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," said Tommy Thiebaud, the Washington Parish director of emergency services.
Isaac's maximum sustained winds had dropped to 35 mph as of Thursday afternoon and it was moving north-northwest at 12 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.
Water levels in most affected areas will gradually subside Friday, the hurricane center said, concluding its reports on Isaac.
Isaac moved into southern Arkansas Thursday afternoon, bringing with it the possibility of flash flooding and tornadoes.
"It's looking more disorganized but it is still putting out quite a bit of rain," said National Weather Service meteorologist Charles Dalton in Little Rock.
Rainfall around Little Rock could total 5 inches by Friday, said Dalton. Higher numbers were expected in the southeastern portion of Arkansas.
More than 827,000 customers -- down from 915,000 earlier in the day -- had no electricity across Arkansas, Alabama, Louisiana and Mississippi, utility companies reported.
Coast Electric Power reduced a high of 30,000 outages to 8,000 by Thursday evening.
"We had a great day primarily because we had better working conditions, and they are getting better by the moment," said spokesman Ron Barnes.
Water-boiling advisories were 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.
"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 3 to 5 feet of water. We saw a big military vehicle stop by to ask us if we are OK."
Tornadoes were suspected to be behind damage in Gulfport and Jackson, Mississippi, and a twister also 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 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, but 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.
National Guard troops and authorities searched St. John the Baptist Parish for people trapped by up to 6 feet of water.
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 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.