Sediments in the central gulf are mainly clay and sandy clay with content of 10% (Srisuksawad et al., 1997;Shi et al., 2015). The grain size composition is similar to those near the Chao Phraya River mouth. While at the boundary of the upper
and central gulf, sediments are mainly sand. Similar clay minerals composition in the sediments of UGOT and adjacent rivers with lower content of smectite (