The water quality of lakes can be degraded by excessive riverine nutrients. Riverine water quality generally varies depending on region and season because of the spatiotemporal variations in natural factors and anthropogenic activities. Monthly water quality measurements of eight water quality variables were analyzed for two years at 16 sites of the Tianmuhu watershed. The variables were examined using hierarchical cluster analysis (HCA) and factor analysis/principal component analysis (FA/PCA) to reveal the spatiotemporal variations in riverine nutrients and to identify their potential sources. HCA revealed three geographical groups and three periods. Two drainages comprising towns and large villages were the most polluted, six drainages comprising widely distributed tea plantations and orchards were moderately polluted, and eight drainages without the factors werethe least polluted. The river was most polluted in June when the first heavy rain (daily rainfall > 50 mm) occurs after fertilization and the number of rainy days is most (monthly number of rainy days > 20 days). Moderate pollution was observed from October to May, during which more than 60% of the total nitrogen fertilizer and all of the phosphorus fertilizer are applied to the cropland, the total manure is applied to tea plantations and orchards, and a monthly rainfall ranging from 0 mm to 164 mm occurs. The remaining months were characterized by frequent raining (i.e., number of rainy days per month ranged from 5 to 24) and little use of fertilizers, and were thus least polluted. FA/PCA identified that the greatest pollution sources were the runoff from tea plantations and orchards, domestic pollution and the surface runoff from towns and villages, and rural sewage, which had extremely high contributions of riverine nitrogen, phosphorus, and chemical oxygen demand, respectively. The tea plantations and orchards promoted by the agricultural comprehensive development (ACD) were not environmentally friendly. Riverine nitrogen is a major water pollution parameter in hilly watersheds affected by ACD, and this parameter would not be reduced unless its loss load through the runoff from tea plantations and orchards is effectively controlled.