By Ellen Wulfhorst and Scott Malone
NEW ORLEANS (Reuters) - Torrential rain dropped by Hurricane Isaac threatened to burst a dam on Thursday, forcing evacuation of up to 60,000 people in Louisiana and Mississippi and leaving large areas of the region flooded and without power.
Isaac, which was downgraded to a tropical depression on Thursday after moving in from the Gulf on Tuesday, left little damage in New Orleans, where stronger barriers were installed after Hurricane Katrina in 2005. Only one fatality linked to the storm has been confirmed so far.
But it left a soggy mess across widespread areas of the U.S. Gulf Coast and could still bring heavy downpours and flooding as it moves into the central United States - where rain is badly needed - over the next few days.
More than 1 million residents of Louisiana and Mississippi were without power due to the storm on Thursday morning, according to the U.S. Department of Energy.
Fears about a possible imminent failure of the Lake Tangipahoa Dam in Mississippi prompted authorities to order the immediate evacuation of 60,000 residents in nearby communities in Louisiana and Mississippi.
The dam, in Pike County, Mississippi, is about 100 miles north of New Orleans. It was damaged by Wednesday night's torrential rains but did not suffer a breach.
"It hasn't failed. They're trying to patch it. They're planning to do a controlled breach if necessary. That would allow them to direct the flow," said Greg Flynn, spokesman for the Mississippi Emergency Management Agency.
If it were to break, low-lying areas - including Kentwood, Louisiana, the hometown of pop singer Britney Spears - could see record flooding, Louisiana Governor Bobby Jindal told a news conference.
The oil and gas industry in the Gulf of Mexico region has so far reported no major storm-related damage to infrastructure although one Louisiana refinery has flooding. Energy production was expected to start ramping up again, after nearly grinding to a halt as Isaac closed in on Louisiana on Tuesday.
Benchmark Brent crude was little changed in Thursday afternoon trading at about $112.75 a barrel.
Multibillion-dollar defenses built to protect New Orleans itself, after it was ravaged by Katrina almost exactly seven years ago, passed their first major test, according to the U.S. Army Corps of Engineers.
"Yes, the levees worked," Jindal said.
SURROUNDING PARISHES HIT
But massive rains and storm surge from the Gulf of Mexico inundated low-lying communities outside the federal flood containment system protecting New Orleans, forcing the evacuation of thousands of people from their homes and some dramatic rooftop rescue operations.
Hardest hit was Plaquemines Parish, southeast of New Orleans, where floodwaters overtopped at least one levee on Wednesday and left many homes under about 12 feet of water.
Local boatmen in Plaquemines plucked dozens from the roofs of their houses after they had decided they could ride out what, compared with Katrina, seemed like a small storm.
Parish President Billy Nungesser said U.S. Army National Guard troops and local sheriff's office officials were going house to house through the area on Thursday to ensure that there were no deaths or injuries.
Clearing weather permitted the use of military helicopters, mostly UH-60 Blackhawks, to aid in the operation.
In St. John the Baptist Parish, northwest of New Orleans, about 3,000 people were forced to evacuate their homes before dawn on Thursday due to storm surges from Lake Pontchartrain and Lake Maurepas, authorities said.
In Slidell, a town of about 27,000 people northeast of New Orleans, the surge from Lake Pontchartrain left the Eden Isle community under about a foot of water.
"You'd have never made me believe a Category 1 would dump this much water," said Sam Caruso, 71," a former mayor of the town who was touring it in his pickup truck.
Emergency services rescued about 350 people from Slidell homes and neighboring communities hit by more severe flooding, local authorities said.
National Guard troops and police moved into the town Thursday afternoon as some local residents navigated flooded streets in boats. Overall, troops have rescued or evacuated more than 3,000 Louisiana residents and three tractor-trailer loads of pets, Jindal said.
'IT WAS SCARY'
Staff Sergeant Denis Ricou, a Louisiana National Guard spokesman, said about 5,800 troops had been deployed due to Isaac and the number could rise to over 8,000 in the coming days.
City officials imposed a dusk-to-dawn curfew on Wednesday to help prevent any repeat of the looting that occurred in New Orleans in the days after Katrina struck in 2005.
New Orleans Mayor Mitch Landrieu warned that the punishment for a looting conviction is harsh - a mandatory three years' hard labor. "If you loot, you'll wear an orange suit," Landrieu told a news conference.
About a dozen looting-related arrests were reported in the city by Thursday morning but the streets were unusually quiet - littered with downed branches, fallen trees and pieces of roofing material.
Power remained out through most of the city, while in the historic French Quarter, a few people were out taking down the boards they had nailed up over store windows. Officials urged patience and good humor during the clean-up.
"Our tempers tend to flare," said New Orleans Councilwoman Cynthia Hedge Morrell at the news conference. "My advice would be sit on the front porch and talk to your neighbors. That's what New Orleans is known for."
As the focus on Isaac shifted from the coast, many in its projected path further north have been praying it will bring rain desperately needed to ease a drought in the central states, where summer crops are drying up and many rivers and dams are critically low.
Meteorologists guess that rains could be too late for most of the stressed corn and soybean crops, but could replenish soil moisture before winter wheat fields are planted.
Isaac never came close to the power of Katrina, which was a Category 3 hurricane on the five-step Saffir-Simpson scale when it smashed into New Orleans on August 29, 2005.
But U.S. President Barack Obama still declared the impact on Louisiana and Mississippi major disasters and ordered federal aid to supplement state and local recovery efforts.
City officials said Louis Armstrong New Orleans International Airport, which closed late on Monday, would remain shut on Thursday until repairs can be made to the damaged lines that supply it with power.
A tow-truck driver, 62-year-old Gregory Alan Parker, died early Thursday after a tree fell on his cab while he was trying to move a large tree from a main street in Picayune, a town in Pearl River County, Mississippi, near the Louisiana border.
Apart from an unconfirmed death in a Louisiana apartment fire, his was the only U.S. fatality blamed on Isaac so far. The storm killed 23 people in Haiti and the Dominican Republic on its way across the Caribbean to the Gulf of Mexico.
The U.S. National Hurricane Center said Isaac, now a tropical storm with top sustained winds of 40 mph, was located about 25 miles southwest of Monroe, Louisiana.
As Isaac faded, Tropical Storm Leslie formed in the Atlantic on Thursday and was located about 1,125 miles east of the Windward Islands, the U.S. National Hurricane Center said. The storm was packing winds of 40 miles per hour (65 km per hour) and was moving west, NHC said.
It is likely to become a hurricane on Saturday but is not expected to threaten the U.S. mainland.
(Additional reporting by Ben Gruber and Kathy Finn in New Orleans, Emily Le Coz in Tupelo, Missisippi, Chris Baltimore in Houston, David Adams and Kevin Gray in Miami; Writing by Tom Brown and Anna Driver; Editing by Jackie Frank and Eric Walsh)