Understanding the spatial and temporal variation of nutrient concentrations, loads and their distribution from upstream tributaries is important for management of large lakes and reservoirs. The Min River is an important tributary of the upper reaches of the Three Gorges Reservoir (TGR) that contributes about 20 % of the inflow water from the Yangtze River to the TGR. It is also one of the main phosphorous sources of the TGR. In this study, total phosphorous (TP) concentrations and hydrological data from 2004 through 2014 for 13 sites in Min River and its tributaries were analyzed. The result suggested that the concentration of TP was comparatively lower in the upstream of the Min River and Dadu River, but tended to increase in recent years. Additionally, the TP pollution status in the Min River basin has gradually evolved from local pollution to whole-basin pollution. The concentration of TP in the Min River main stream increased, decreased and then increased again along the waterway. Pollution from nonpoint sources was the main contribution in the upstream portion and Outer River basin of Min River. Pollution from point sources was more serious than that from nonpoint sources in Chengdu, Meishan and Leshan segment. Specifically, TP pollution in Min River of Chengdu segment (Huanglongxi, Inner River) was primarily affected by urbanization, while Leshan segment (Yuebo) was mainly affected by glyphosate production. Controlling pollution from nonpoint sources and improving the sewage collection rate to reduce TP loads will be the primary task in the upstream portion and Outer River basin. Point source pollution controlling will still be the primary task associated with water quality management in the Inner River (Chengdu segment) and the downstream (Leshan segment) basin of Min River for the foreseeable future. Pollution from both point sources and nonpoint sources should be cared in the other area of Min River basin.

Understanding the spatial and temporal variation of nutrient concentrations, loads, and their distribution from upstream tributaries is important for the management of large lakes and reservoirs. The Three Gorges Dam was built on the Yangtze River in China, the world’s third longest river, and impounded the famous Three Gorges Reservoir (TGR). In this study, we analyzed total nitrogen (TN) concentrations and inflow data from 2003 till 2010 for the main upstream tributaries of the TGR that contribute about 82% of the TGR’s total inflow. We used time series analysis for seasonal decomposition of TN concentrations and used non-parametric statistical tests (Kruskal-Walli H, Mann-Whitney U) as well as base flow segmentation to analyze significant spatial and temporal patterns of TN pollution input into the TGR. Our results show that TN concentrations had significant spatial heterogeneity across the study area (Tuo River> Yangtze River> Wu River> Min River> Jialing River>Jinsha River). Furthermore, we derived apparent seasonal changes in three out of five upstream tributaries of the TGR rivers (Kruskal-Walli H ρ = 0.009, 0.030 and 0.029 for Tuo River, Jinsha River and Min River in sequence). TN pollution from non-point sources in the upstream tributaries accounted for 68.9% of the total TN input into the TGR. Non-point source pollution of TN revealed increasing trends for 4 out of five upstream tributaries of the TGR. Land use/cover and soil type were identified as the dominant driving factors for the spatial distribution of TN. Intensifying agriculture and increasing urbanization in the upstream catchments of the TGR were the main driving factors for non-point source pollution of TN increase from 2003 till 2010. Land use and land cover management as well as chemical fertilizer use restriction were needed to overcome the threats of increasing TN pollution.