We have been conducting water quality research of rivers in Kofu including Nigorigawa basin 6 times a year for more than 20 years. In the last three years we carried out more detailed water quality research in Nigorigawa basin. Using research data of Nigorigawa, we show the present status and long-term tendency of water quality in Nigorigawa basin. In Nigorigawa basin environmental water quality standard category is set to rank D at Nigorigawa-bashi, practically the lowest place of the river. The research reveals the magnitude of influence of the branches on water quality of the lowest place of the river. Additionally, we point out that there would be some problems to face if environmental water quality standard category for Nigorigawa-bashi would be set one rank up.