The objective of this study is to simulate the impact of point source and non-point sources of pollution to the river tributary in University Sains Malaysia (USM) by using QUAL2K. Three parameters are used in simulation which are Biochemical Oxygen Demand (BOD), Ammoniacal Nitrogen (NH3-H), and Total Suspended Solid (TSS). Chemical Oxygen Demand (COD) and Dissolved Oxygen (DO) are measured in order to calculate the Water Quality Index of the river. Trial and error method is used to calibrate the model parameters until the simulated results agreed with the observed data. Then, the low flow scenario is simulated to observe the impact of pollution on the river during low flow. Results shows Water Quality Index (WQI) of river tributary in USM was classified as Class III, which agree with Environmental Quality Report 2018. The main pollutants of the river were Ammonium Nitrogen (NH3-H) which the concentration of NH3-H of all sampling stations exceed 2.7 mg/L. Low flow simulation shows BOD and NH3-H of the sampling station exceed 2.7mg/L (Class IV) at the downstream of the river while TSS concentration maintain Class I at the downstream of the river