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