(CNN) -- Isaac slashed its way further inland Thursday, claiming its first fatality and stranding residents in flood-prone areas even as it threatened to wreak more havoc in the days to come.

Mississippi and Louisiana announced mandatory evacuations for all low-lying areas along the Tangiaphoa River.

A dam at 700-acre Lake Tangipahoa has not breached, "but has been badly damaged by heavy rains," the Pike County Emergency Management Agency said.



Louisiana Gov. Bobby Jindal announced that Mississippi officials planned an intentional breach at the dam to help prevent it from breaking.

The river was observed at more than 17 feet Thurday morning -- more than two feet above flood stage, the National Weather Service said, predicting that the river will go as high as 19.5 feet by Friday.

Near Kentwood, Louisiana, the river was at 12 feet Thursday morning and could go as high as 17 feet -- four feet above flood level in the area, the weather service said.

Officials predict that a planned breach would send much of the water into a heavily forested area and not flood Louisiana towns, Jindal said.

However, Tangipahoa Parish President Gordon Burgess called for a mandatory evacuation in the area. Jindal said the move was a precaution because if the dam were to break it would only take 90 minutes for flood waters to get to Kentwood.

The parish said the evacuation could involve 40,000 to 60,000 people, but those are "very rough estimates," Jindal said.

Kentwood's population is just over 2,000.

On the Mississippi side, 12 homes were evacuated, emergency management officials said.

Although Isaac weakened Thursday, the hurricane-turned-tropical-storm was still dumping heavy rains, creating dangers along the northern Gulf Coast.

The storm has had a "major impact" on Mississippi, Gov. Phil Bryant told CNN in Gulfport. "This is a man-made beach," he said, indicating the area where he was standing. "Most of that sand is gone. Thousands of homes have been damaged; people have been out of their homes and will be."

A tow truck driver attempting to clear debris on a road in Mississippi was struck by a falling tree, officials said. The incident took place at midnight, said Amanda Harris, deputy director of the Pearl River County Emergency Management office. The National Weather Service said it received reports of the fatality in Picayune, Mississippi.

On Louisiana's border with Mississippi, residents of Washington Parish were alerted that the Bogue Chitto River was expected to rise by 14 feet overnight.

To the southwest, in St. John the Baptist Parish, National Guard troops looked for people stranded after thousands were forced to flee when a surge forced water over the banks of Lake Pontchartrain.

"What we're doing is we have got law enforcement and fire personnel who are going door to door to notify people," Tommy Thiebaud, the Washington Parish director of emergency services, told CNN.

Some areas could see 25 inches of rain, the National Weather Service said.

Isaac's maximum sustained winds had dropped to 40 mph as of 10 a.m. CT (11 a.m. ET) and it was moving north-northwest at 9 mph, the service said.

"On the forecast track, the center of Isaac will continue to move over Louisiana today, over Arkansas on Friday, and over southern Missouri Friday night," forecasters said.

In Plaquemines Parish, southeast of New Orleans, 3,000 people remained in an area close to an 8-foot-tall levee that was threatened by rising waters, Jindal's office said.

About 100 people who had ignored an evacuation order were rescued Wednesday in a flooded part of the parish, officials said. On Thursday, authorities from the sheriff's office and National Guard "will begin looking house to house to make sure they got everyone," Parish President Billy Nungesser said.

"We didn't have time to panic," Rafael Delgadillo of Braithwaite, Louisiana, told CNN on Thursday. He, his wife and daughter were rescued by a neighbor Wednesday. "We were in survivor mode," he said.

Nearly a million customers had no electricity across Arkansas, Alabama, Louisiana and Mississippi. Utility companies reported more than 978,000 customers without power.

Water boiling advisories were being issued in a number of towns and cities along the Gulf Coast.

A flash flood emergency was issued for Slidell, Louisiana. Surge flooding from Bayou Bonfouca and the W-14 canal gushed into parts of the city, and sudden inundation of up to 5 feet was possible in low-lying areas, the National Weather Service said.

