Authorities in two states along the US Gulf Coast urged residents to seek shelter Thursday amid fears a dam could fail, as a weakening Isaac doused an already drenched region, AFP reports.
Authorities in Mississippi called for precautionary evacuations of the area near Lake Tangipahoa after the Percy Quin Dam near the border with Louisiana showed signs of damage due to the storm.
But they insisted the dam had not been breached, showing instead "impaired portions," as crews began a controlled release of water into the river below to ease pressure on the dam.
"Crews from federal, state and local response agencies will be working through the night," Mississippi Governor Phil Bryant said on Twitter.
In Louisiana, Tangipahoa Parish President Gordon Burgess called for a mandatory evacuation for anyone living within a mile of the Tangipahoa River on either side.
Between 40,000 and 60,000 people could be affected by flooding if the dam breaks, Governor Bobby Jindal's office said.
He said officials aimed to allow about eight feet (2.5 meters) of water to come out before reinforcing the dam's structural integrity, and warned that without a breach of the dam, water levels could reach up to a near-record 17 feet downriver.
"Make no mistake; it would be a significant flooding event for Tangipahoa Parish," Jindal said.
The dam scare came as forecasters warned of life-threatening floods, tornadoes and storm surges due to the slow-moving and ferocious mix of wind and rain generated by Isaac, which made landfall as a hurricane on Tuesday but has since lost steam.
At 2100 GMT, the National Hurricane Center said Isaac had weakened further to a tropical depression with maximum sustained winds of 35 miles (55 kilometers) per hour.
"Even though Isaac is no longer a tropical storm, dangerous hazards from storm surge, inland flooding and tornadoes are still occurring," a statement from the Miami-based center said.
The area hardest hit by the storm was coastal Plaquemines Parish, where officials have ordered the evacuation of some 3,000 people.
The storm has revived memories of Hurricane Katrina, which devastated the region exactly seven years ago, flooding New Orleans and killing at least 1,800 people in the broader Gulf Coast area.
This time around, new multi-billion-dollar post-Katrina flood defenses appeared to be holding, though authorities still urged residents to stay indoors and three regional airports remained closed.
Rescue workers headed to Slidell on the north shore of Lake Pontchartrain after floodwaters began rising rapidly. About 100 people were rescued with high-water trucks and boats.
Officials said some areas were under about five feet of water after a canal swollen by two days of heavy rains overtopped raised railway tracks.
Dozens more remained trapped in their homes as roads became impassable. "We weren't expecting it," police chief Randy Smith said.
Latoya Sanders, 26, said she was ready to move back to California after riding out her first hurricane in Slidell.
"I'm scared," she said after police carried her five children aged one to seven into a truck.
Isaac could dump up to 25 inches (65 centimeters) of rain over parts of Louisiana and much of Mississippi and southwestern Alabama through Friday.
The storm may wind up causing as much as $2.5 billion in damage in and around Louisiana and in the offshore oil sector in the Gulf of Mexico, according to early estimates from natural disaster modeler Eqecat.
Louisiana authorities urged people in about a dozen areas to boil their tap water before cooking with it, drinking it or using it to brush their teeth. The Mississippi River flowed backward for nearly 24 hours due to pressure from Isaac, the US Geological Survey said.
The waterway similarly went into reverse gear during Katrina, cresting at 13 feet above its previous level.
More than 760,000 people were still in the dark in Louisiana, Mississippi, Arkansas and Texas, local power company Entergy said as the weather system snapped utility poles and downed transmission lines.
Across Louisiana, more than 4,000 people were crammed into shelters.
Dozens of nursing home residents, many in wheelchairs, were among those taken to higher ground by the National Guard in high-water trucks.
More than 20 people were killed when Isaac slammed into Haiti and the Dominican Republic last week.