The early morning breach occurred in the upper of two reservoirs at the hydroelectric plant run by St. Louis-based utility AmerenUE, company officials said.

One person had been feared missing, but was later accounted for, authorities said.

Gov. Matt Blunt said three or four family members were hospitalized after water swamped their home at a state park where the father is superintendent.

Three children with critical injuries were being transported to a hospital in St. Louis, 120 miles to the northeast. A spokesman for Cardinal Glennon Children's Hospital did not have the names of the children and could not confirm if they were related to the park superintendent. He said a 7-month-old suffered from hypothermia, and a 3-year-old and 5-year-old had breathing problems.
The Federal Energy Regulatory was investigating the cause of the breach at the Taum Sauk Lake Hydroelectric Plant. AmerenUE officials said there was no sign of foul play. The reservoir sits near a fault line in the Ozark mountains, but Blunt said there was no seismic activity Wednesday morning.
National Weather Service meteorologist Joe Pedigo said rain was not a factor in the break. The region received only about one-tenth of an inch of rain overnight, he said.

Conditions along the Black River, where the plant is situated, were considered dangerous, the weather service said.

"The Lesterville area and areas south along the Black River are in extreme peril," said Missouri State Highway Patrol Sgt. Marty Elmore. "We need to make every effort to have folks get to higher ground."

Pedigo said rescue teams searched for people believed to be trapped in cars, especially along Highway N near the reservoir shortly after the breach. Pedigo said a house, a mobile home, several cars and a tractor-trailer were reported washed away.

The town of Lesterville, with about 150 residents, was under a voluntary evacuation order, said Reynolds County emergency management director Terry Sanders. She didn't know how many people were forced out.

Reynolds County has about 6,700 residents.

The plant was built in 1963. AmerenUE officials said the breach occurred at the northwest corner of the reservoir that holds back 1.5 billion gallons of water from the Black River.

About a billion gallons of water — nearly 80 percent of what was stored there — leaked from a v-shaped breach in the wall of the reservoir, which is dug out of the top of a mountain, said Gary Rainwater, president and CEO of AmerenUE.

The water drained down the side of the reservoir into a valley, then flowed back to the Black River, he said.

"A number of AmerenUE engineers and specialists are investigating the incident; clearly, public safety is our top concern," plant superintendent Rick Cooper said.

AmerenUE spokeswoman Susan Gallagher said the plant has four chief features — the upper reservoir atop Proffit Mountain, a 7,000-foot-long shaft and tunnel, a powerhouse with two reversible pump turbine units and a lower reservoir formed by a dam across the Black River's east fork.

During times of peak demand for electricity, water released from the upper reservoir rushes down the shaft and through the tunnel. As it passes through the powerhouse, the water spins the turbines to generate electricity, then is retained in the lower reservoir.

Rainwater said the lower reservoir was safe.