Evacuations were under way in the city Thursday morning.

Authorities expected some street flooding in a few subdivisions on the south side of town, "but at present, that street flooding is minimal," the city said in a statement Thursday morning.

"There is water all around me," Vincent Molino, who lives in the area, told CNN in an iReport. "It looks like my home is in a lake. The area is completely flooded ... pretty much the whole neighborhood has three to five feet of water. We saw a big military vehicle stop by to ask us if we are OK.

"It is strange that the water is going down really slowly.

"The most amazing thing is that I still have power," Molino said.

Mississippi had 70 rescues along the Gulf Coast overnight, Gov. Bryant said.

Throughout parts of the Gulf Coast, the National Weather Service warned, "life-threatening hazards from storm surge and inland flooding are still occurring."

Isaac is believed to have spawned three tornadoes overnight in Mississippi and Alabama, the weather service said.

Tornadoes are suspected to be behind damage in Gulfport and Jackson, Mississippi, while a twister is blamed for knocking down power lines and damaging a home in Geneva, Alabama.

President Barack Obama signed major disaster declarations for Louisiana and Mississippi after slow-moving Isaac pummeled the Gulf Coast, dropping more than 20 inches of rain in some locations and creating a dangerous storm surge.

Isaac made landfall as a Category 1 hurricane seven years after Hurricane Katrina swept ashore in Louisiana and Mississippi. Katrina is blamed for the deaths of 1,800 people, most in New Orleans after the levee system failed and the city flooded.

Isaac's greatest punch bypassed New Orleans.

The city reported relatively minor damage from the storm, and officials said there were at least a dozen incidents of looting. New Orleans Police said arrests were made in each case, but didn't specify how many people were involved or where the arrests occurred.

"Our crews have already hit the streets this morning to assess damage and start clean-up," New Orleans Mayor Mitch Landrieu tweeted.

Still, New Orleans was expected to face some pounding rain throughout Thursday.

"This storm took forever to get here, and now it doesn't want to leave," said Lt. Col. Jerry Sneed, deputy mayor in charge of public safety.

National Guard troops and authorities searched St. John the Baptist Parish for people trapped by up to 6 feet of water.

By late Wednesday, about 1,500 people had been evacuated from the parish, and another 1,500 were expected to leave their homes, Jindal's office said.

Dozens of buses moved residents out of flooded portions of the parish, while authorities worked to rescue others.

"We're continuing to rescue people from different areas throughout the parish," Paige Falgoust, communications director for St. John the Baptist Parish, said early Thursday. "Our main focus right now is getting people out of their homes."

The storm surge from Lake Pontchartrain came quickly and "in a different way from what we were expecting," Falgoust said.

The surge was unusually bad in LaPlace, about 25 miles northwest of New Orleans, where many people had been rescued or still needed to escape rapidly rising water.

By Thursday morning, at least 200 rescues had taken place, parish officials said.

Dusk-to-dawn curfews were in effect from New Orleans to Baton Rouge, where authorities were urging residents to stay inside and ride out the slow-moving storm.

In Plaquemines Parish, an initial estimate showed that as many as 800 homes may have received significant water damage, Jindal said.

The U.S. Army Corps of Engineers reported a significant storm surge in the parish, which was the scene of many rescues. One involved National Guard troops who moved 112 residents from the Riverbend nursing home to another facility.

Officials were considering intentionally breaching the levee downstream to allow some of the floodwater to flow back out of the inundated area, Jindal said.

That could happen Saturday, said Nungesser, the parish president. The Times-Picayune reported that digging could begin as early as Thursday.

Airports from Baton Rogue to Biloxi were closed until at least Thursday as the stubborn storm continued to saturate the region.

CNN's Leslie Tripp, Brian Todd, Soledad O'Brien, Ed Lavandera, Martin Savidge, John Zarrella, Chandler Friedman, Anika Chin, Mike Ahlers, Aaron Cooper and Ed Payne contributed to this report.