Using Google Earth Engine to assess cumulative impacts of cascade dams on river surface morphology in mountainous watersheds- Case of the lower Yalong River

Abstract

Cascade dam construction significantly alters river surface morphology, posing challenges for sustainable river management. This study investigates the cumulative morphological impacts and restoration potential in the lower Yalong River using topographic maps, satellite imagery, and field surveys. In contrast to previous research focused on local hydrodynamics or sediment processes, this work emphasizes large-scale, long-term river surface evolution. Five key morphological indicators—river width, width ratio, shape index, sinuosity index, and stability—were extracted from multi-temporal Landsat imagery using the Modified Normalized Difference Water Index (MNDWI). A dynamic rate method was used to quantify the average annual change of each indicator during different dam construction stages. The results show that cascade dams substantially reshaped water surface patterns: 67 % of river segments widened upstream to form reservoirs, while 6 % experienced downstream narrowing due to flow reduction. Pre-dam morphological changes extended up to 134 km, while post-dam impacts were limited to within 15 km. The most significant changes occurred during the construction of the first and fourth dams, while the fifth dam had minimal influence due to its smaller scale. Correlation and principal component analyses identified river width and stability as the most sensitive indicators of cumulative impacts. These findings demonstrate that cascade dams induce pronounced and phase-specific alterations to river morphology, with both localized and far-reaching upstream-downstream interactions. The study improves understanding of geomorphic responses to multi-dam systems and offers a quantitative basis for evaluating ecological resilience and guiding sustainable river basin management in mountainous regions.