Unlike developed countries, China has a nationally unified water environment standard and a specific watershed protection bureau to perform water quality evaluation. It is a major challenge to assess the water quality of a large watershed at a wide spatial scale and to make decisions in a scientific way. In 2016, weekly and real-time data for four monitoring indicators (pH, dissolved oxygen, permanganate index, and ammonia nitrogen) were collected at 21 surface water sections (sites) of the Yangtze River Basin, China. Results showed that one site had a relatively low Site Water Quality Index and was polluted for 12 weeks meanwhile. By using expectation-maximization clustering and hierarchical clustering algorithms, the 21 sites were classified. Variable spatiotemporal distribution characteristics for water quality and pollutants were found; some sites exhibited similar water quality variations on the weekly scale, but had different yearly grades. The results revealed polluted water quality for short periods and abrupt anomalies, which imply potential pollution sources and negative effects on water ecosystems. Potential spatio-temporal water quality characteristics, explored by machine learning methods and evidenced by time series and statistical models, could be applied in environmental decision support systems to make watershed management more objective, reliable, and powerful. and to evaluate alternative hypotheses regarding important contaminant sources and watershed properties that control transport over large spatial scales. However, there is no unified model available for the Chinese government to make decisions regarding large watershed management [12]. Local governments have a variety of environmental models that they can choose from, and may select diverse models that do not allow meaningful comparisons with the results of models chosen in other areas [13]. Even in the same area, different departments of the same local government use different models with diverse data to assess the water quality of the same river basin, resulting in a huge amount of variability in the water quality evaluation reports, which often fail to reach the same conclusion [3,4].Thanks to the increased collection and use of data, data-driven approaches have been playing an increasingly important role in water management [14]. Statistical and numerical models enable environmental decision support systems (EDSS) to be more reliable and powerful in coping with real-world environmental systems [15]. Real-time data are widely used in urban water management and by water utilities in developed countries [16][17][18][19][20], but rarely in rural watershed management, especially in large watershed management [21,22]. In China, the rapidly growing economy and population is generating widely distributed polluted surface water throughout the country. Thus, there is an increasing need for online data for large watershed management to meet the objectives of early warning monitoring of surface water quality, and for monitoring and control of tot...