Three young siblings whose home was swept away when a mountaintop reservoir collapsed remained hospitalized Thursday, but the conditions of two of them had been upgraded.

Tara Toop, 3, and her 7-month-old brother, Tucker, have been upgraded from critical to serious condition, said Bob Davidson, spokesman for Cardinal Glennon Children's Hospital in St. Louis. Their 5-year-old brother, Tanner, remained in critical condition.

The children were caught up in a billion-gallon torrent of water released before daybreak Wednesday by the collapse of a stone retaining wall around the upper reservoir of a southeast Missouri hydroelectric plant.

The roughly 50-acre reservoir drained like a bathtub in 12 minutes, turning lower ground into a landscape of flattened trees and clay-covered grass. Two homes were swept away, including that of Jerry Toops, a state park superintendent, and his wife, Lisa.

A truck driver and a fire chief first heard Jerry Toops' cry for help. Rescuers searched for an hour before finding the couple's children in a muddy field 500 yards from where their home had once stood.

The children -- the only people hospitalized because of the deluge -- were suffering from hypothermia and breathing problems.

"Pretty much all of them were in shock, honestly," said Lesterville Fire Department Chief Ben Meredith.

The V-shaped, 600-foot-wide breach opened up just after 5 a.m. at the plant operated by St. Louis-based utility AmerenUE.

AmerenUE chairman and chief executive Gary Rainwater said it appeared that the plant's automated instruments had pumped too much water into the reservoir and caused it to rupture. A backup set of instruments should have recognized the problem but didn't, and the utility is trying to figure out why, AmerenUE said.

Trucker Greg Coleman was hauling a load of zinc when a wall of water emerged from the darkness and slammed into his truck near the Taum Sauk Lake Hydroelectric Plant.

"I had no idea where it was coming from -- I travel this road every day," Coleman said.

The water hit Coleman's truck, splashing through the windows. He climbed onto the roof and saw that another truck and a car were also submerged, with the drivers also on the roofs. The water receded within minutes.

It was then that Coleman said he heard Jerry Toops' cries for help.

The water eventually flowed back into the Black River. Residents of Lesterville, a community of about 150 people about 15 miles downstream, were urged to move to higher ground for several hours.

Emergency workers north of Lesterville said they saw two tractor-trailers pushed about 150 yards off the road, two pickup trucks and a car tossed into a field, and a house that paramedic Chris Hoover described as "just totally gone."

The reservoir, built in 1963, was dug out of the top of 1,590-foot Proffit Mountain, with huge, sloping, 90-foot-high walls made of stone removed from the peak. The reservoir -- the upper of two reservoirs at the hydroelectric plant -- was lined with concrete and asphalt. A plastic liner was added two years ago because of minor leaks, Rainwater said.

Gov. Matt Blunt said AmerenUE would be held responsible for flood damage if an investigation finds the company is liable. The company said it would respond to the flooded community's needs.

J. Mark Robinson, director of the office of energy projects at the Federal Energy Regulatory Commission, said the plant, including the reservoir, was inspected most recently in August and found to be properly operated and maintained.