To analyze the status of spawning habitat for the four major Chinese carps in the lower reaches of the Ganjiang River under varying hydrodynamic conditions, a physical habitat model for fish spawning habitat was established by coupling the two-dimensional hydrodynamic model and the habitat suitability model based on the fuzzy logic method. This model was used to simulate the spatial distribution of suitable spawning areas for the four major carps in the lower reaches of the Ganjiang River under different flow rates, to analyze the variation patterns of the weighted usable area (WUA) of spawning habitat, and to determine the appropriate ecological flow required for spawning. The results show that the overall suitability of the main branch was higher than that of the middle branch and the south branch when the flow rate was low. When the flow was high, the overall suitability of the main branch decreased obviously while the suitability of the middle branch and the south branch increased. The WUA of spawning habitat increased initially and then decreased with increasing flow rate. Compared with the low-flow condition, the WUA decreased faster under the high-flow condition. When the flow rate was 4200 m³/s, the WUA was the largest, accounting for 22.97% of the study area. The suitable ecological flow range of spawning habitat for the four major Chinese carps was between 3500 and 4800 m³/s, and the optimum ecological flow was 4 200 m³/s.

To investigate the hydraulic characteristics of fish-friendly culverts, a numerical model of a fish-friendly culvert with triangular lateral baffles was developed based on the large eddy simulation method. Three-dimensional numerical simulation of flow within the culvert was conducted under various submergence levels, and the accuracy and reliability of the numerical model were validated through flume experiments. The results indicate that at half the height of the baffle, flow separation occurs along the outer edge of the baffle, and the size of the recirculation zone increases as the submergence level decreases. The intensity of the shear layer and the turbulence around the baffle are negatively correlated with the submergence level. Under shallow flow conditions, vortex shedding becomes more chaotic, with stronger and more numerous vortex structures near the water surface compared to those near the bed.

To analyze the influence of the water temperature change in the lower reaches of Jinsha River on the spawning period of typical fishes before and after the completion of the Xiangjiaba Dam, the interannual water temperature trend and the variation range in recent years are analyzed and studied based on the measured data; and then the suitable water temperature range of 14 typical fish species in the historical spawning period is projected to the change curve of current water temperature, so as to obtain the variation of spawning period. The influence of water temperature change on the spawning activity is quantitatively evaluated by the proposed index of time overlap. The results show that the rise period of temperature in the lower reaches of Jinsha River before the dam construction is from February to June, and the annual variation range is 11.8 to 23.2℃. After the dam is built, it is postponed to April-July, with the variation range of 14.0 to 23.1℃, and the temperature fluctuations show a decreasing trend. Under the water temperature condition after the dam construction, the main spawning period of typical fishes in the lower reaches may move from early March-late July to late March-early August.