In highly urbanized plain river network regions, the effects of land use on water quality exhibit pronounced scale dependence, yet the characteristic response scale and associated source–sink mechanisms remain insufficiently understood. Taking the Wenruitang River Basin as a case study, this research integrates high-resolution land-use data with multi-site water quality monitoring to explore scale-dependent land use–water quality relationships. Circular buffer zones of 100 m, 300 m, and 500 m were delineated around monitoring sites, and Spearman correlation analysis and RDA were applied. The results indicate that land-use explanatory power follows a clear inverted U-shaped pattern with increasing scale, peaking at the 300 m buffer, where land-use composition explained 96.56% of the variation in water quality. This scale represents the characteristic response distance of water quality to landscape structure in plain river networks. At this scale, built-up land shows significant positive correlations with NH₄⁺–N, TP, and CODMn, acting as the dominant pollution source, whereas forest and grassland exhibit significant sink functions. However, the strong dominance of built-up land relative to limited ecological land creates a source-dominated and sink-deficient configuration, constraining water quality improvement. These findings emphasize the importance of scale-aware management and suggest that water quality control should extend beyond narrow riparian buffers to the 300 m core influence zone through impervious surface regulation and low-impact development measures.