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Chao Phraya River serves as the raw water source and also serves as the main drainage channel for the wastewater from all activities simultaneously. However, in each province where the river flows, there are differences in the number of population, land used and economic activities. It causes the difference in rate and the BOD load of wastewater discharges. In this study, the contaminant sources along the Chao Phraya River have been investigated during the dry season with used of UAV. The potential sources have then been confirmed by the water quality analysis results of water samples taken along the 270-km of the river from Chainat to the Gulf of Thailand. It has been found that the wastewater discharges from households strongly deteriorates the river water quality. Thus, the river water quality along Pathumthani, Nontaburi, Bangkok and Samutprakarn Provinces are poor and worsen than the water quality standard. Moreover, the wastewater from pig farms and industrial activities also shows contamination potential. However, the wastewater from aqua culture is not such dirty; therefore it dilutes the river water, instead.