The "Mighty Muddy" at Memphis is indeed both mighty and muddy. The river is extremely dangerous here, high speed currents with hidden sandbars and eddies, underwater man-made jetties, huge objects like trees, barges, houses, and lost containers floating down river. I wouldn't venture onto "old man river" with less than two big engines, radar, and an experienced river pilot.
Farther north, like St. Louis and above, the Mississippi river can be used for recreation, by Memphis, it's a raging beast. If you visit Memphis, don't be tempted to wade in the shallows along the river park downtown. Every year someone (sometimes a whole family) is lost this way.
