Modelling and Analysis of Water Exchange between the Oxbow Lake and Main River Channel in the Middle Reaches of the Yangtze River

The oxbow lake located in the middle reaches of the Yangtze River (YR) serves as a primary relocation and protection area for nationally protected species such as the Yangtze finless porpoise, and is also an important natural gene pool for biodiversity conservation in the middle reaches of the Yangtze River (MYR). The operation of the Three Gorges Dam (TGD) has altered the frequency and intensity of water exchange between the oxbow lake and the YR, affecting the stability of its morphology and the maintenance of its ecological functions. Statistical modeling reveals that the duration of continuous rise in water level in the MYR follows a Gaussian distribution function. Following the full impoundment operation of the TGD, there is a significant increase in the frequency of short-duration rises in water level, a decrease in the frequency of medium-duration rises, and a decline in the frequency of long-duration rises. Moreover, the rate of water level change in the MYR follows a power function, with a decreasing trend before and during the initial operation of the TGD, and an overall increasing trend after full impoundment, significantly lower than the pre-operation average. By employing threshold regression models and Langmuir equations, a formula for calculating water exchange intensity has been established. The average annual water exchange intensity shows a significant overall decrease from the period after the operation of the TGD to the full impoundment operation phase. However, following full impoundment, there is a significant upward trend in water exchange intensity, reaching levels comparable to the pre-operation period. During the emptying and pre-release period of the TGD, water exchange intensity increases by 5.5 times, while during the seedling flood of the four major Chinese carps in the MYR, water exchange intensity decreases by 70%. Conversely, during the reservoir storage and utilization phase, water exchange intensity remains relatively stable.