Impact of a dam on lateral hydrological connectivity in a river–floodplain system and the optimal threshold for vegetation growth

ABSTRACT Lateral hydrological connectivity critically regulates river–floodplain ecosystems, yet current investigations into optimal connectivity thresholds remain insufficient. This study integrated remote sensing interpretation with an improved graph theory model to quantitatively assess lateral hydrological connectivity and determine the optimal connectivity thresholds favorable for vegetation growth in the Huayuankou and Jiahetan river–floodplain systems (Lower Yellow River, China) from 1986 to 2021. The operations of the Xiaolangdi Dam enhanced the flood-season lateral hydrological connectivity (4.26–27.27%) and increased vegetation coverage in both flood and non-flood seasons (17.73–164.82%), but reduced non-flood seasons connectivity by over 20%. The influence pattern of connectivity on vegetation growth was similar at the same water level. The identified optimal connectivity thresholds for vegetation revealed that higher connectivity (0–0.5) is beneficial at low water levels, whereas lower connectivity is preferred at high water levels. At medium water levels, the optimal values exhibited regional and seasonal variation, ranging from 0.4–0.5 (flood season) and 0.6–0.7 (non-flood season) at Huayuankou, and from 0.45–0.5 (flood season) and 0.2–0.3 (non-flood season) at Jiahetan. The overall optimal hydrological connectivity threshold for vegetation growth was 0.3–0.4. These findings offer important insights for conserving and managing river–floodplain systems.