The synergistic response between temperature, flow field and nutrients in the tributary disturbed by the Three Gorges reservoir

The changes of hydrodynamic conditions and the related eutrophication have been observed among reservoir-impacted regions. However, the evolution and impacts of the hydrodynamic characteristics of tributaries under the interference of the reservoir still need further explanation. The three-dimensional Environmental Fluid Dynamics Code (EFDC) model coupling with the watershed hydrological model was built to explore the special flow field of the tributaries in the Three Gorges Reservoir Region. The results showed that there was significant laminar flow pattern driven by water temperature, except during low level and storage periods. During the rising level period, the difference in response to temperature changes between the main stream and tributaries caused the invading flow to converge in bottom layer. The convergence of cold and warm peaks resulted in water masses flowing out in opposite directions from surface layer. During this process, there existed transition from horizontal circulation to vertical circulation, breaking up vertical differences of 0.40 mg/L of nitrogen, aggravating eutrophication in surface layer during the low level period. Instead, the low velocities in stagnant areas did not lead to significant accumulation of nutrients. However, the variation of nutrients in the annular section exhibited a short-term lag compared to the longitudinal profile. Finally, the stepped tides by interrupting the continuous raise processes of water level was more efficient for controlling eutrophication than general regulating rules during the impoundment period. The results could be used for managing eutrophication in similar regions.