Spatially Differentiated Assessment of River Health in an Arid Seasonal Basin: A Case Study of the Yarkant River

River health assessment in arid seasonal rivers is challenging because strong hydrological seasonality, spatial heterogeneity, and intensive human intervention can produce substantial differences among river reaches. Conventional basin-scale assessments may obscure localized ecological and management problems when indicators with different spatial characteristics are evaluated using a uniform spatial unit. This study developed a spatially differentiated river health assessment approach for the Yarkant River Basin, an arid seasonal river basin in northwestern China. River health was considered an integrated condition encompassing basin structure, water conditions, aquatic biota, and socio-economic service functions. A multi-dimensional indicator system comprising 12 indicators across four criteria was established. The main methodological feature is that indicators were evaluated using spatial units consistent with their physical meanings, monitoring characteristics, and available data. Reach-based indicators were calculated separately for the upper, middle, and lower reaches, whereas ecological-flow and water-quality-related indicators were evaluated using hydrological control sections and water function zones, respectively, and then linked to the corresponding reaches for aggregation. Fish retention, public satisfaction, water supply reliability, and drinking-water-source compliance were also calculated separately for the three reaches using reach-specific data. A composite weighting method integrating a guideline-based least-squares weighting component and an entropy-based objective weighting component was used to aggregate the indicator scores. The Yarkant River obtained an overall River Health Index (RHI) of 84.24, corresponding to the “healthy” category under the adopted classification scheme. The upper, middle, and lower reaches scored 83.23, 87.03, and 83.32, respectively. However, the Biota criterion scored only 68.00, substantially lower than the Water (88.47) and Socio-economic service (90.41) criteria. This contrast indicates that the composite RHI should not be interpreted as evidence of uniformly good ecological integrity, particularly because the biological assessment is represented by a single fish-based indicator. The results highlight the value of retaining indicator-specific spatial information and interpreting the composite RHI together with its individual ecological and functional dimensions.