Water Environment Variation in the Three Gorges Tributary and Its Influencing Factors on Different Scales

Long River is a small tributary typical of the Three Gorges Reservoir. In order to clarify the influence of hydro-meteorological factors and human regulation of the reservoir water level on the water environment, we monitored water environment parameters at six sites along Long River for three years, and were able to assess the degree to which variation in the water environment was affected from the micro-scale to macro-scale processes. We found that the water environment parameters have obvious seasonal variation and spatial distribution patterns. From upstream to the estuary, the influence of the backwater was gradually enhanced, whereas the influence of inflow gradually weakened. The distance coefficient between the samples in the backwater area and control sites in the upstream and the Yangtze River had a significant longitudinal gradient pattern (ANOVA, p < 0.05). Through redundancy analysis (RDA) forward selection, we found that cumulative precipitation, water level, cumulative net radiation, daily average temperature, daily net radiation, and daily precipitation all had significant influences on the water environment of the Long River (global permutation test, p < 0.05). The perennial backwater area was mostly affected by water level fluctuations (explaining 39.9% of data variation), whereas the fluctuation backwater area and the upstream inflow area were most affected by cumulative precipitation (explaining 42.9% and 44.0% of data variation, respectively). On the macroscale, the contribution rate of monthly change to the variation of water environment variables reached 54.9%, and the contribution rate of geographical change to the variation of water environment variables was only 7.2%, indicating that the water environment of Long River was mainly affected by exogenous input factors driven by the monsoon climate. Furthermore, we showed that the spatial interpolation method combined with the distance coefficient can easily and efficiently describe the complicated dynamic influences of the upstream inflow and the Yangtze River backwater on the water environment in the Long River bay. The conclusions are helpful in explaining the driving mechanisms of the water environment in the tributaries of the Three Gorges Reservoir, which can provide reference for water environmental protection and management in the Three Gorges Reservoir.