Abstract: Assessment of spatial and temporal changes in surface water is an important aspect for evaluating temporal variations of the river’s pollution due to natural and anthropogenic input of point and non point sources. MERI has been implementing a seasonal water quality study from 14 sampling sites for 22 parameters collected since 1993 in the Hackensack River and its tributaries. This study assessed variation of water qualities, classification of monitoring sites and examined interrelationship between different variables. Spatial distributions identified three different zones by Ward’s Hierarchical agglomerative clustering analysis (CA). A decision tree was also constructed to see the relationship between each water quality parameters and each cluster as an alternate statistical approaching method. Conductivity, sulfate, BOD, NH4, DO and Iron had the highest influence in order on the water quality among 22 parameters to separate three clusters.