Effects of the Three Gorges Reservoir impoundment on the hydrological conditions for potamodromous fish spawning

Accurate assessment of the effects of reservoir impoundment on the downstream hydrology and ecology is a prerequisite for the restoration of impaired rivers. However, little research has been conducted to directly link hydrological processes to biological processes. To address this knowledge gap, we used 15-year spawning records for four Chinese major carps in the middle Yangtze River; downstream of the Three Gorges Reservoir, China; and the hydrological data during these spawning events to identify the impaired components of the linkage between the hydrological processes and these fish spawning events due to reservoir impoundment. Results of this study show that the reservoir impoundment has led to a 14-day delay in the start of the spawning season, 10 fewer annual spawning days, a 2-day shortening of the duration of spawning events, and the disappearance of one peak spawning period. The delay in the spawning start date is connected to the decrease in water temperature caused by the cooling effect of the reservoir water released during the early spawning season, and the lower spawning activity throughout the spawning season is attributed to the insufficiency of the flow pulses, especially the decreases in both the duration and the rate of the flow increases. The acceptable and optimum flow criteria deduced from the pre-impoundment flow-spawning linkage can provide process-explicit environmental flow criteria for the spawning function restoration. This research presents an approach based on the flow-biotic process linkage for the assessment of impoundment-induced hydrological and ecological effects, and the process-based information can reduce the uncertainty in environmental flow management.