Natural organic matter (NOM) is found in all surface, ground and soil waters. NOM in water has a significant effect on drinking water treatment. The presence of NOM can create a need for increased coagulant doses in drinking water treatment. Humic and fulvic materials represent up to 70% of dissolved organic carbon (DOC) and are major components of NOM. This work evaluated the effect of humic acids on the particle size distribution of flocs and settled effluent turbidity for a synthetic surface water treated with polyaluminum chloride (PACl) as coagulant. Results obtained from this study indicate that the presence of NOM increased the concentration of flocs and shifted the particle size distribution toward smaller particle sizes with a concurrent increase in the effluent turbidity. A mechanistically based hydraulic flocculation model, which takes effects of humic acids into account, was developed in this research based on observations of residual turbidity. The model was validated by successfully predicting data from independent experiments. The predictive model provides a useful guideline for effective coagulant dosages in water treatment.