## 4 Results and Analysis  
### Spatiotemporal Evolution Characteristics of Thalweg  
The main channel of the upper reaches of the Tarim River (above Yangijibazha) is relatively stable in the dry season, and the riverbed is not bifurcated. The water flow is basically limited in the main channel, and there is no obvious bifurcation or beach segmentation. At this time, the thalweg is approximately distributed along the central axis of the main channel. The geometric shape of the main channel in the dry season is easy to determine by remote sensing images. The central line of the main channel is obtained by extracting the main channel in the dry season, and the central axis of the main channel in the dry season is approximated to the thalweg of the river. The thalweg line refers to the continuous connection of the deepest point along the longitudinal direction of the riverbed, which usually represents the path of the deepest water flow in the river channel. It usually corresponds to the mainstream trajectory (the main water flow axis), and is also the area with the fastest flow rate and the strongest erosion capacity. As shown in Figure 4, the thalweg length of the Xinqiman-Kelelik river segment increased from 142.9 km to 171.9 km between 1993 and 2024, and the curvature coefficient rose from 1.95 to 2.34. This indicates that the curvature of the river has increased significantly, and the side erosion is the dominant driving force. The deposition of the beach makes the river pass passively as the secondary driving force in the active development of the meandering river. Combined with the changes in annual runoff and sediment load (Figure 2), the evolution of the thalweg can be divided into three phases: The first stage, the rapid development period of the river (1993-1995): Runoff increased from 1.646 \(\times\) 10\({}^{9}\) m\({}^{3}\) to 3.341 \(\times\) 10\({}^{9}\) m\({}^{3}\), and the sediment load rose from 7.127 \(\times\) 10\({}^{5}\) tons to 1.4467 \(\times\) 10\({}^{6}\) tons. The strengthened flow triggered intensified erosion, leading to an increase in thalweg length and enhanced river channel meandering. The second stage, the river fluctuation adjustment period (1996-2011): Runoff (8.37 \(\times\) 10\({}^{9}\)-6.839\(\times\) 10\({}^{9}\) m\({}^{3}\)) and sediment discharge (3.623 \(\times\) 10\({}^{7}\)-2.9613 \(\times\) 10\({}^{7}\) tons) were characterized by violent fluctuations, and the peak appeared in 2010 (runoff 6.839 \(\times\) 10\({}^{9}\) m\({}^{3}\), sediment discharge 2.9613 \(\times\) 10\({}^{7}\) tons). During this period, there were four bends (1998, 2001, 2002, 2006), resulting in frequent river channel adjustment. The third stage, the secondary development period of the river (2012-2024): The thalweg was extended by 17.9 km, and the bending coefficient increased by 0.24. With the periodic growth of runoff and sediment transport, sediment deposition formed a beach body, forcing the water body to bypass and extend the thalweg. Figure 4: Trends in thalweg length and curvature coefficient of the Xinqiman–Kelelik river segment from 1993 to 2024. The river sections with severe evolution in typical years were selected for comparative analysis of thalweg changes, namely Tarim Farm Area 5, the Paman Reservoir river section in Tarim Township, the Pama river section, the Kuerqiaoer river section, and the Moon Bay scenic spot river section, as shown in Figure 5. The maximum offset distance of the thalweg was calculated respectively, and the results are shown in Table 2.  
As shown in Figure 5 and Table 2, the thalweg of the Tarim Farm Area 5 reach and Paman Reservoir reach has exhibited a significant northward migration trend over the past 32 years (Figure 5c,d). Specifically, the former has migrated a total of 801.2 m northward over 32 years, with the peak migration rate and distance occurring between 2004 and 2008, at a rate of 324.6 m/a and a distance of 1298.2 m. The latter has migrated a total of 1209.9 m northward, with the northward migration trend continuing to dominate after 2000.