| Abstract | Chao Phraya river is the most important river in Thailand and especially upper Chao Phraya river has been facing increasing anthropogenic activities such as domestic waste discharge, agriculture run off, industrial wastes discharges and so on and river sediments and top soils play essential roles to determine the environmental quality and to identify the potential sources of heavy metals from anthropogenic sources. This study was established to investigate spatial distribution and risks of heavy metals in surface sediments and top soils in the upper Chao Phraya watershed due to their potential hazards to environment. ICP-MS was used to determine heavy metals concentration and interpret using Interpolation methods (IDW and kriging) with Arc GIS program. In addition, integrated risk assessments are used to determine the environmental risks associated with the heavy metals. Moreover, multivariate analysis methods were used to determine the potential sources of heavy metals. The results of surface sediment samples indicated that concentration in upstream of Ping (As, Cd, Cr, Cu, Hg, Pb, Ni, Zn), Nan (As, Cr, Cu, Ni) and Wang (As, Cr, Cu, Pb, Zn), in downstream of Yom (As, Cr, Cu, Zn, Ni) and Sakae Krang (Cr) exceeded TEC values of Thailand standard; Ni and Cr concentration in surface sediments from Nan watershed are found higher than PEC value. The spatial distribution of top soils showed that majority of high heavy metals concentration was found near Chiang Mai’s city, Mae Moh power plant and agriculture land areas of watersheds. According to risk assessment of surface sediment samples, Ping watershed showed the severe and major risk in the upstream of Ping and major risks were found upstream of Nan and agriculture land in downstream of Yom. The results of risk assessments in top soil samples showed that major risk is found at Ping forest and Wang watershed near Mae Moh power plant while considerate RI is found in forest land in Nan watershed and Sakae Krang watersheds. In surface sediments, three metals clusters were identified by the results of PCA and Cluster analysis. Cd, Zn, Cu, Cr, Ni were dominated with PC1, probably might be from agriculture sources such as fertilizers, pesticides, and mineral resources. Hg and Pb were associated with PC2, possibly might come from the urbanization activities such as municipal waste, atmospheric deposition from road dust, traffic activities and vehicle emission. As was associated with PC3, probably might come from coal burning, and/or natural sources since arsenic can be found in the earth crust. In addition, there can be three groups in top soils: Cd, Pb, Cu and Zn were dominated to PC1, probably might contribute from urbanization activities such as highways, road dust, traffic activities, and municipal waste. Ni and Cr were dominated to PC2, probably might contribute from agriculture source and natural sources. PC3 (Hg and As) might contribute from both natural sources and anthropogenic sources |