Evaluation of river longitudinal connectivity based on landscape pattern and its application in the middle and lower reaches of the Yellow River, China

River connectivity plays an essential role in maintaining the health and stability of river basin ecosystems. It is of great significance to protect river ecosystems to clarify the effect of water conservancy project construction and operation on river hydrological connectivity. The longitudinal connectivity is affected by the landscape patterns of river, such as the convergence and dispersion of the mid-channel bars and the river areas. This study aims to analyze the impacts of construction and regulation of Xiaolangdi Dam on the connectivity of the middle and lower reaches of the Yellow River from the perspective of landscape pattern. An improved longitudinal river connectivity evaluation method was proposed by accounting for the influence of the landscape pattern represented by mid-channel bars based on barrier coefficient method, and then was applied to analyze the connectivity pre- and post-dam construction. The results show that the amplitude and frequency of the oscillation of the river were greatly reduced and tended to be stable. The aggregation degree of mid-channel bars was reduced, and the distribution of mid-channel bars was more dispersed. The river longitudinal connectivity before and after the construction of the Xiaolangdi Dam were 1.35 and 1.50 respectively, indicating an increased river longitudinal connectivity. Overall, there are differences in connectivity before and after Xiaolangdi Dam construction, and connectivity fluctuates after dam construction. Because of the dam regulation of water and sediment, the river connectivity during the flood season increased significantly, and was greater than that before and after the flood season. The longitudinal connectivity evaluation method established in this study is accurate and efficient, and provides an intuitive and reliable new method for quantitatively analyzing the changing laws and characteristics of river connectivity.