## 4 Results and Analysis
### Spatiotemporal Evolution Characteristics of Thalweg
This phenomenon is primarily driven by the transverse circulation of the meander, where the concave bank is deepened by high-velocity spiral flow erosion, while the convex bank is elevated due to sediment deposition from low-velocity flow [49; 50; 51; 52]. Together, these processes drive the northward migration of the thalweg. After 2008, the construction of the north bank revent significantly slowed the migration rate of the thalweg. The average annual migration rate of the Tarim Farm Area 5 segment decreased from 49.11 m/a to 4.03 m/a, while the rate for the Paman Reservoir segment decreased from 64.03 m/a to 15.59 m/a. However, the northward migration trend of the thalweg has not been fully restrained. The continuous northward migration threatens the flood control safety of the northern bank villages in Tarim Farm Area 5 (with an increased risk of overtopping at sharp bends) and the stability of the bank foundation at the Paman Reservoir. To address the threats, the following governance measures are suggested for the two river sections. Tarim Farm Area 5 reach: Extend the bank protection system upstream. Install downstream slope erosion resistance--pointing spur disks to divert flows southward, and adding vegetation buffer strips to enhance bank. Pam Reservoir Section: Construct permeable spur dikes and diversion walls on the north bank to curb erosion. Optimize the cross-section morphology (deeper north, shallower south) to weaken lateral circulation. Combine these efforts with sediment promotion projects on the south bank and reservoir sediment water regulation to stabilize the river channel. The thalweg evolution in the Pama Reach (Figure 5e) and the Kuerqiaoer Reach (Figure 5f) both exhibit the characteristic of \"early oscillation-later directional migration\". In the early stages (Pama Reach 1993-2004, Kuerqiaoer Reach 1993-2012), due to the wide and shallow channel and the dispersed flow, the main flow dynamics axis was unstable, leading to frequent north-south oscillations of the thalweg. In the later stages, driven by the high runoff year (with runoff of 4.411 \(\times\) 10\({}^{9}\) m\({}^{3}\) in 2006 for the Pama Reach and 5.242 \(\times\) 10\({}^{9}\) m\({}^{3}\) in 2016 for the Kuerqiaoer Reach), the river underwent meander cutoffs and straightening. It is inferred that the narrowing of the new channel is due to the continuous erosion of the concave bank (the north bank of the Bama River and the south bank of the Kurjoer River) caused by the transverse circulation, which forms a deep-water channel and drives the directional migration of the thalweg. The thalweg of the Pama Reach exhibited a cumulative northward migration of 2143.4 m toward the Tianshan Mountains side between 1993 and 2024. Notably, between 2004 and 2008, the thalweg migrated 1892.6 m northward toward the Tianshan Mountains. This is mainly related to the high runoff of the river in 2006, which led to the river turning and straightening, resulting in the northward movement of the river. After 2006, the thalweg oscillation stabilized. The migration rate dropped from 111.56 m/a to 29.38 m/a, and it showed a slow northward migration trend. The thalweg of the Kuerqiaoer Reach migrated a total of 2619.4 m southward from 1993 to 2024. Notably, between 2012 and 2016, the thalweg migrated 2507.7 m southward. This is due to the high runoff event in 2016, which led to the bending and straightening of the river channel, resulting in the southward movement of the river channel. The new river path moved farther away from the north bank farmland, affecting the irrigation water supply for the farmland.