<p>This study assessed surface water quality across the Osun River and its tributaries, focusing on spatial and seasonal variations, pollution intensity, and ecological implications. Parameters analyzed were pH, turbidity, dissolved solids, electrical conductivity (EC), dissolved oxygen, nutrients, trace metals, and microbial counts. Most values complied with World Health Organization standards; however, elevated phosphate levels in several locations suggested anthropogenic influence, while Osunjela exhibited high microbial counts linked to fecal contamination. Correlation analysis revealed strong associations among EC, dissolved solids, and metals, indicating shared pollution sources. Principal Component Analysis (PCA) identified four dominant drivers: industrial/agricultural runoff, domestic effluents, livestock waste, and geological weathering. This study takes a different approach compared to previous research on Nigerian rivers, which primarily focused on describing water quality. Here, we combine the Water Quality Index, Metal Pollution Index, and PCA to give a more comprehensive picture of pollution levels and their sources. By comparing the Osun basin to global contexts, we highlight the importance of connecting local insights to the broader issues faced by urban rivers in developing areas. The implications for policy are clear: places like Osunjela require immediate action, including better sanitation facilities, stricter controls on waste discharge, and community-led waste management efforts. It is essential to establish routine monitoring and pair it with watershed management strategies that balance urban growth with environmental protection. These findings go beyond just academic interest; they offer practical recommendations for regulators and local leaders to help protect public health and maintain the vital ecological functions of River system.</p>