Study on fish habitat creation in meandering braided reaches of the three gorges fluctuating backwater area

The Chaotianmen to Fuling reach in the Three Gorges Reservoir's fluctuation zone is located within the National Germplasm Resource Protection Area for the four major Chinese carps in the upper Yangtze River.During the impoundment period of the fluctuating backwater area of the Three Gorges Reservoir, the water depth is large and the flow velocity is low, which is suitable for the survival of the four major Chinese carps.During the drawdown and flood seasons of the Three Gorges Reservoir, the river channel restores its natural river characteristics, which is manifested in reduced water depth and increased flow velocity, thereby narrowing the scope of suitable habitats for the four major Chinese carps in the upper .The sharp-bend and branching reaches in the fluctuating backwater area of the Three Gorges Reservoir feature special geomorphic characteristics: after the sharp bends, the river channel is mostly in a widened and branched form, with a large number of stone beams distributed at intervals along the nearshore area, which provides a natural topographic basis for the construction of continuous shoal-bar and deep pool habitats.In the process of reef removal and slag disposal in the river section, the waste slag is placed in the inshore deep pool area, which has a certain impact on the four major Chinese carps in the upper habitats.To address this, relying on the 4.5m-depth waterway project from Chaotianmen to Fuling in the fluctuating backwater area of the Three Gorges Reservoir, and taking the Dajiantan sharp-bend branching reach as a typical example, a scheme is proposed to construct the four major Chinese carps in the upper habitats using the reef-removal slag from the waterway regulation project.A 2D planar mathematical hydrodynamic model for the project reach was established and validated using MIKE21, and the hydrodynamic distributions of the reach under different hydrological combinations were calculated.Based on the Habitat Suitability Index (HSI) model, water depth and flow velocity were selected as key influencing factors to analyze the improvement effect of the shoal-bar and deep pool habitat construction scheme in the sharp-bend branching reach on the hydrodynamic conditions of the four major Chinese carps in the upper habitats.The results show that, in the sharp-bend and branching reaches of the fluctuating backwater area of the Three Gorges Reservoir, habitat construction using shoal bars can increase the Habitat Suitability Index (HSI) by up to 0.4 compared with the pre-scheme condition. This significantly improves the habitat quality of the four major Chinese carps in the upper habitats during the drawdown and flood seasons, and can provide a reference for habitat restoration in similar river courses.