Mississippi is bracing for potentially historic flooding that could swell rivers and flood farmland. More than 20 counties declared states of emergency.

With dozens of rivers approaching record highs, the Mississippi was expected to crest on Thursday at its highest point in more than four decades.

Authorities are bracing for the water's impact beyond the waterways: The Mississippi will soon swamp farmland from Louisiana to Alabama.

"That water will make its way back down to the Gulf and eventually flow into the wetlands," David Studdert, a hydrologist at the National Weather Service in Jackson, said. "That could be devastating to the wetlands, which are now struggling to survive from climate change."

Officials are planning a massive evacuation of areas facing the waters. Nearly 200 volunteers converged in Hattiesburg on Friday morning to go door to door, helping residents clear homes and businesses of valuables and pets.

"We were stuck on U.S. 64 for two hours last night," said Brent Butler, 28, whose car stalled near the submerged U.S. 231 bridge near the interstate. "Every time we tried to turn around, we had to go stop on another bridge. It's crazy."

Officials are asking people to leave their homes, but have not opened more evacuation centers. People living along the Mississippi are receiving sandbags, and shelters are being opened to accommodate evacuees.

The Mississippi River, already sitting at near record highs, has been flowing at its highest rate in the history of the Southwest Pass during the past two weeks.

Winds are blowing sand over rail tracks behind the Mississippi, drying it out and setting a new record for the river.

Tom Leonard, a university professor in Louisiana who was at the historic Shreveport recreation area on Friday, said there is a "deep climate of uncertainty" about what will happen to the Mississippi River.

"But I think it's not likely that it's going to be anything like, say, the flooding that happened in the Mississippi in 1973," he said.

That flood was caused by a deep freeze that set records for low temperatures in the winter. It resulted in about 200 deaths, damaged large areas of shantytowns and wiped out large swaths of rural areas in central and southern Louisiana.

While today's weather conditions are unlikely to be as severe, scientists expect the Mississippi River will continue to rise for several weeks.

Even with Monday's historically low temperatures, forecasters were still expecting the river's flow to push more water into Louisiana, Mississippi and Tennessee, forecasters said.