A 2-D CE-QUAL-W2 hydrodynamic and water quality model was established from 2008 to 2018 in.Xiangxi bay (XXB). Recently built upstream Xiluodu Dam Wudongde Dam Xiangjiaba Dam and Beihtan dam have altered ecohydrology of mainstream and tributaries, so nutrients regime during pre and post dames stages are comparatively investigated here. Although, TP increased at lower reach but eutrophication remained TP limited with few exceptions. Maximum supply from main stream, potential of water exchange on surface corresponding to increased overflow intrusion facilitated increased average Total Nitrogen (TN) at euphotic zone in XXB by 0.1 mg /l, 0.3 mg/l and 0.6 mg/l respectively since 2014. Eutrophication and algal growth in XXB remained TP limited with few exceptions at upper reach. Average TN increased by 0.1 mg/l, 0.3 mg/l and 0.5 mg at upper, middle and lower reaches respectively in Late March since 2014 compared to those in early years. It increased by 0.2 mg/l, 0.4 mg/l and 0.6 mg/l at upper, middle and lower reaches in Early April since 2014. It increased by 0.3 mg/l, 0.6 mg/l, and 0.9 mg/l at upper, middle and lower reaches in mid April since 2014. It increased a bit at upper reach, 0.6 mg/l at middle reach and 0.9 mg/l at lower reach during late April. It increased a bit at upper reach and by 0.3 mg/l at middle reach and by 0.5 mg/l at lower reaches corresponding to either of the patterns. During mid May, it remained same at upper reach, decreased by 0.2 mg/l at middle reach and 0.5 mg/l at lower reache. During late May, it remained same at upper reach, decreased by 0.4 mg/l at middle reach and 0.6 mg/l at lower reaches. These statistics indicate average increase in TN in spring except in late May .TN concentration was beyond danger limit sustaining and prolonging eutrophication status in spring. As overflow, interflow and upper interflow patterns are prevalent in random order so nutrients could not be predicted theoretically, however nutrients management from non-point sources and main supplier TGR is advisable.

A 2-D CE-QUAL-W2 hydrodynamic model was established to simulate evolutionary mechanisms and shifting trends of flow patterns per annum and over seasons from 2008 to 2018 in a reflective tributary Xiangxi Bay (XXB) of the Three Gorges Reservoir, China. Reasons behind shifting trends of flow patterns were also investigated. Model performance was validated and simulated data was synchronous to observed data. In general, percentage of Pattern (6) was 14%, 20%, 17%, 12% and 11% per annum and in spring, summer, autumn and winter respectively by 2013. It was increased by 26%, 30%, 22%, 25% and 35% per annum and in spring, summer, autumn and winter respectively since 2014. Increased temperature and flow dynamics (such as 10,000 m3 s−1 in spring) in Three Gorges Resevoir (TGR) since 2014 were underlying shifting trends of density current patterns. Correlation among patterns prior to and after newly built upstream reservoirs was novel and innovative in finding hydrodynamic thresholds to increase effectivity. Particulars elaboated and associated with respective density current patterns indicate increased surface velocity and water exchange with increased overflows. This could help understanding hydrodynamics and ecological variations in TGR and XXB. Thermal establishment and flow dynamics in TGR triggering overflow intrusion in XXB are required to be achieved. A hydrodynamic and water quality model of XXB coupled with TGR mainstream is recommended to correlate additive impacts of advantageous and disadvantageous patterns and to evaluate hydrodynamical thresholds triggering advantageous patterns in XXB.