The picture below shows a floodplain alongside a river channel. a floodplain is a flat landform created by deposition on the bottom of a wide river valley. over time, a river gradually shifts its position back and forth across a valley bottom. as it shifts, it drops sediment to gradually build up a flat floodplain. as the landform's name implies, another process plays a role in floodplain formation. which of the following describes this process? glaciers move through a river valley every winter and flatten out the landforms.  a.  glaciers move through a river valley every winter and flatten out the landforms.  b.  a river may flood its valley and form a permanent inland sea.  c.  acid rain weathers large rocks until they form a broad, flat plain.  d.  a river floods over its channel banks and deposits sediment on the wide valley bottom.