Information on water quality and its temporal variations is necessary for sustainable water management. The focus of this research is identification of the long-term trends of water quality for the Chehelchay River, Golestan province. The data sets of twelve water quality variables recorded at the Lazoore river gauge station (located in the outlet of the Chehelchay watershed) during the period of 1976-2008 were analyzed. Non-parametric statistical methods (Mann-Kendall and seasonal Kendall tests) were employed for trend detection. The results indicated that, seven variables (sodium, chloride, SAR, sulfate, potassium, EC and TDS) demonstrate statistically significant increasing trend, four variables show no significant trend, and only one variable (bicarbonate) has a decreasing trend. Increasing trend of variables and thus river water quality deterioration could be attributed to factors such as land-use change, soil erosion and human wastes. Our study on land use changes, occurred in the Chehelchay watershed during the period of the study, showed that 2537 hectares of forested areas has been converted to croplands. Agreement between land-use change and water quality trends supports the hypothesis that land-use change is one of the most important factors causing deteriorating trends in the river water quality. This research reveals an urgent need for implementation of necessary management actions in the Chehelchay watershed to control pollution sources and protect river water quality.