Assuming the river floods most every December - I'd suggest an annual planting each year in the lowland plots. A clover like Crimson clover - is a great fall planting (Late August - Early Sept in AL) and it is economical to plant if you have to do it 2x per year. Maybe mix in some cowpeas too.

When the flood waters receed for the last time - over seed the are with an anual clover again - It might last through a good part of the summer.

You might try mixing in a white clover - like Ladino - to see how well it had handles the flooding - but basically - most clovers (and crops in general) cannot take more than 3-4 days underwater.

Good luck

FH