Why is there no loess on a floodplain?

Answer 1

Loess is wind-deposited soil that occurs in dune-like formations.
Drift is often carried by wind off of floodplains and deposited in adjacent hills.
However, if the drift is deposited on the floodplain, then by definition it will be eroded by flooding or blown further on within time, in order for the landform to continue to meet the definition. Once the loess "sticks" and persistent dunes form, it is no longer a floodplain, but a loess hill.

There is one semantic distinction to consider, than being streambeds that flow through loess hills. The Little Sioux River flows through the Loess Hills of western Iowa, and in that section of river, the entire landform is referred to as loess. The floodplain through the Loess Hills is still loess soil, however it is not primarily aeolian (wind-deposited), but is secondarily deposited after erosion by water.

http://pubs.usgs.gov/info/loess/


.

Answer 2

It would simply be washed away -- it is very light.

http://www.museum.state.il.us/exhibits/larson/loess.html