Assessing cumulative impacts of cascade dams on vegetation in the Reservoir-Hydropower impact zone.

Abstract

Vegetation, a key indicator of watershed health, is increasingly degraded due to the compounded effects of cascade dam construction, especially in mountainous regions. This study examines vegetation dynamics within the Reservoir-Hydropower Impact Zone (RHIZ) of the lower Yalong River using a novel three-dimensional spatial framework defined by a 5 km horizontal buffer, longitudinal dam segments, and 2 km elevation zones. By integrating remote sensing with machine learning, we analyzed land use changes from 1986 to 2021, focusing on forests, grasslands, and farmland. GeoDetector and path analysis were used to disentangle the combined impacts of cascade dam development and climate change. Key findings include: (1) During dam expansion, forests, grasslands, and unused lands in RHIZ declined, while farmland, urban land, and water bodies expanded, particularly near riverbanks. (2) Cumulative impacts of cascade dams caused more substantial changes in RHIZ vegetation than single dams. Following the first dam's construction, dynamic rates for water bodies, urban areas, and farmland rose to 7 %, 10 %, and 14 %, respectively, while forests and grasslands remained stable. The fourth dam triggered the highest land use dynamic rates, with water bodies, grasslands, and forests reaching 40 %, -17 %, and -8 %, respectively. After all dams became operational, grassland degradation continued, while other land types gradually stabilized. (3) Cascade hydropower had stronger effects than climate, both through direct land cover change and indirect microclimate shifts. This study introduces an integrated spatial-analytical approach for RHIZ vegetation assessment and highlights the need to account for cumulative dam impacts in ecosystem management and sustainable hydropower planning.