Updated at 6:52 p.m. ET
(CBS/AP) MCCOMB, Miss. - While Isaac has now been downgraded to a tropical depression, the effects of the storm continue to threaten thousands of residents in the Gulf South. Louisiana officials say a controlled release of water has begun at an Isaac-threatened, endangered dam at a southwest Mississippi state park. Mississippi Gov. Phil Bryant said in a news conference later Thursday that officials are clearing an area to do a controlled breach of the dam. At the same time, crews began intentionally breaching a levee stressed by the storm's flooding in Louisiana's hard-hit Plaquemines Parish south of New Orleans.The 2,300-foot-long earthen dam in Mississippi on the Tangipahoa River about 50 miles north of New Orleans threatened to break Thursday. Louisiana Gov. Bobby Jindal said at an afternoon news conference that the dam was damaged at Lake Tangipahoa in Mississippi's Percy Quinn State Park.
Tangipahoa Parish President Gordon Burgess issued an emergency alert warning of an "imminent failure" at the dam earlier Thursday. Burgess said between 50,000-60,000 people had 90 minutes to evacuate, CBS New Orleans affiliate WWL-TV reports.
The National Weather Service warned that a break in the dam would raise the level of the already swollen Tangipahoa River from around 11 feet to 17 feet in Kentwood, La., which is near the state line and best known outside the state as the hometown of pop singer Britney Spears. Jindal called for an evacuation of Kentwood after he and Burgess flew over Tangipahoa Parish in a helicopter, WWL-TV reports.
Mayor Whitney Rawlings of McComb, Miss., which is north of the park, told CBS News that there was a "50-50 chance" of the dam failing. He urged people south of the dam to evacuate.
"People need to be moving," Rawlings told CBS News.
With water still trapped between two floodwalls in Plaquemines Parish, a sparsely populated area south of New Orleans that is outside the federal levee system, officials began letting the standing water drain back into the Mississippi River and marshland.
Resident David Newman was frustrated the U.S. government spent billions reinforcing flood defenses for New Orleans after Katrina, and now he had the water.
"The water's got to go somewhere," he said. "It's going to find the weakest link, and with the wind directions, we was ground zero."
Meanwhile, Isaac dumped unrelenting rain and flooded areas around New Orleans and the Mississippi Gulf Coast on Thursday as people who thought they could ride out a lowest-category hurricane faced quickly rising waters. Hundreds of homes were swamped, about 500 people had to be rescued and half of Louisiana was without power. At least two people were killed.
Even as Isaac weakened on its slow trek inland, it continued to spin off life-threatening weather including storm surges, inland flooding from torrential rain and potential tornadoes. By mid-morning Thursday, Louisiana's Public Service Commission said 871,000 homes and businesses around the state are without power.
New Orleans itself was protected by newly fortified flood defenses put in place after Hurricane Katrina's devastation seven years ago.
"Hopefully, as far as the city of New Orleans is concerned, the worst is behind us," New Orleans Mayor Mitch Landrieu said.
But residents outside the city were scrambling as the slow-moving Isaac, now a tropical storm, kept dumping rain. Along the shores of Lake Ponchartrain just north of New Orleans, officials sent scores of vehicles to help evacuate about 3,000 people.
In the second reported death tied to Isaac, a tow truck driver was killed Thursday morning when a tree fell on his truck in Picayune, Miss., just across the state line from Louisiana. Authorities said Isaac was causing heavy rain and strong winds at the time. Pearl River County Coroner Derek Turnage identified the victim as 62-year-old Gregory Alan Parker of Picayune.
A canal near Slidell that helps Bayou Bonfouca drain into Lake Pontchartrain was being hit with a strong wind from the south, forcing water to pour into southern portions of the city, WWL-TV reports. Slidell Mayor Freddie Drennan told WWL-TV Thursday morning that water levels had reached 4 feet high in some neighborhoods.
"The real problem now is nobody can tell us how much water we're going to get," Drennan said.
Rescuers patrolled through Slidell's neighborhoods in heavy-duty trucks meant for driving through flooded areas, offering rides to people where water levels reached their waists in some areas and were above cars in others, WWL-TV reports.
"The husband and wife and their two dogs were in an area where a lot of houses washed away," said Lt. Cmdr. Jorge Porto. "They used a flashlight inside the house as a signaling device, which made all the difference in locating them effectively."
The floodwaters "were shockingly fast-rising, from what I understand from talking to people," Lt. Gov. Jay Dardenne said. "It caught everybody by surprise."
Isaac's maximum sustained winds had decreased to 40 mph and the National Hurricane Center said it was expected to become a tropical depression by Thursday night, meaning its top sustained winds would drop below 39 mph. The storm's center was on track to cross Arkansas on Friday and southern Missouri on Friday night, spreading rain as it goes.
The hurricane center said in an advisory at 4 p.m. ET that Isaac was about 35 miles west-northwest of Monroe, La., and was heading north-northwest at 12 mph.
