The purpose of research is to assess the seasonal changes of water quality in mainstream and in specific tributaries in the Ca River Basin. Water samples were collected in the dry season (May 2021) and in the beginning of the rainy season (September 2021) from 4 sites located on Ca River’s mainstream and 5 sites located on Ca River’s tributaries to analyze water quality parameters. Results indicated that physical parameters (pH, EC, salinity, DO) and and heavy metals (As, Cu) are within the standard values according to the National Technical Regulation on Surface Water Quality QCVN 08-MT:2015/BTNMT for domestic water supply purposes (Columne A1). The concentration of nutrients measured in the tributaries is higher than those in the mainstream due to the contribution of nutrients and organic materials from agricultural runoff. In addition, the water flow in the tributaries is lower than in the mainstream, as a result, the concentration of nutrients tends to be higher in the tributaries. The samples revealed that the concentration of nutrients (NO3-, TN, PO43-, TP) and chemical oxygen demand (COD) appeared higher in the beginning of the rainy season than in the dry season. The increase of nutrient concentrations can be explained by the leaching of nutrients into the river. In general, the water quality of Ca River’s tributaries during the rainy season have a high nutrient concentration and also the risk of eutrophication.