The stone retaining wall around a huge mountaintop reservoir in the Ozark Mountains collapsed before daybreak, releasing a billion-gallon (3.8-billion-liter) torrent of water that swept away at least two homes and several vehicles and critically injured three children, authorities said. The V-shaped, 600-foot (180-meter)-long breach opened up just after 5 a.m. Wednesday at a hydroelectric plant run by St. Louis-based utility AmerenUE, and in 12 minutes the 50-acre (20-hectare) reservoir had emptied itself out with terrifying effect, turning the surrounding area into a landscape of flattened trees and clay-covered grass. 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. Rescuers searched for an hour before finding the family of Jerry Toops, superintendent of a state park near the reservoir. The four were huddling silently at the far end of a muddy field 500 yards (455 meters) from where their home had once stood. "Pretty much all of them were in shock, honestly," said Lesterville Fire Department Chief Ben Meredith. The three children - ages 7 months, 3 and 5 - were listed in critical condition at a hospital in St. Louis, 120 miles (195 kilometers) to the northeast. The two older children had breathing problems; the baby suffered from hypothermia, authorities said. The reservoir, built in 1963, was dug out of the top of 1,590-foot (477-meter) Profit Mountain, with huge, sloping, 90-foot (27-meter)-high walls made of the 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. Normally, water released from the reservoir rushes down a 7,000-foot (2,100-meter) shaft and tunnel and spins the turbines to generate electricity. In Wednesday's accident, water gushed through the breach and streamed down the side of the mountain and into a valley, draining the reservoir like a bathtub. At 5:12 a.m., the water level in the reservoir was high, according to AmerenUE. By 5:24 a.m., it registered as low. The water eventually flowed back into the Black River. Soon after the break, police and the National Weather Service urged the 150 residents of Lesterville, about 15 miles (24 kilometers) downstream in the sparsely populated area, to move to higher ground. But by midday, once the water had flowed back into the river, the danger had passed. 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. AmerenUE serves 2.3 million customers in Missouri and Illinois, and the plant provides about 2 percent of its total electric generation. The floodwaters knocked over some power poles, causing scattered outages, but there was no widespread interruption of power because of the breach.