Although most river bedforms are composed of silt and sand, some rivers with gravel or cobble beds can also create dune-shaped bedforms.
(most river bedforms; are composed; of silt and sand) (some rivers with gravel or cobble beds; can also create; dune-shaped bedforms)
most river bedforms some rivers with gravel or cobble beds
are composed can also create
of silt and sand
dune-shaped bedforms