Three-decadal dynamics of mid-channel bars in downstream of the Three Gorges Dam, China

Abstract. The downstream of the world’s largest Three Gorges Dam (TGD) along Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). The morphodynamics of these MCBs are crucial to the river’s hydrological processing, local ecological functioning, and socioeconomic development. However, a systematic understanding of such dynamics and their linkages to the TGD across the entire downstream reach remain largely unknown. Using Landsat archive images and a thematic extracting scheme, the work presents comprehensive monitoring of MCB dynamics in terms of number, area, and shape, in the downstream of the TGD during 1985–2018. Total 140 MCBs were extracted and grouped into four types to represent small size (< 2 km²), middle size (2 km²–7 km²), large size (7 km²–33 km²) and extra-large size (> 33 km²) MCBs, respectively. Most of the MCBs in terms of number (70 %) and total area (91 %) were concentrated in the lower reach (Hankou to Estuary). The number of small size MCBs decreased after TGD closure and most of such decreasing events happened in the lower reach. Although all four types of MCBs showed overall increasing trends in area, small MCBs had smaller rate and received more influence by the TGD operation than the large MCBs. Small size MCBs tended to become relatively shorter and wider whereas the others became slimmer after TGD operation. Impacts from the TGD operation could decrease along with the increasing distance from TGD to Hankou (for shape dynamics) or to Jiujiang (for area dynamics). The quantified longitudinal and temporal dynamics of MCBs across the whole downstream of the TGD provides a crucial monitoring basis for continuous investigations of changing mechanisms in the morphology of the Yangtze River system.