Wet–Dry Seasonal Dynamics of River-Network Structure and Connectivity in the Poyang Lake Basin (1990–2025)

Seasonal hydrological fluctuations can substantially reshape river–lake networks, but changes in water extent, network morphology, and structural connectivity may not occur synchronously. This study investigated wet–dry seasonal variations in the Poyang Lake Basin using Landsat imagery from eight representative years between 1990 and 2025. Surface water was extracted using a consistent NDWI threshold, and 30 m DEM-derived channel information was used to correct interrupted, duplicated, and spurious river centerlines before Horton–Strahler ordering and graph-theoretic analysis. Mean water area increased from 3407.14 km2 in the dry season to 6872.87 km2 in the wet season, while mean river-network density increased from 0.0348 to 0.0461 km/km2. The maximum Strahler order increased from III in the dry season to IV in the wet season. River-network complexity increased from a dry-season mean of 12.85 to 58.38 in the wet season, and the river-network development coefficient increased from 3.28 to 13.59, indicating a substantially more developed tributary and hierarchical structure under wet-season conditions. In contrast, the mean α, β, and γ indices increased by only 9.8%, 4.6%, and 4.3%, respectively, and dry-season values exceeded wet-season values in some years. From 1990 to 2025, wet-season water area increased by 11.11%, whereas river-network length decreased by 2.77%. These results demonstrate that seasonal changes in water extent, river-network morphology, and structural connectivity are asynchronous, highlighting the need to distinguish these complementary dimensions when evaluating large river-connected lake systems.