SLIDELL, Louisiana (CNN) -- Nearly a week after Hurricane Isaac slammed into the Gulf Coast, the effects of the storm were still being felt Monday: From the thousands forced into shelters by flooding to the tens of thousands still living without power in sweltering conditions.

Evacuation orders, most voluntary, remained in place in a number of parishes as authorities grappled with new threats posed by rain swollen rivers and lakes.

In St. Tammany Parish, north of Lake Ponchartrain, authorities were grappling with two potential threats -- one from a weakened lock on a canal and the other from the rain swollen Pearl River.

Parish officials warned people to stay away from the area, even as a mandatory evacuation was lifted after authorities opened the lock to relieve pressure.

"As there is still a potential threat, even though reduced, a voluntary evacuation remains in place until the Army Corps of Engineers deems the lock stable and safe," Pat Brister, the president and sheriff of St. Tammany Parish, said Sunday. "Please stay vigilant."

Forecasters, meanwhile, predict the Pearl River will crest Monday at 19.5 feet, more than five feet about flood stage, posing a potential threat to up to several thousand homes in St. Tammany.

President Barack Obama was set to visit the state on Monday to get a first-hand look at recovery efforts, which will include a tour of St. John the Baptist Parish where thousands were forced from their home after Isaac's storm surge pushed water over the banks of Lake Ponchatrain.

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 Mississippi.

A flood warning was issued for Mississippi's rain swollen Wolf River, north of Gulfport, where it was expected to crest Tuesday more than eight feet about flood stage, the weather service said.

More than 3, 500 people were in shelters across the state on Sunday, according to Gov. Bobby Jindal's office. In Mississippi, roughly 100 people remained in shelters, state officials said.

In St. James Parish, between New Orleans and Baton Rouge, a dusk to dawn curfew was imposed after the Blind River crested at flood stage, flooding nearly two dozen homes. National Guard troops were deployed to the area to help with security and possible evacuations, Jindal's office said.

Most of the areas hit hard by Isaac were outside the new federal levee system that was reconstructed at a price of $14.5 billion following Hurricane Katrina.

Crews in Lafitte, on the outskirts of New Orleans, were considering intentionally breaching two spots in a levee along Bayou Barataria on Monday to help drain up to five feet of flood waters brought by the storm surge, officials said.

State officials have promised that money garnered from fines paid by BP over the Gulf oil spill will be used to reinforce the area levees, Lafitte Mayor Tim Kerner said.

But so far, he says, that hasn't happened.

"Yeah, it's frustrating," Kerner told CNN affiliate WWL-TV. "It makes you feel like you're not doing your doggone job. But I can't help it if the corps actually looks me in the face and promises that we're going to get things and we don't."

As many in Louisiana entered their sixth day without power, frustration with the pace of restoration efforts also grew.

At the height of the storm, more than 850,000 customers were reportedly without power in Louisiana, Mississippi, Alabama, Texas and Arkansas. By Monday, there were roughly 129,000 customers without power in Louisiana, according to Entergy Louisiana.

"Some areas are delayed due to high water conditions," the power company said on its website.

But for Tyrone Wilson, who relies on an electric scooter for transportation, the return of power means the return of his mobility.

"I got to go put I up because I got no power," Wilson told WWL. "I have no way to get around. I have to medicine and go to the doctor. I have no way to get there."

CNN's Greg Botelho, Chelsea J. Carter, Matt Smith and David Ariosto contributed to this report.