Eric Gay, Associated Press
A sherriff's vehicle sits in flood waters caused by Isaac, Thursday, Aug. 30, 2012, north of LaPlace, La, off Lake Pontchartrain. Isaac's maximum sustained winds had decreased to 45 mph and the National Hurricane Center said it was expected to become a tropical depression by Thursday night. The storm's center was on track to cross Arkansas on Friday and southern Missouri on Friday night, spreading rain as it goes.

ROBERT, La. — Louisiana officials ordered evacuation Thursday of low-lying areas along the Tangipahoa River because a lake swollen by rain from Tropical Storm Isaac was threatening to break a 2,300-foot-long earthen dam at a state park in southwest Mississippi.

Mississippi's Department of Environmental Quality asked the Army Corps of Engineers to bring in pumps and said it probably will dig out part of the bank of Lake Tangipahoa near the dam structure to allow a controlled release of water into the river. The dam will not be breached.

"The dam is holding," said Richard Ingram, director of the department's Office of Land and Water Resources. But he said some dirt slid off at two places and engineers want the lake's water level lowered 8 feet to relieve pressure on the dam.

Mississippi officials said the release should relieve the pressure without significantly affecting the river's water level.

The 700-acre lake is part of Percy Quin State Park in Pike County.

If the dam collapsed, the flood could be as bad as those of 1983 and 1990, said Ken Graham of the National Weather Service. Louisiana Gov. Bobby Jindal said those are the worst floods recorded in Tangipahoa Parish.

Tangipahoa Parish President Gordon Burgess asked residents in mostly rural areas along the river, from the Kentwood community south to Robert, to leave because of uncertainties over the dam situation. The evacuation order did not include the parish's major city, Hammond, which has about 20,000 people and is home to Southeastern Louisiana University.

Jindal said residents should take the evacuation order seriously.

"If this dam were to break, it would take about 90 minutes for the water to get to Kentwood," not far from the state line, Jindal said.

Ken McCollum said he grabbed a few personal items and fled his home in Robert after he heard a false report that the dam holding back the Tangipahoa River had been breached. He couldn't believe his situation could get any worse after Isaac pounded his town with rain and high winds for two days.

McCollum said he didn't know where he would go in case the dam did fail, but he parked himself on the edge of the highway, ready to bolt in case it did.

"We're going to get a ton of water if it does. I went through Katrina, so I don't think anything would be worse than that, but all that water coming in at once. A dam break could be pretty serious," he said. "I don't want to be trapped."

With a controlled release, Mississippi officials "don't estimate there would be a water impact in any Louisiana parish. But obviously that's based on early modeling," Jindal said.

In Mississippi, Pike County firefighters pumped water from the lake over the dam for a time Thursday but withdrew because of water on the road at the site.

Louisiana officials were concerned that more water entering the Tangipahoa might spell flood problems downriver. The Tangipahoa rises in Mississippi as a narrow river and widens as it flows about 120 miles south to empty in Lake Pontchartrain.

Because the population of the area is sparse in many places, officials did not have an immediate estimate of how many homes or people were affected by the evacuation order. Mississippi officials did not order an evacuation and said they don't believe the volume of water in the lake would threaten communities downstream.

Louisiana State Police Superintendent Mike Edmonson said a Louisiana National Guard flight over the dam shortly before 11 a.m. CDT showed the structure was still holding. Mississippi officials confirmed the dam was secure.

Louisiana state troopers and National Guard troops were heading to the state line to be ready if the situation changes.

"It's speculation, but we have to at least prepare for it," Edmonson said.

Mississippi Emergency Management Agency spokesman Greg Flynn said prisoners sandbagged an area where part of the dam's base had slid away. He said there are no breaks, no water is coming through and he's been told officials at the scene don't think there's any immediate danger of it breaking.

The park is in an agricultural area. Flynn said any water pouring through would affect no more than 12 homes in Pike County.

Park manager Will Busby said in June that Lake Tangipahoa would be drawn down 6 to 10 feet for repairs on the dam and spillway valve — a project that would take all winter.