OF THE  MAJOR  RIVERS OF THE  WORLD,  THE  NILE  CASTS  A romantic spell from an ancient past, the Amazon conjures up images of adventure and danger, while the Yangtze entwines the soul with the mysteries of the Orient. Images of pharaohs lounging on royal barges rowed by a hundred men past the pyramids come to mind... the Spanish conquistadors struggling and dying in a green hell... Chinese junks and sampans crowding water turned yellow-brown with flowing silt. But it is the Mississippi that truly captures the imagination.

Thanks to the stories of Mark Twain of big side-paddle riverboats coming around the bend with whistles blowing as they passed Huck Finn and Tom Sawyer on a raft, and of battles up and down the river by Union and Confederate ironclads during the Civil War, the Mississippi's past seems so near that one has but to pierce a thin veil to experience it.

"The Father of Rivers," as the Indians called it, the Mississippi is the only river in North America that ranks in the top ten of the world. Third in length, third in drainage, fifth in volume, it stretches from the headwaters in Montana of its longest tributary, the Missouri, 3,484 miles south to the Gulf of Mexico.

Almost as fluid as mercury, always searching for the path of least resistance, the Mississippi has changed course many times throughout the last five thousand years, especially after the seas finally reached their present levels at the end of the last ice age. Between 1900 B.C. and 700 B.C. it flowed almost forty miles west of its present course. Restlessly, the river shifted back and forth across the state of Louisiana, carving a channel before migrating and carving another. Almost half of Louisiana was formed by the Mississippi depositing tremendous amounts of silt and clay carried from as far north as Minnesota and Montana.

"The water looks quiet today," said a man in an elevated seat who gazed from the pilothouse of the George B. Larson, an Army Corps of Engineers survey boat.

Standing at the control console, the boat's captain, Lucas Giraud, merely nodded as he piloted the craft past the cattle grazing on the levees of the Mississippi River in southern Louisiana.

This was Cajun country, the last outpost of French Acadian culture. Pickup trucks parked under spreading trees next to tar-papered cabins raised on pylons. Nearby, small Baptist churches rose from the damp countryside, their paint-peeled wooden sides overlooking cemeteries with weathered tombs rising above the ground. Soybeans and corn rose from the rich soil between man-made ponds for the farming of catfish. Little hardware and feed stores stood across narrow roads from auto garages surrounded by rusting wrecked cars half-buried in green underbrush that sprouted through their broken windows.