The distribution and characteristics of sediments in the UGOT are first associated to its source. The UGOT is surrounded by land in three directions. Thus, its sediment distribution is strongly influenced by terrestrial material input from the Mae Klong, Tha Chin,Chao Phraya, and Bang Pakong Rivers. The four rivers have an annual water discharge of about 12.7 109 m3 (Table 2). The Mae Klong and Chao Phraya Rivers annually deliver 30 and 8.1 million tons of sediment to the sea, respectively (Winterwerp et al., 2005; Milliman and Farnsworth, 2011). However, data on sediment discharge in the Tha Chin and Bang Pakong Rivers are lacking. Sediment load is inferred to be relatively small compared with those of the Mae Klong and Chao Phraya Rivers because the annual discharge of these two rivers are 3.7 and 9.02 m3 are less than 10% of those of Chao Phray and Mae Klong(Table 2). Emery and Niino (1963) considered that origin of sediment are mainly modern detrital in the UGOT, and the sediment at the bottom is relict in nature. As shown in Fig. 3B, the silt sediment with Mz < 40 lm is near the north coast (water depth less than 15 m) of the UGOT. This result may imply that fine-grained
sediment with Mz < 30 lm from the Chao Phraya River and Bang Pakong River is mostly deposited near the north coast, whereas coarse-grained sediment with Mz > 55 lm from the Mae Klong River and Tha Chin River is accumulated near the west coast.