Isaac persisted along its treacherous path through the South Thursday, flooding wide areas, stranding residents and claiming its first fatality.

While the hurricane-turned-tropical storm Isaac weakened and is expected to dwindle into a tropical depression by Thursday evening, heavy rain is still making its mark on areas of the Gulf Coast.

Mandatory evacuations for all low-lying areas along the Tangipahoa River in Mississippi and Louisiana were announced as a dam at the 700-acre Lake Tangipahoa "has been badly damaged by heavy rains," the Pike County, Miss., emergency management officials said.

Officials caused an intentional levee breach by pumping water over the top to relieve pressure on the dam and digging a hole in it, the Mississippi Emergency Management Agency said.

While authorities did not expect the situation to turn catastrophic, MEMA spokesman Greg Flynn said the state had "a good number of prisoners there ready to begin sandbagging if it comes to it."

Mandatory evacuations were also called downriver in low-lying areas of Louisiana's Tangipahoa Parish. Louisiana Gov. Bobby Jindal said the move was a precaution because if the dam were to break it would only take 90 minutes for floodwaters to get to Kentwood, a town of about 2,200 residents.

As of Thursday morning, the National Weather Service reported that the Tangipahoa River was observed at more than 17 feet, more than two feet above flood stage, and predicted that it would go as high as 19.5 feet by Friday.

Jindal said a planned breach would send much of the water into a heavily forested area and not flood Louisiana towns. The evacuation could potentially involve 40,000 to 60,000 people, but those are "very rough estimates," he added.

In Mississippi, Gov. Phil Bryant told CNN that the storm has had a "major impact" on low-lying areas of the state.

"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."

In the storm's first known fatality, a tow truck driver attempting to clear debris on a road in Mississippi was struck by a falling tree, officials said.

Amanda Harris, deputy director of the Pearl River County Emergency Management office, told reporters the incident took place at midnight in Picayune, according to CNN.

National Guard troops continued their search in St. John the Baptist Parish, La., on Thursday, looking for stranded residents 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 forecast.

As of 1 p.m. CT (2 p.m. ET) on Thursday, Isaac's maximum sustained winds had dropped to 40 mph moving north-northwest at 9 mph, the National Hurricane Center 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.

"Isaac is expected to become a tropical depression this afternoon or tonight."