- Mandatory evacuations are in place in St. Tammany Parish, north of New Orleans
- A canal lock is in danger of failing, officials say
- The Pearl River, swollen by rain, is also threatening to flood area homes, officials say
- Flash flood warnings are issued for portions of Tennessee and Kentucky, forecasters say
(CNN) -- As the remnants of Isaac threatened flash flooding in portions of Tennessee and Kentucky on Sunday, residents in Louisiana and Mississippi grappled with the aftermath of the storm that again brought mandatory evacuations amid flood warnings.
Mandatory evacuations were issued Saturday night for portions of Louisiana's St. Tammany Parish after the Pearl River swollen by days of rain threatened to overwhelm a lock in one area and overflow its banks in another, potentially flooding hundreds of homes.
The most pressing issue was the potential failure of a lock on a man-made canal that juts off the Pearl River.
After getting permission from the U.S. Army Corps of Engineers, authorities on Saturday "opened some valves" to prevent the lock from failing, St. Tammany Parish spokeswoman Suzanne Stymiest said.
"We are beginning to relieve some of the pressure, and we are hopeful," Stymiest said. "However, the mandatory evacuation is still in place. We cannot determine (yet) that we have been successful."
People living in the parish, which sits on the north shore of Lake Ponchartrain between the towns of Bush and Hickory, will not be allowed to return until the structural integrity of the lock is assured, parish officials said.
Meanwhile, the parish was also contending with another potential flooding problem. The Pearl River, which flows from Mississippi into Louisiana, crested Monday at 19.5 feet, according to a special notice posted Saturday night on the St. Tammany Parish website.
"We have a plan in place and will provide you with any and all information as this situation progresses," the notice said.
At least 35 subdivisions, four mobile home parks and one campsite were alerted to possible flooding, though authorities did not immediately call for evacuations.
The remnants of Isaac, meanwhile, moved to the northeast early Sunday, after scattering much needed rain on parts of the drought-ridden lower Midwest. Missouri and Illinois both saw up to six inches of rain in some areas on Saturday.
The National Weather Service issued a flash flood warning early Sunday for portions of Kentucky and Tennessee as well as the Central Appalachian Mountain region.
Isaac is blamed for at least 19 deaths in Haiti and at least four more in Louisiana and Mississippi after making landfall last week as a Category 1 hurricane -- on the seventh anniversary of Hurricane Katrina -- near the mouth of the Mississippi River.
The storm posed the first real test to New Orleans following a $14.5 billion federal effort to reconstruct the city's flood control system after it failed during Katrina in 2005. Katrina killed nearly 1,800 people, most when the storm overwhelmed the levee system and flooded the city.
Though much weaker than Katrina when it came ashore, Isaac moved slowly and dumped enormous amounts of rain on Louisiana and Missouri.
Louisiana's Plaquemines Paris was overwhelmed by Isaac, which caused a parish-maintained back levee to fail and contributed to the flooding of businesses and homes.
Officials were intentionally breaching levees in strategic areas in the parish, in hopes of getting "the bulk of this water out in five to seven days," Plaquemines Parish President Billy Nungesser said Saturday.
Hundreds of thousands in the region were without power early Sunday, including at least 320,000 Entergy Louisiana customers, officials said
Residents began returning home over the weekend in Helena, Mississippi, where National Guard troops rescued hundreds trapped by rising waters brought on by Isaac.
Annie Judge returned home Saturday to find more than four feet of water inside her home.
"Clothes, my laptop, a bunch of baby stuff, pictures," all ruined, she told CNN affiliate WLOX. "All the furniture is ruined."
CNN's David Ariosto contributed to this report.