(CNN) -- Louisiana officials took steps Saturday to ease pressure on a lock near the Pearl River and keep it from failing, one of several crises brewing days after Isaac barreled through the area as a hurricane.
Earlier in the day, the state's office of Homeland Security and Emergency Preparedness posted on Twitter that "Pearl River Lock No. 2 is in danger of failing" and urged "anyone living in the mandatory evacuation area to leave immediately." St. Tammany Parish warned on its website at 1:15 p.m. that "failure of Lock 2 is imminent!"
The lock is on a man-made canal off the Pearl River.
The evacuation order applies to those living in a stretch from Bush to Hickory, an area Lindsey deBlieux from the Louisiana Recovery Authority described as fairly rural. Buses were sent to the area to pick up residents, and evacuees were urged to call 211 to get in contact with a local Red Cross shelter.
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 told CNN on Saturday evening.
"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."
St. Tammany Parish also warned residents of another potential mass flooding problem -- the Pearl River itself.
That river is expected to crest early Monday at 18.5 feet and could rise even higher, the parish said on its website. It then listed 35 subdivisions, four mobile home parks and one campsite in an area that extends south to Lake Pontchartrain that may be "potentially affected" by rising waters.
Meanwhile, what's left of Isaac continued to chug northward Saturday, scattering rain on parts of the Midwest where farmers and ranchers have been facing a drought so severe that it prompted emergency disaster assistance last month.
The storm system moved into western Missouri and Illinois, bringing much need rain to isolated areas, forecasters said.
"Come take a visit," said farmer Brad Detring of Farmington, Missouri, referring to the remnants of Issac.
Earlier Saturday, the U.S. Coast Guard said it had fully reopened the lower Mississippi River -- an area that stretches from Baton Rouge to the Gulf of Mexico.
The channel was opened at 10 p.m. Friday, after it had been partly opened for large vessels the evening before, the Coast Guard reported.
"The Mississippi River system is a vital part of the U.S. economy, so we are very pleased we can open the river to those who depend on it," said Capt. Peter Gautier.
Coast Guard teams are continuing to monitor the area and respond to a "number of ship groundings and barge strandings along the riverbank caused by the river surge and high winds of the storm."
Isaac contributed to at least 19 deaths in Haiti, then caused at least four more total in Louisiana and Mississippi after hitting the United States earlier this week.
While most people have their lights back on, hundreds of thousands were still without power Saturday night, including about 320,000 Entergy Louisiana customers.
Isaac also brought flooding because of storm surges and heavy rains, including as much as 20 inches in New Orleans.
Mississippi's Homochitto National Forest reported closures in several areas as a result of flooding.
"Safety remains and is always a high priority for us," said Bruce Prud'homme, Homochitto district ranger. "Closing these areas is necessary for public safety."
South of New Orleans, Plaquemines Parish was among the hardest-hit areas as fast-rising waters swamped scores of homes and businesses.
Officials were intentionally breaching levees in strategic areas, in hopes of getting "the bulk of this water out in five to seven days," Plaquemines Parish President Billy Nungesser told CNN.
He acknowledged, though, that drying the east and west banks of the Mississippi River in the parish could take 17 days.
Gina Meyer, the parish's emergency medical services superintendent, said her home suffered wind damage during Hurricane Katrina seven years ago, and flooded this time as waters rose 14 feet higher than normal.
Beyond efforts to lower water levels, Meyer said that residents could rely on each other to weather this ordeal.
"Trust in God, rely on your family, and it's not just the people that are blood related to you, it's the people you have grown up with all your life and that's what gets you through something like this," she said.
CNN's George Howell, Susan Candiotti, Stephanie Gallman contributed to this report.