There is a civilization indigenous to an area like the whitewater flooded forests of the southern half of the Amazon basin. This civilization is in the Bronze age.
The forests are in a six month wet-dry cycle, with water level in forest going up and down by about 10m. In the 'dry' season, the people plant maize, beans, and sweet potatoes on terra firma on the edge of the floodplain as soon as the water levels drop. In the 'wet' season, these people will make floating gardens in which they plant arrowroot, leren, and runner beans (as well as vegetables, like tomato and chilis).
The selected of plants will need about 1 foot of soil to grow, preferably 2 feet. The floating farm would be filled with soil each year as the water levels rise, and then emptied during the dry season. A re-usable structure would be best. These floating farms are kept in the flooded forest area outside of the main streams, 'moored' to various tall trees; Acai or Kapok are often planted for this purpose. Stream flow rates are negligible for most of the season, its more like a lake, so the structure does not have to be that strong.
My question is: how would a floating farm be built, and how large would these farms be?