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(1) Background: Small- and medium-sized rivers in urban areas are unique environments that serve as blue-green corridors for urban residents. The relationship between land-use types and water quality in these rivers provides important information for effectively addressing urban river restoration and pollution management. However, not much attention has been paid on these small- and medium-sized rivers, especially in large urban agglomerations with dense river networks. (2) Methods: This study undertook a field investigation on 130 sampling small- and medium-sized rivers during the late summer and applied data-driven water quality index and landscape analysis techniques to evaluate the direct impacts of riparian land-use types on the summertime water quality in Shanghai’s small- and medium-sized rivers. Riparian land-use types were derived from OpenStreetMap (OSM) datasets, including industrial, commercial, residential, and green spaces. (3) Results: Residential and green space are located closer to these sampled rivers than industrial and commercial land types, suggesting a tentative link between anthropogenic activities and water quality. Further analysis concluded that urban resident settlements, characterized by specific land-use types, DMSP-OLS nighttime lights, OSM road density, and OSM river density, strongly affected the water quality at the sub-catchment scale. We further determined the critical radii for impacts of land use types on urban rivers. Industrial types may influence water quality within a maximum radius of 5 km, followed by green space (4 km), residential areas (3 km), and commercial developments (2 km). These mathematically and statistically computed radii provide updated visions for river health assessment. For a specific land-use type, the assessed water quality index will be biased by using an assessment area with a radius higher or lower than the above-estimated radii. The study also quantified the spatial extent and transmission efficiency of non-point source pollution in a super built-up area of central Shanghai. We observed that contaminants transported by river pathways can reach a larger area than those transported by roads. (4) Conclusions: The high-quality environments in small- and medium-sized rivers are tightly linked to riparian landscape patterns. It is therefore urgent to control domestic pollutions as part of the restoration of megacity’s urban rivers and grapple with the complex challenges of risks to water supply. This study elaborates the importance of integrating land-use planning and water-quality management to maintain the functions and services of small- and medium-sized urban rivers.
