Post-Regulation Hydrological and Channel-Planform Changes in the Urban Reach of the Lhasa River

Hydrological regulation and local engineering can modify the planform dynamics of wandering rivers, but separating their effects is difficult where pre-regulation morphology and synchronous hydraulic observations are unavailable. This study integrates discharge and water-level records (2000–2023), SRTM-derived surface topography, and nine December satellite-image composites (2015–2023) for the urban Lhasa River reach between Gates 1 and 5. Within the available 24-year discharge record, Pettitt tests identify a mid-2000s distributional shift at the three stations; sensitivity tests place the detected change in 2005–2006. Because the pre-change segment contains only six years and no precipitation-based natural-flow control was available, the result is interpreted as temporally consistent with, but not proof of an effect of, Zhikong Hydropower Station commissioning. The 2013 Pangduo project is treated separately as an engineering timeline marker because no distinct annual mean breakpoint was detected at that date. Mapped water area increased by 45.39% between 2015 and 2023, whereas exposed bar–island and riparian-land classes decreased by 36.29% and 61.13%, respectively. These values are reported as a water-surface expansion signal consistent with gate impoundment and/or acquisition-stage differences, not as confirmed sedimentary or geomorphic conversion. The comparison also contains sensor-dependent positional uncertainty: approximately ±15 m for the 2015 and 2018 Landsat-8 inputs and ±5 m for Sentinel-2 inputs. Centerline length and sinuosity varied by less than 1%. The nearest-neighbor bankline metric declined by 9.53%, whereas the node-mean channel-regime-center metric declined by 81.99%; the latter is retained only as an exploratory indicator because it is sensitive to centerline-node sampling. Overall, the observations are consistent with reduced lateral activity in an engineered reach, but the available evidence does not isolate reservoir regulation, gate operation, bank protection, climate, land use, or sediment-supply effects. Stronger causal inference requires a pre-regulation morphological baseline, synchronous discharge and water-level observations at image acquisition, fixed-interval centerline resampling, sediment data, and surveyed hydraulic geometry.