Flash-flood watches were in effect from eastern Texas through Louisiana and up into the Mississippi River Valley -- including Arkansas, Tennessee, Missouri and southern Illinois.
Some bayous and creeks near Shreveport are expected to crest at levels not seen since 1991, according to CNN meteorologist Michael Guy.
In Mississippi, officials warned of flash flooding
. Gov. Phil Bryant declared a state of emergency.
"The water's still rising," Washington County Emergency Management Director David Buford told CNN.
State offices in 17 Louisiana parishes were closed Thursday, according to CNN affiliates KTAL and KMSS
. The northwest part of the state could see another 8 to 10 inches on top of the drenching it received Wednesday, when some isolated locations got more than 14 inches.
Residents also may be in for a soggy weekend. The current system, which has caused this wet-weather pattern, is expected to remain over the lower Mississippi River Valley through the weekend, according to Guy.
That means the region is at risk of further flooding until Monday.
At least four people have been killed in storms across the region, officials said. In Texas, a man died after his kayak capsized in Dickinson Bayou, near Galveston, police said.
Three people have died in Louisiana, Gov. John Bel Edwards said. In one case, a driver died when his vehicle was swept off the road in floodwaters in Bienville Parish, a spokesman for the state's Office of Homeland Security and Emergency Preparedness said.
Edwards declared a state of emergency for the state, and the National Weather Service issued a flash flood emergency for several parishes
Officials warned that floodwaters could rise above a levee and place thousands of homes in jeopardy.
Officials in Bossier Parish said they had closed at least 100 roads. On Wednesday, they issued a mandatory evacuation order for residents of 3,500 homes that could be at risk if floodwaters keep rising. They expanded that evacuation order Thursday
, warning that levees will likely overtop by Friday morning, putting even more homes in jeopardy.
"We've had folks who had to be rescued off rooftops, people rescued from cars, clinging to trees," sheriff's spokesman Bill Davis said.
The sheriff's office posted video showing water rescues in flooded neighborhoods.
Photos showed some roads covered with water, and others that were apparently torn apart in the storm.
Cathy Little of Shreveport said she's seen flooding there before, but never like this.
She posted videos of flooding in the area on Instagram. One showed a neighbor's home surrounded by water.
Parts of Arkansas and Texas have endured heavy rainfall.
More than 10 inches of rain has fallen in Searcy, Arkansas, since Monday. Longview, Texas, has seen more than 8 inches, according to the National Weather Service.
Forecasters warned people to stay off the roads in areas facing heavy rains.
"Most flood deaths occur in vehicles," the National Weather Service said.
Flooding is the leading weather-related cause of death over the last 30 years, according to CNN meteorologists Jennifer Gray and Monica Garrett