The present study is an attempt to show how changes in land use and land cover would change the quantity of surface water resources in a river basin in northwestern Iran. In order to detect the changing trend of surface water quantity in the river basin, the long-term statistic data of sediment load and river discharge were gathered over the period between 1987 and 2013. For land use change detection of the river basin, the land use land cover maps of the study area in the years of 1987, 1998, 2002, 2009, and 2013 were prepared from Landsat satellite images using supervised classification method. The changing trend of river discharge showed a significant and positive relationship with rain-fed agriculture (R2 = 0.8152), poor rangeland (R2 = 0.7978), and urban areas (R2 = 0.8377). There was also a strong negative correlation between water discharge and irrigated agriculture (R2 = 0.7286) and good rangeland (R2 = 0.8548). In conclusion, increasing the area of rain-fed agriculture, good rangeland (type IV), and urban land uses, due to their effects on increasing the runoff, have caused an increase in the water flow of Zanjanroud River.