(5:14 p.m. EDT) -- Near-record breaking Mississippi River water levels have prompted two cruise lines to alter itineraries. The National Weather Service has described the flood, caused by heavy rains across the central United States, as the longest-lasting since the "Great Flood" of 1927.

American Queen Steamboat Company has adjusted the docking locations for two of its ships, American Queen and American Duchess, both scheduled to depart on June 2 from Memphis and New Orleans, respectively. Dates also have been rearranged.

A statement issued today by the line read:

"Due to historic high water on the Mississippi River and inaccessible ports, we must alter our schedule for your upcoming voyage.

"Rest assured that you will still have the opportunity to visit each port previously offered, either by way of our alternate dock locations, special included excursion or on another day during the cruise."

American Queen's modified itinerary includes New Roads, La. (in lieu of St. Francisville, La., previously Natchez, Miss.) on June 6; Natchez (formerly Baton Rouge, La.) on June 7; and Baton Rouge from June 8 to 9 instead of St. Francisville and Nottoway Plantation.

American Dutchess will spend June 4 leisurely sailing along the river, in place of calling at Nottoway Plantation; it also will substitute St. Francisville with New Roads, on June 5.

The line added that any affected, pre-booked premium shore excursions will be automatically rebooked for their newly scheduled dates.

Meanwhile, a spokesperson for American Cruise Lines -- another river cruise operator with ships on the Mississippi -- told Cruise Critic it has needed to make only minor itinerary changes.

"We have been able to go to nearly all our regular ports along our many Mississippi River itineraries," said the line's spokesperson, Alexa Paolella. "We have made a few minor modifications on certain cruise dates as needed, but the shore excursions and activities along the Mississippi have remained unchanged."

Water levels are expected to continue to rise in early June, according to multiple weather forecasts. Mandatory evacuations have been issued for parts of the region.

Cruise Critic will continue to update this story as more information becomes available.