The progress in the contamination of rivers makes it impossible to use surface water from these sources or constitutes a risk to the health of those who do so. The high presence of organic matter in rivers can cause its decomposition to cause a decrease in dissolved oxygen in the water, reaching septic conditions, eutrophication of water, and methane emission if there are anaerobic processes; Likewise, the presence of pathogenic organisms (bacteria, viruses) makes the water unfit for human consumption and may be a route of disease transmission. The objective of the research was to reduce organic matter and bacteria in river water using chitosan nanoparticles. The treatment was verified by taking into account the Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) as indicators of organic matter and contamination by pathogenic organisms by the level of total coliforms. The results obtained were a reduction of BOD by 73.541% with a dose of 12 g, COD by 67.75% with a dose of 10 g, and Fecal Coliforms 95.6%, with a dose of 8 grams. The methodology is viable to improve the surface water quality of rivers in an easy way.