Abstract: During the operation of the Hanjiang Xinglong hydro-junction, the water level below the Xinglong dam has experienced a cumulative decrease of 2.47-2.55 m under the flow rate of 500-800 m³/s, unfavorable for safety and operation. A comprehensive analysis reveals that the declining water level can be attributed to several factors. The Middle Route of the South-to-North Water Transfer Project and its associated compensation measures gave rise the changes of water and sediment conditions in the middle and lower reaches of the Hanjiang River. This resulted in riverbed scouring due to the clear water discharge. Additionally, the construction of the Xinglong hydroproject caused adjustments to the local river regime. Consequently, the downstream thalweg migrated back and forth, leading to bending and lengthening of the mainstream line. Furthermore, the discharge area and channel volume experienced significant increases. The implementation of waterway regulation measures also resulted in the shrinkage of the original shoal and the rapid deepening of the new main channel. Last but not the least, water diversion project from the Yangtze River to the Hanjiang River intensified the scouring force of water flow during medium and dry seasons. These factors collectively contributed to the noticeable decline in low water level in the downstream of the Xinglong dam.

Key words: decline of low water level, riverbed scouring, river regime adjustment, Hanjiang Xinglong Reach, downstream of hydro-junction