Distribution characteristics and stability thresholds of channel-floodplain morphology in the wide floodplain zone of the Songhuajiang River

The wide floodplain area is an important zone for flood detention and sediment deposition in alluvial compound channels, as well as a crucial carrier of wetland and cultivated land resources. Balancing the functions of flood detention, sediment deposition, agriculture, and ecology constitutes a core challenge in floodplain management. Based on remote sensing image interpretation and statistical analysis of measured cross-sectional data from the Songhuajiang River, this study reveals the interrelationships among characteristic parameters such as channel–floodplain dimensions, distribution patterns, river-regime changes, and floodplain area. (1) The mainstream channel width and floodplain width of the Songhuajiang River exhibit a trend of being larger upstream and smaller downstream, with relatively wide variation ranges. In contrast, the main channel width shows the opposite trend—smaller upstream and larger downstream—though its overall variation range is limited. The ranges of channel width, main channel width, and floodplain width are 2.0–12.9 km, 0.5–2.4 km, and 0.5–11.7 km, respectively. The water depth of the main channel and floodplain, as well as the elevation difference between them, also display a trend of being larger upstream and smaller downstream, although the overall variation range is small. The maximum water depth ranges of the main channel and floodplain are 8.9–22.5 m and 1.6–5.2 m, respectively, while the elevation difference between them generally falls within 5.0–20.5 m. (2) The river regime of the Songhuajiang River mainstream remains generally stable, with an average annual migration amplitude of approximately 110 m. Specifically, the average annual migration amplitude is 70–210 m in the Zhaoyuan reach, 20–110 m in the Harbin reach, 70–160 m in the Jiamusi reach, and 40–280 m in the Fujin reach. (3) The total floodplain area of the Songhuajiang River mainstream is about 3,105 km², including approximately 1,396 km² on the left bank, 1,050 km² on the right bank, and 659 km² of mid-channel bars. The wide floodplain zones are mainly distributed in the upstream and downstream reaches. Among these, the Zhaoyuan–Harbin reach has the largest floodplain area per kilometer, followed by the Jiamusi–Tongjiang reach, while the Yilan–Jiamusi reach has the smallest. Through graded statistics of main-channel width and floodplain water depth, the intrinsic relationships between the channel-to-floodplain width ratio and main-channel width, as well as between the channel-to-floodplain water-depth ratio and floodplain water depth, are revealed. The stability thresholds of the channel–floodplain system under long-term water–sediment dynamics are determined as follows: (1) A strong correlation exists between the main-channel width and the channel–floodplain width ratio in the Songhuajiang River mainstream. The width ratio decreases as the main-channel width increases, but the gradient of change is considerably steeper when the main-channel width is less than 1,000 m than when it exceeds 1,000 m. (2) The stability thresholds for channel width, main-channel width, floodplain width, and channel–floodplain width ratio in the Songhuajiang River mainstream are 6,500 m, 1,000 m, 5,500 m, and 5.5, respectively. (3) A strong linear relationship exists between floodplain water depth and the channel–floodplain water-depth ratio in the Songhuajiang River mainstream, with the ratio increasing as floodplain water depth increases. (4) The stability thresholds for floodplain water depth, main-channel water depth, elevation difference between channel and floodplain, and channel–floodplain water-depth ratio in the Songhuajiang River mainstream are 3.5 m, 13.5 m, 10 m, and 0.26, respectively. The characteristic parameters and stability thresholds of the Songhuajiang River's channel–floodplain cross-sections are derived from statistical analysis of long-term hydro-sedimentary dynamics, providing a technical basis for defining the optimal management width of channel–floodplain systems in wide floodplain zones.