This study assessed the water quality of the Tahuando River as it flows through Ibarra and analyzed its relationship with human activities over time. Seven monitoring points—representing upstream, midstream, and downstream conditions—were sampled over three consecutive years (2021–2023). Water quality was determined using the Water Quality Index (WQI) based on Brown’s parameters: pH, turbidity, temperature difference, dissolved oxygen, biochemical oxygen demand (BOD), nitrates, phosphates, total dissolved solids, and fecal coliforms. The WQI classified water quality from excellent to poor, allowing an evaluation of contamination levels and potential uses. Results showed that upstream sites consistently maintained good or acceptable quality, while urban and downstream sections exhibited progressive deterioration. From 2021 to 2023, WQI values declined significantly, with the final downstream site (“Tahuando Final Point”) reaching a poor classification despite the presence of a wastewater treatment plant. The continued discharge of untreated effluents and diffuse pollution impeded the river’s self-purification, rendering water unsuitable for consumption, recreation, or irrigation—except in the upper reaches. These outcomes were consistent with findings from other tropical Andean basins linking urbanization and untreated wastewater to declining WQI values. Overall, the study highlights that anthropogenic pressures, particularly from urban wastewater and land-use change, are the main drivers of degradation in the Tahuando watershed, threatening aquatic biodiversity and the long-term sustainability of this vital water resource.