Abstract: A study was conducted to assess the physico- chemical effects of sand harvesting in River Thwake, Machakos County- Kenya. A total of 8 water samples from a sand harvesting site (point A) and no-sand harvesting site (point B) were collected and analyzed for; temperature, Hydrogen Potential, turbidity, color, electrical conductivity, Lead, Copper, Zinc, Iron, Biological Oxygen Demand and Chemical Oxygen Demand. There were significant differences in the means of point A and B water samples as regards to; color (p=0.001), turbidity (p=0.001), Iron (p=0.001) Chemical Oxygen Demand (p=0.001) and Zinc (p=0.02) which was associated with sand harvesting activities at point A. However, Copper (p=0.54), conductivity (p=0.17), temperature (p=0.93), Hydrogen Potential (p=0.09), Lead (p=0.85), Biological Oxygen Demand (p=0.63) mean scores were not significantly different despite sand harvesting activities. The results were compared with WHO guidelines, 2009 and Kenya Bureau of Standards 2006 water guidelines to ascertain its suitability for domestic use.