CBS News hurricane consultant David Bernard reports from CBS Miami station WFOR-TV that Isaac's final rainfall totals for southern Louisiana could be 18-30 inches. Bernard reports that the storm's feeder bands still posed a flooding threat for the Louisiana capital of Baton Rouge, north of New Orleans.
The rain fell almost constantly for more than a day, flooding neighborhoods in a rural part of the state and in neighboring Mississippi. Officials had to respond quickly because the waters were rising fast -- even as Isaac meandered slowly northward Thursday on a path toward Arkansas.
In Plaquemines Parish, dozens of people in flooded coastal areas had to be rescued after the storm pushed water over an 18-mile levee.
Officials rushed to evacuate more than 100 nursing home residents from Plaquemines Parish, an area with a reputation for weathering storms and perhaps the hardest hit by Isaac. In this hardscrabble, mostly rural parish, even the sick and elderly are hardened storm veterans.
"I don't think we had to evacuate to begin with," said Romaine Dahl, 59, as he sat in a wheelchair. "The weather was a hell of a lot worse last night than it is now. And I got an idea that after all this is said and done they're going to say everything is over with, go on back home."
Other residents in the Riverbend Nursing and Rehabilitation Center were loaded into ambulances and taken to a nearby naval station. Residents had their names and birth dates attached to their shirts.
Josephine King, 84, handled the move well, waiting in a wheelchair. "I'm feeling good," she said.
Isaac arrived seven years after Hurricane Katrina and passed slightly to the west of New Orleans, where the city's fortified levee system easily handled the assault.
"Unfortunately, that's not been the case for low-lying areas outside the federal system, in particular lower Jefferson and Plaquemines parishes," said Louisiana Democratic U.S. Sen. Mary Landrieu. "Hurricane Isaac has reinforced for us once again just how vulnerable these critical areas are. We must re-engage the Corps of Engineers on this."
Jerry Larpenter, sheriff in nearby Terrebonne Parish, said he thinks the storm's impact took many by surprise.
"I think a lot of people were caught with their pants down," he said. "This storm was never predicted right since it entered the Gulf. It was supposed to go to Florida, Panama City, Biloxi, New Orleans. We hope it loses its punch once it comes in all the way."
New Orleans' biggest problems seemed to be downed power lines, scattered tree limbs and minor flooding. One person was reported killed, compared with 1,800 deaths from Katrina in Louisiana and Mississippi. And police reported few problems with looting. Mayor Landrieu ordered a dusk-to-dawn curfew.
President Barack Obama declared federal emergencies in both Louisiana and Mississippi, according to a statement from the White House, freeing up federal aid for affected areas.
Forecasters expected Isaac to move farther inland over the next several days, dumping rain on drought-stricken states across the nation's midsection before finally breaking up over the weekend.
Even at its strongest, the storm was far weaker than Hurricane Katrina, which at its peak was a Category 5 and hit New Orleans as a Category 3. Isaac came ashore late Tuesday as a Category 1 hurricane, with 80 mph winds near the mouth of the Mississippi River. It drove a wall of water nearly 11 feet high inland. Because Isaac's coiled bands of rain and wind were moving at only 5 mph -- about the pace of a brisk walk -- the threat of storm surges and flooding was expected to linger Thursday as the immense system crawled across Louisiana.
In coastal Mississippi, officials used small motorboats Wednesday to rescue at least two dozen people from a neighborhood Isaac flooded in Pearlington. In addition, the National Weather Service said there were reports of at least three possible tornadoes touching down in coastal counties. No injuries were reported.
None of the reports had been confirmed. Until the weather clears, there is no way for survey teams to assess the area to determine whether damage was done by tornadoes or straight-line winds, said NWS Meteorologist Shawn O'Neil.
Back in New Orleans, the storm canceled remembrance ceremonies for those killed by Katrina. Since that catastrophe, the city's levee system has been bolstered by $14 billion in federal repairs and improvements. The bigger, stronger levees were tested for the first time by Hurricane Gustav in 2008.
In Vermilion Parish, a 36-year-old man died after falling 18 feet from a tree while helping friends move a vehicle ahead of the storm. Deputies did not know why he climbed the tree.
The storm stalled for several hours before resuming a crawl inland, and forecasters said that was in keeping with its erratic history. The slow motion over land means Isaac could be a major soaker, dumping up to 20 inches of rain in some areas. New Orleans reported at about 10 inches in some places as rain continued to fall late Wednesday.
As hard wind and heavy rain pelted Melba Leggett-Barnes' home in the Lower 9th Ward, an area leveled during Katrina, she felt more secure than she did seven years ago.
"I have a hurricane house this time," said Barnes, who has been living in her newly rebuilt home since 2008. She and her husband, Baxter Barnes, were among the first to get a home through Brad Pitt's Make It Right program.
Her yellow house with a large porch and iron trellis was taking a beating but holding strong.
"I don't have power, but I'm all right," said Barnes, a cafeteria worker for the New Orleans school system.