrete-and-rock embankment spilled 1.5 billion gallons (6.8 billion liters) of water in 30 minutes, officials said.

"What we know so far is (the water) went across a county road, then through a park to the Black River. There was a 2-foot (0.6-meter) surge through Lesterville," though the water stayed within the river's banks and the town was not affected, said J. Mark Robinson, director of energy projects for the Federal Energy Regulatory Commission.

The agency sent investigators to seek the cause of the accident.

One hundred people were evacuated from the rugged area, said Spence Jackson, a spokesman for Missouri Governor Matt Blunt.

Besides the family that was rescued, campgrounds at a state park were submerged but no one was registered as staying there, he added.

It was unclear what might have caused the breach that sent water cascading into a lower reservoir which then overflowed, said Ameren spokeswoman Susan Gallagher. The plant is inspected at least once yearly, she said.

Though the weather was overcast at the time, minimal precipitation had fallen in recent days and did not appear to be a factor, officials said.

The lower reservoir - which is nearly six times larger than the now-empty 55-acre (22 hectare) reservoir on top of Proffit Mountain - is intact, company officials said. There was no effect on Ameren's power output, as the plant is used only during peak demand periods.

When the Taum Sauk Hydroelectric Plant is operating, water from the upper reservoir is fed through tubes inside the mountain to twin turbines. During low demand, water is pumped back up into the reservoir.