Buffalo River. 130 miles of up to class 2 + below Ponca. Class 3 and 4 above Ponca.
There is plenty of info if you Google Buffalo River.
It can be cold in March. At the end of the month is a little better. Early on there is likely to be snow and freezing temperatures.
Until you get to Florida things are not really warm in March though Louisiana is close. Temps even in north Florida in March can get down to just above freezing.
Buffalo National RIver